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(Ritchie, Hanway, Thompson, & Benson, 1994; Smit, 2000:9). ...... without causing important reductions in grain yield (Strydom, Bennie, van Rensburg.
EFFECTS OF SOIL WATER AVAILABILITY AND PLANTING DENSITY ON THE GRAIN YIELD OF SOYA BEANS [Glycine max (L.) Merrill]

by

ERIC NEMAKHAVHANI NDOU

Submitted in partial fulfillment of the requirements for the degree

MAGISTER TECHNOLOGIAE: AGRICULTURE

in the

Department of Crop Sciences

FACULTY OF SCIENCE TSHWANE UNIVERSITY OF TECHNOLOGY

Supervisor: Prof W Van Averbeke

November 2006

DECLARATION

“I hereby declare that the dissertation submitted for the degree M Tech: Agriculture, at the Tshwane University of Technology, is my original work and has not been previously submitted to any other institution of higher education. I further declare that all sources cited or quoted are indicated and acknowledged by means of a comprehensive list of references”.

E.N. Ndou

Student number: 99005149

Copyright© Tshwane University of Technology 2006 i

DEDICATION

This manuscript is dedicated to my beloved mother, Mmbadi Thivhonali Gladys, who passed away before its completion. Ndi a livhuwa u aluswa, u dzheniswa tshikilo, ţhuţhuwedzo na lufuno lwevha sumbedza sa mubebi. Ndi do vha elewa misi yoţhe.

ii

ACKNOWLEDGEMENTS

I thank God, the Almighty, for the strength and protection he provided me with throughout the study.

I would like to express my sincere gratitude to Prof W Van Averbeke for his keen supervision, suggestions and support throughout the study and to Prof PJ Jansen van Vuuren, the Head of the Department of Crop Sciences, for providing me with facilities, research equipment and implements on the Research Farm.

I am also grateful to The National Research Foundation, the Research Committee of the Faculty of Agricultural Sciences and the Tshwane Nutrition Project for their financial support towards my work and to the Monsanto (Sensako) Seed Company for supplying and delivering, free of charge, the SNK 500 seed that was used in the 2002/03 experiment.

My thanks are also due to Dr B Eisenberg for providing me with advice and assistance during the statistical analysis of my data, Mrs F Mahlangu for her assistance in accessing information in the TUT library and Mr G de Nysschen of the ARC-ISCW for his assistance with the climatic data.

iii

I acknowledge Mr FK Khangale, Mr AI Ntho, Mr P Moila, Mr MN Fihla, Mr A Nyamela and Ms L Sibeko and particularly Mr ZM Sosibo, who all helped me at one stage or another with the work in the field or in the laboratory.

I also wish to thank Mr D Mfolo and the staff at the TUT Research Farm for their assistance with field preparation.

My appreciation also goes to Dr MA Smit of the SA Sugar Research Institute and to Mr AJ de Lange, Dr AJ Liebenberg and Mr GP de Beer of the ARC-Grain Crops Institute for their valuable advice during the course of this study. I would also like to thank my friends whose love, encouragement and support gave me the inspiration until the end of my study.

Last but not least, I am indebted to my parents (Mmbangiseni and the late Thivhonali) for their words of encouragement and support.

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ABSTRACT

The current study investigated interaction effects between soil water availability and planting density on the grain yield of soya beans [Glycine max (L.) Merrill] for the purpose of refining the existing planting density recommendations for local use. A factorial experiment involving a split-plot design with soil water availability as main plot treatment and planting density as split-plot treatment was conducted during the 2002/03 and 2003/04 summers. Three soil water availability treatments and planting densities ranging from 5 plants m-2 to 80 plants m-2 were employed. Overall, the study confirmed existing theory that describes soil water availability and planting density relationships in soya beans. In the 2002/03 experiment, which was defoliated by a hail storm, only planting density had a statistically significant effect on grain yield, whilst in the 2003/04 experiment, the effects of soil water availability, planting density and their interaction were significant. In both experiments, the increase in grain yield obtained by raising planting density was primarily due to an increase in the number of pods per unit area and number of seeds per unit area, whilst other yield components were less affected. In the 2003/04 experiment the interaction effect between soil water availability and planting density was statistically highly significant.

v

TABLE OF CONTENTS PAGE DECLARATION

i

DEDICATION

ii

ACKNOWLEDGEMENTS

iii

ABSTRACT

v

TABLE OF CONTENTS

vi

LIST OF FIGURES

ix

LIST OF TABLES

xiv

LIST OF SYMBOLS AND ABREVATIONS

xix

LIST OF APPENDICES

xxiii

CHAPTER ONE 1

INTRODUCTION

1

CHAPTER TWO 2

REVIEW OF LITERATURE

5

2.1

Physiological and agronomic characteristics of soya beans

5

2.2

Effect of planting density and spacing on the growth and yield of soya beans

2.3

Effect of soil water availability on the growth and yield of soya beans

2.4

Planting density and soil water availability interaction effects on the growth and yield of soya beans

8 13

15

vi

2.5

2.6

Southern African perspectives on planting density and soil water availability effects in soya beans

16

Conclusions

17

CHAPTER THREE 3

MATERIAL AND METHODS

19

3.1

Introduction

19

3.2

Description of the study site

19

3.3

Experimental procedures

27

3.3.1

General procedures

27

3.3.2

Specific procedures that applied to the 2002/03 experiment

31

3.3.3

The 2003/04 experiment

37

CHAPTER FOUR 4

RESULTS AND DISCUSSION

40

4.1

Introduction

40

4.2

The 2002/03 experiment

40

4.2.1

Conditions during the growing season

40

4.2.2

Water

41

4.2.3

Leaf area development

47

4.2.4

Grain yield

49

4.2.5

Yield components

52

4.3

The 2003/04 experiment

60

vii

4.3.1

Conditions during the growing season

60

4.3.2

Water

62

4.3.3

Leaf area development

67

4.3.4

Grain yield

69

4.3.5

Yield components

71

4.4

Summary of the findings

80

CHAPTER FIVE 5

CONCLUSIONS

85

REFERENCES

87

APPENDICES

99

viii

LIST OF FIGURES PAGE FIGURE 1.1:

Area planted to soya beans in South Africa and total production during the period 1970-2003 (National Department of Agriculture, 2004:19)

FIGURE 2.1:

Effect of planting density and soil water availability on the grain yield of soya beans after Elmore (1998)

FIGURE 3.1:

21

Map of the City of Tshwane showing the location of Honingnestkranz

FIGURE 3.4:

20

Map of Gauteng Province showing the location of the City of Tshwane

FIGURE 3.3:

15

Map of South Africa showing the location of Gauteng Province

FIGURE 3.2:

2

22

Map of Honingnestkranz showing the location of the TUT Research Farm

23

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FIGURE 3.5:

Estimated mean daily evapotranspiration (mm) by soya beans planted on 1 November at Roodeplaat using the procedures of Green (1985)

FIGURE 3.6:

A water meter with accuracy of 1litre was used to measure the amount of irrigation water applied to the plots

FIGURE 3.7:

32

The bridge that was used during thinning and weed control in the high planting density treatments

FIGURE 4.1:

29

View of the level basins used to maintain the high water (HW) and intermediate water (IW) treatment

FIGURE 3.8:

25

35

(a) The 2002/03 experiment a few hours before the hailstorm occurred (61 DAP); (b) the experiment the day after the hailstorm damage (63 DAP); (c) close view of the hail damage to the soya bean plants in the low planting density treatment (10 plants m-2, 63 DAP); (d) plants showing remarkable recovery from the hail damage by producing a new flush of leaves (high planting density treatment, 84 DAP)

41

x

FIGURE 4.2:

Comparison of the total weekly amounts of water supplied to the high water (HW) treatment with the expected crop evapotranspiration estimated using the procedures described by Green (1985:6, 33) in the 2002/03 experiment

FIGURE 4.3:

42

Comparison of the total weekly amounts of water supplied to the intermediate water (IW) treatment with the expected crop evapotranspiration estimated using the procedures described by Green (1985:6, 33) in the 2002/03 experiment

FIGURE 4.4:

43

Comparison of the total weekly amounts of water supplied to the low water (LW) treatment with the expected crop evapotranspiration estimated using the procedures described by Green (1985:6, 33) in the 2002/03 experiment

FIGURE 4.5:

Effect of planting density on leaf area index of soya beans in the high water (HW) treatment of the 2002/03 experiment

FIGURE 4.6:

44

48

The interaction between planting density and water levels on the grain yield of soya beans in the 2002/03 experiment

51

xi

FIGURE 4.7:

From emergence until 60 days after planting (DAP), water stress negatively affected growth of the soya bean plants in the low water (LW) treatment as is shown here for the highest planting density treatment (80 plants m-2) 56 DAP

FIGURE 4.8:

61

View of soya bean plants in the lowest planting density treatment (5 plants m-2) in the low water (LW) treatment 56 DAP

FIGURE 4.9:

61

Comparison of the total weekly amounts of water supplied to the high water (HW) treatment with the expected crop evapotranspiration estimated using the procedures described by Green (1985:6, 33) in the 2003/04 experiment

FIGURE 4.10:

62

Comparison of the total weekly amounts of water supplied to the intermediate water (IW) treatment with the expected crop evapotranspiration estimated using the procedures described by Green (1985:6, 33) in the 2003/04 experiment

FIGURE 4.11:

63

Comparison of the total weekly amounts of water supplied to the low water (LW) treatment with the expected crop evapotranspiration estimated using the procedures described by Green (1985) in the 2003/04 experiment

65

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FIGURE 4.12:

Effect of planting density on leaf area index of soya beans in the high water (HW) treatment of the 2003/04 experiment

FIGURE 4.13:

68

The interaction between planting density and water levels on the grain yield of soya beans in the 2003/04 experiment

70

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LIST OF TABLES PAGE TABLE 2.1:

Vegetative and reproductive stages of soya bean plants (after Fehr and Caviness, 1977)

TABLE 2.2:

Effect of planting density on the grain yields and yields components of soya beans after Board (2000)

TABLE 2.3:

6

12

Effects of soil water availability on growth and yield of soya beans during different reproductive (R) stages of growth after Ritchie et al. (1994)

TABLE 2.4:

Effect of planting density on soya beans grain yield (after Seed Co., SA)

TABLE 3.1:

14

17

Summary of climatic variables recorded at the Roodeplaat Agricultural Research Station over a period of 45 years (after ARC-ISCW, 2005)

TABLE 3.2:

24

Plant spacing and equivalent planting densities used in the 2002/03 and 2003/04 soya bean experiments

31

xiv

TABLE 3.3:

Chemical properties of the topsoil at the site of the 2002/03 experiment before the experiment was planted and after it was harvested

TABLE 3.4:

Chemical properties of the topsoil at the 2003/04 experimental site

TABLE 4.1:

33

38

Weekly amount of rainfall received at the experimental site during the growing period of soya beans in the 2002/03 experiment

TABLE 4.2:

Weekly amount of irrigation applied to the different water treatments in the 2002/03 soya bean experiment

TABLE 4.3

47

Effect of planting density and soil water availability on the grain yield of soya beans in the 2002/03 experiment

TABLE 4.5:

46

Mean total amount of water recorded for the different treatments in the 2002/03 experiment

TABLE 4.4:

46

50

Effect of planting density and soil water availability on the number of soya bean pods plant-1 in the 2002/03 experiment

52

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TABLE 4.6:

Effect of planting density and soil water availability on the number of soya bean pods per unit area in the 2002/03 experiment

TABLE 4.7:

Effect of planting density and soil water availability on the number of soya bean seeds plant-1 in the 2002/03 experiment

TABLE 4.8:

54

55

Effect of planting density and soil water availability on the number of soya bean seeds per unit area in the 2002/03 experiment

TABLE 4.9:

Effect of planting density and soil water availability on the number of soya bean seeds pod-1

TABLE 4.10:

58

Effect of planting density and soil water availability on the mass of 100 soya bean seeds in the 2002/03 experiment

TABLE 4.11:

57

59

Weekly amount of rainfall received at the experimental site during the growing period of soya beans in the 2003/04 experiment

TABLE 4.12:

66

Weekly amount of irrigation applied to the different water treatments in the 2003/04 soya bean experiment

66

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TABLE 4.13:

Mean total amount of water recorded for the different treatments in the 2003/04 experiment

TABLE 4.14:

Effect of planting density and soil water availability on the grain yield of soya bean in the 2003/04 experiment

TABLE 4.15:

67

69

Effect of planting density and soil water availability on the number of soya bean pods per plant in the 2003/04 experiment

TABLE 4.16:

72

Effect of planting density and soil water availability on the number of soya bean pods per unit area in the 2003/04 experiment

TABLE 4.17:

74

Effect of planting density and soil water availability on the number of soya bean seeds per plant in the 2003/04 experiment

TABLE 4.18:

75

Effect of planting density and soil water availability on the number of soya bean seeds per unit area in the 2003/04 experiment

76

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TABLE 4.19:

Effect of planting density and soil water availability on the number of soya bean seeds per pod in the 2003/04 experiment

TABLE 4.20:

77

Effect of planting density and soil water availability on the seed mass (100 seeds) of soya beans in the 2003/04 experiment

TABLE 4.21:

78

Summary of the results obtained in the 2002/03 and 2003/04 soil water availability x planting density experiments with soya beans in Pretoria

81

xviii

LIST OF SYMBOLS AND ABREVATIONS

AGB

Above-ground biomass

ANOVA

Analysis of variance

AP

After planting

ARC-ISCW

Agricultural Research Council - Institute for Soil, Climate and Water

Ca

Calcium

CGR

Crop growth rate

cm

Centimetre

cmol(+) kg-1

Centimol positive charge per kilogramme soil

CWU

Consumptive water use

DAP

Days after planting

Eto

Reference crop evapotranspiration

Evap.

Evapotranspiration

FSSA

Fertilizer Society of South Africa

g

Gramme

g m-2

Gramme per square metre

GY

Grain yield

ha-1

Per hectare

Hr day-1

Hour per day

HW

High water treatment

IW

Intermediate water treatment

K

Potassium

xix

kg

Kilogramme

kg ha-1

Kilogramme per hectare

kg m-3

Kilogramme per cubic metre

kg P ha-1

Kilogramme phosphorus per hectare

km

Kilometre

km/day

Kilometre per day

LAGR

Leaf area growth rate

LAI

Leaf area index

LSD

Least Significant Difference

LW

Low water treatment

m

Metre

m2

Square metre

Mg

Magnesium

mg kg-1

Milligramme per kilogramme

mg P kg-1

Milligramme phosphorus per kilogramme soil

mm

Millimetre

mm day-1

Millilitre per day

N

Nitrogen

Na

Sodium

NAR

Net assimilation rate

NSD

Non significant difference

o

Degree Celsius

C

P

Phosphorus

xx

PAWC

Profile available water capacity

pHwater

pH measured in water

plant-1

Per plant

Pod-1

Per pod

R

Reproductive stage

R1

Start of flowering stage

R2

Full flowering stage

R3

Start of pod formation stage

R4

Full pod formation stage

R5

Start of seed formation stage

R6

Full seed development stage

R7

Start of maturing stage

R8

Full maturity stage

SAS®

Statistical Analysis Software

Seed-1

Per seed

T

Temperature

t ha-1

Ton per hectare

TUT

Tshwane University of Technology

USA

United States of America

V

Vegetative stage

V (n)

nth node stage

V1

First node stage

V2

Second node stage

xxi

V3

Third node stage

V4

Fourth node stage

V5

Fifth node stage

VC

Cotyledon stage

VE

Emergence stage

WAP

Weeks after planting

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LIST OF APPENDICES PAGE APPENDIX A1:

Description of the soil profile of the Hutton Ventersdorp soil at the TUT Research Farm (Bon Accord, Tshwane)

APPENDIX A2:

Analytical data for the Hutton Ventersdorp type soil at the TUT Research Farm

APPENDIX B:

99

100

Leaf area index of the 2002/03 and 2003/04 planting density x soil water availability soya beans experiment

101

TABLE B1:

Leaf area index values, 2002/03 experiment

101

TABLE B2:

Leaf area index values, 2003/04 experiment

101

APPENDIX C:

Analysis of variance of the 2002/03 and 2003/04 planting density x soil water availability soya beans experiment

TABLE C1:

Analysis of variance for grain yield of soya bean, 2002/03 experiment

TABLE C2:

102

102

Analysis of variance for number of soya bean pods plant-1, 2002/03 experiment

103

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TABLE C3:

Analysis of variance for number of soya bean pods m-2, 2002/03 experiment

TABLE C4:

Analysis of variance for number of soya bean seeds plant-1, 2002/03 experiment

TABLE C5:

106

Analysis of variance for grain yield of soya bean, 2003/04 experiment

TABLE C10:

105

Analysis of variance for consumptive water use of soya bean, 2003/04 experiment

TABLE C9:

105

Analysis of variance for 100 seed mass of soya bean, 2002/03 experiment

TABLE C8:

104

Analysis of variance for number of soya bean seeds pod-1, 2002/03 experiment

TABLE C7:

104

Analysis of variance for number of soya bean seed m-2, 2002/03 experiment

TABLE C6:

103

106

Analysis of variance for number of soya bean pods plant-1, 2003/04 experiment

107

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TABLE C11:

Analysis of variance for number of soya bean pods m-2, 2003/04 experiment

TABLE C12:

Analysis of variance for number of soya bean seeds plant-1, 2003/04 experiment

TABLE C13:

108

Analysis of variance for number of soya bean seeds pod-1, 2003/04 experiment

TABLE C15:

108

Analysis of variance for number of soya bean seeds m2, 2003/04 experiment

TABLE C14:

107

109

Analysis of variance for seed mass (100 seeds), 2003/04 experiment

109

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CHAPTER ONE 1 INTRODUCTION The soya bean plant [Glycine max (L.) Merrill] is an annual summer legume that belongs to the Fabaceae family. It is probably the most important legume in the world. As with other members of this family, it bears its seed in pods. The crop is thought to be indigenous to Manchuria, China, where it has been utilised as a food from about 2500 BC (Smit, 2000:6). In the United States, soya beans were introduced as hay and forage crop, but it became increasingly important as an oil and protein meal seed crop during the 19th century. Towards the end of the 20th century, the United States of America was the world’s largest producer of soya beans (Hume, Shanmugasundaram & Beversdorf, 1985:391 and Smit, 2000:6). According to Smit (2000:6) the first South African report on soya beans is found in the Cedara Memoirs of 1903. Poor germination and shattering of pods experienced in initial trials lead to the implementation of a breeding programme at the Agricultural Research Station in Potchefstroom during the early 1950s. This programme resulted in the development of varieties that were adapted to local conditions and no longer shattered prematurely. Production of soya beans in South Africa remained localised and relatively unimportant until the early 1970s, when soya beans were incorporated in a rotation with wheat on land where irrigation was available (Smit, 2000:6).

Soya beans play an important role in human and animal nutrition. They contain about 40 % protein, which is of excellent quality and digestibility. They are also a good source of energy and fatty acids for animals (Smit, 2000:3). Commercially, the value of soya beans is further enhanced by the use of the crop as a raw material for a wide

1

range of applications. Important products derived from soya beans are protein meal (about two-thirds of total world consumption) and edible oil (just over a quarter of world consumption). The crop also has many applications in the human food and nutrition sector and in the chemical industry (Smit, 2000:3).

Relative to other parts of the world, especially in Asia, the production of soya beans in South Africa is relatively new. Some 35 years ago, less than 10 000 ha were planted to the crop. Since then, production has increased considerably, especially during the last 15 years (Figure 1). In 2002/03, about 100 000 ha were planted to soya beans in South Africa, producing a grain yield that exceeded 120 000 tons.

Area

180 160

Production

140 120 100 80 60 40

2002/03

1998/99

1994/95

1990/91

1986/87

1982/83

1978/79

1974/75

20 0 1970/71

Area planted (x1000 ha) and Production (x1000 t)

200

Years

FIGURE 1.1: Area planted to soya beans in South Africa and total production during the period 1970-2003 (National Department of Agriculture, 2004:19)

Despite the substantial increase in local production, South Africa remains an importer of soya beans. In 2002, the country imported about 0.9 million tons of soya bean oil

2

cake (National Department of Agriculture, 2004:89) demonstrating that local production still does not meet the national demand.

Local soya bean production may be increased by expanding the area planted to the crop, or by improving its management on existing land. The current study seeks to make a contribution to improved management of the crop. Its focus is on tailoring planting density in soya beans to the expected availability of soil water during the growing season. According to De Beer (2002) and Smit (2003) the existing planting density guidelines in local soya bean production are useful approximations, but they are still subject to refinement.

Experience elsewhere has shown that optimizing planting density in soya beans to suit a particular set of growing conditions such as climate, nutrient availability and row spacing may increase bean yields by 5 to 15 % (Duncan, 1986; Ethredge, Ashley & Woodruff, 1989; Board, Kamal & Harville, 1992; Egli, 1994). Many factors affect optimum plant population density, but there is no doubt that water availability is among them (Taylor, 1980; Graterol, Elmore & Eisenhauer, 1996; Elmore, 1998). By investigating the interactions between water availability and planting density, the current study seeks to contribute to the refinement of existing guidelines for planting density in local soya bean production.

The purpose of the study was to contribute to knowledge aimed at increasing the yield of soya beans obtained by South African farmers by refining existing planting density recommendations for different levels of soil water availability.

The specific

objectives of the study were to empirically determine the response of soya beans to

3

planting density at different levels of water, and to investigate the possible interaction between these two factors. The hypothesis of the study was that “grain yield of soya beans is subject to a positive interaction between soil water availability and planting density”.

The study delimitations were that the experiment was conducted at a single site over a period of two years only. Two different cultivars with similar growth habits were used as test crops (one cultivar per season). Soil water availability was not monitored during the growing season, but gravimetric samples were taken at planting and at harvest and water application to the different water treatments was recorded, enabling the estimation of total consumptive water use during the growing season in each of these treatments. Leaf area development of the soya bean plants was monitored in the high water treatment only.

4

CHAPTER TWO

2 REVIEW OF LITERATURE

2.1 Physiological and agronomic characteristics of soya beans

Soya beans are photoperiodic and are known as quantitative short-day plants. The main effect of day length on soya bean development is that of flowering induction. Reduction in day length triggers the onset of the reproductive stage (Norman, 1978; Farias, 1994; Smit, 2000:9).

Soya bean varieties are categorised in several ways. One of these is growth habit. There are two types of soya bean varieties, namely determinate and indeterminate. In determinate varieties vegetative growth is terminated by the onset of flowering and the growth tip ends in a pod-bearing raceme, a short, stem-like structure that produces flowers and later on pods along its length. Varieties with an indeterminate growth habit develop flowers and form pods simultaneously and over an extended period (Ritchie, Hanway, Thompson, & Benson, 1994; Smit, 2000:9).

Smit (2000:10)

reports that South African soya bean varieties are further subdivided into three maturity classes based on their time to maturity. Short, medium and long growth period varieties are identified.

During its development, soya beans pass through different growth stages. Fehr and Caviness (1977) divide the growth stages of soya beans into two main stages, namely,

5

vegetative and reproductive, as illustrated in Table 2.1. In a stand a growth stage is reached when 50 % of the plants are at or past that particular stage of development.

TABLE 2.1: Vegetative and reproductive stages of soya bean plants (after Fehr and Caviness, 1977)

Vegetative stages

Reproductive stages

Stage no

Stage description

Stage no

Stage description

VE

Emergence

R1

Start of flowering

VC

Cotyledon

R2

Full flowering

V1

First node

R3

Start of pod formation

V2

Second node

R4

Full pod formation

V3

Third node

R5

Start of seed formation

V4

Fourth node

R6

Full seed development

V5

Fifth node

R7

Start of maturing

V (n)

nth node

R8

Full maturity

Soya bean development is sensitive to environmental factors. These include soil type, pests and diseases, wind, light, temperature and water (Ritchie et al., 1994). Water use by soya beans varies with climatic conditions, management practices and length of growing season (Whigham & Minor, 1978; Weiss, 1983; Farias, 1994). In South Africa soya beans can be grown successfully in areas where the mean annual rainfall exceeds 600 mm (Smit, 2000:4).

Soya beans are tolerant to a wide range of soil conditions, but they favour a deep, well-drained fertile soil with a high water holding capacity. An optimum pHwater of 6.0 to 6.5 is desirable, but soya beans can tolerates pH levels ranging between 5.2 and 6

7.5. The crop can be grown successfully on heavy clay soils and sandy soils. In general, soya beans can be produced successfully in areas where maize is grown (Weiss, 1983; Fertilizer Society of South Africa, 1989:187; Smit, 2000:16).

Soya beans are best described as a weakly tap-rooted crop. The taproot gives rise to many lateral roots in the top 30 cm of the soil profile. In highly compacted soils soya beans produce short plants with restricted root development. In conditions where soil compaction is not a limiting factor, soya bean taproots may reach depths exceeding 1.5 m (Weiss, 1983; Gazzoni, 1994; Smit, 2000:7).

Soya beans develop well under a wide range of temperatures. The minimum and maximum temperatures for soya bean growth are 10 0C and 40 0C, respectively. The optimum temperature for growth is 25 0C.

Regions where the mean monthly

temperature of the warmest month is less than 20 0C are considered poorly suited for soya bean production (Farias, 1994; Smit, 2000:16).

Soya bean seeds are borne in pods. The pods contain one to four seeds (Smit, 2000:7). According to Gazzoni (1994) and Smit (2000:8), average grain mass of soya beans ranges between 12 g and 25 g per 100 seeds. Differences in seed mass among varieties do not necessarily affect grain yield. Depending on the cultivar, planting density and growing conditions, a single plant can bear from three to 350 pods (Gazzoni, 1994; Smit, 2000:7).

7

2.2 Effect of planting density and spacing on the growth and yield of soya beans

The growth of plants is studied by means of growth analysis (Hicks, 1978). Growth analysis involves the measurement of plant growth in terms of net assimilation rate (NAR), leaf area growth rate (LAGR), and crop growth rate (CGR) (Barnes & Beard, 1992:17). The purpose of growth analysis is to investigate relationships between these variables over time (Barnes & Beard, 1992:17). CGR is defined as the increase in biomass per unit time. CGR is a function of NAR and leaf area index (LAI). NAR is the increase in biomass per unit of assimilatory material per unit time. LAI is the leaf area of the plants per unit soil surface area (Hicks, 1978; Barnes & Beard, 1992:17).

Grain yield in soya beans is closely related to CGR (Shibles & Weber, 1966). NAR in soya beans declines as LAI increases, because leaf development gives rise to selfshading of the lower leaves. CGR, on the other hand, increases as LAI increases (Hicks, 1978). Generally, CGR of soya beans increases during the first 50 to 60 days after planting until the onset of the flowering (R1) stage and declines thereafter (Buttery as cited by Hicks, 1978).

As planting density increases, light interception per plant decreases, but total light interception per unit ground area increases. A reduction of light interception reduces leaf area and above-ground biomass (AGB) per plant, but increases leaf area and AGB per unit area (Board, Kamal & Harville, 1992; Board & Harville, 1992; Board, 2000).

8

Grain yield of soya beans is the product of several components, namely, number of pods per plant, number of seeds per plant, number of seeds per pod, seed mass and number of plants per unit area (Lehman & Lambert as cited by Tudor-Owen, 1978:4; Ritchie et al., 1994). The grain yield of soya beans is described by equation 1.

GY = No plants m-2 x No pods plant-1 x No of seeds pod-1 x Mass seed-1

equation 1

whereby GY

= grain yield of soya beans expressed in g m-2

No plants m-2

= planting density expressed as mean number of plants m-2

No pods plant-1

= mean number of pods plant-1

No of seeds pod-1

= mean number of seeds pod-1

Mass seed-1

= mean seed mass expressed in g

Generally, research on the effect of planting density on the yield of soya beans grown in the United States shows that grain yield increases as planting density is raised until an optimum is reached, and that increasing planting density in excess of the optimum brings about a decline in grain yield (Wright, Shokes & Sprenkel, 1984; Moore and Longer, 1987; Board, 2000). One of the reasons for grain yield to decline when planting density exceeds the optimum is an increase in lodging (Cooper, 1971; Boquet, 1990). Grain yield in soya beans is a function of LAI, and LAI is a function of planting density (Shibles & Weber, 1965; Board, Kamal & Harville, 1992; Browde, Pedigo, Owen & Tylka, 1994; Grymes, Griffin, Boethel, Leonard, Jordan & Russin, 1999).

9

Increasing planting density reduces the time needed by the crop to develop a canopy that intercepts 95 % of the available light at which CGR is expected to be optimal. In soya beans interception of 95 % of the available light occurs at LAI values of 3.5 to 4 (Shibles & Weber, 1965; Board, Kamal & Harville, 1992; Board & Harville, 1992; Board, Weir & Boethel, 1997; Board, 2000). Shibles & Weber (1966) contend that soya beans produce optimum grain yield when they attain the critical LAI of 3.5 to 4.0 by the time they reach developmental stages R2 to R5. Duncan (1986), on the other hand, argues that optimum-planting density for grain yield coincides with the planting density that produces maximum AGB at the time of flower initiation. This means that differences in grain yield of soya beans due to differences in planting density are established during the V stages and early R1 stage, and not during the R2 – R7 stages of growth (Duncan, 1986; Elgi, 1988). As a result, the optimum planting density for grain yield may well be above that required to achieve 95% light interception at full leaf development (Duncan, 1986; Elgi, 1988; Board, Kamal & Harville, 1992).

The response of soya beans to planting density is essentially a function of variability in the number of pods per unit area, which, in turn, is a function of the number of fertile nodes (Board, Kamal & Harville, 1992). The number of pods and seeds per plant represent the size of the assimilate sink during reproductive growth. Increasing planting density and narrowing the spacing cause a reduction in the number of pods per plant (Enyi, 1973). Several studies have shown that whereas the number of pods and seeds plant-1 decrease with increasing plant density, they increase per unit area (Enyi, 1973; Tudor-Owen, 1978:31; Elgi, 1988; Ethredge, Ashley & Woodruff, 1989). Planting density and row spacing do not affect the number of seeds pod-1 in

10

soya beans (Enyi, 1973; Tudor-Owen, 1978:55; Ikeda, 1992) and they appear to have a limited effect on seed mass. Enyi (1973), Tudor-Owen (1978:55) and Taylor (1980) report an increase in seed mass when plant spacing was widened, but Chapman (1987), Egli (1988), Ethredge, Ashley and Woodruff (1989) and Board (2000) failed to record a response in seed mass to variability in plant spacing.

Research in the USA has also shown that the relationship between grain yield and planting density in soya beans is subject to considerable variation. Board and Harville (1992) report that when planting density is kept constant, equidistant spacing increases LAI and light interception during the vegetative stages and early reproductive stages. According to these authors, grain yields were achieved that are higher than those obtained when spacing is not equidistant. Similarly, Board, Kamal and Harville (1992) show that when planting density is kept constant, narrowing the row spacing increases light interception and grain yield in soya beans. However, Wells (1991) points out that the advantage of narrowing rows, which results in more equidistant plant spacing, occurs primarily during the early part of the season, when it brings about a reduction in competition among plants within the row. This increases total biomass, but not necessarily grain yield. Examining four planting density and row width combinations in soya beans, Wells (1991) recorded differences in total biomass yield among the treatments that were statistically significant, but the differences in grain yield among the treatments were not. Chapman (1987) points out that soya bean plants have the ability to fill the available space around them by producing lateral branches, which results in a similar number of nodes per unit area being produced over a range of planting densities. The ability of a soya bean plant to adapt its development in terms of number of lateral branches to the available space is

11

probably one of the factors that explain the absence of substantial differences in grain yield among different planting density treatments that were observed in some studies.

Cultivar is another factor that affects the relationship between grain yield and planting density in soya beans.

Tudor-Owen (1978:58) reports that grain yield of both

determinate and indeterminate cultivars peaks at a density of 40 plants m-2. Egli (1988, 1994) disagrees. He argues that determinate and indeterminate soya bean cultivars differ in their response to planting density. He reports that determinate cultivars tend to have a distinct optimum density for grain yield, but indeterminate varieties do not.

Recent work by Board (2000), summarized in Table 2.2,

demonstrates that even determinate cultivars may lack a distinct optimum planting density. His work shows that planting density effects over a wide range of densities (8 to 39 plants m-2) can be eliminated when using determinate cultivars that partition a high proportion of their dry matter into the branches.

TABLE 2.2:

Effect of planting density on the grain yields and yields

components of soya beans after Board (2000)

Planting density

Seed yield

Number of seeds m-

(no plants m-2)

(g m-2)

2

Number of seeds pod-1

Number of pods m-2

8.0

406.6

2459

1.84

1336

14.5

415.3

2505

1.81

1384

39.0

396.1

2386

1.82

1311

Mean

406.0

2450

1.82

1344

12

2.3 Effect of soil water availability on the growth and yield of soya beans

Research into the effects of water stress on soya beans shows the crop to be fairly drought resistant, but not insensitive to water deficits (Korte, Williams, Specht & Sorensen, 1983a and 1983b; Smiciklas, Mullen, Carlson & Knapp, 1992; Batchelor, 1998; Paz, Batchelor & Seidl, 2000).

Soya beans germinate best when planted in moist soil. Irrigation or rain after planting reduces the final plant population, primarily because of soil crusting. Excessively high water contents in the surface layer of the soil also have a negative effect on germination rate (Smit, 2000:8).

Generally, soya bean seed yield is least sensitive to water stress during the vegetative stages.

Insufficient soil water availability during vegetative growth limits crop

growth and may have a negative effect on grain yield.

Adequate soil water

availability during the V stages of growth results in optimum vegetative growth of the soya bean plant, but this does not always result in grain yields that are higher than those obtained under conditions where soil water was less available (Stanley, Kaspar & Taylor, 1980; Smit, 2000:54-55).

Soya beans are most sensitive to water stress during flowering, pod set, and particularly during pod fill (Constable & Hearn, 1980). The important effects of water stress on the growth and yield of soya beans during the different reproductive stages of growth are shown in Table 2.3. When soya beans are subjected to water

13

stress during the early reproductive stages (R1, R2 and R3) LAI is reduced and flower and pod abortion is increased. This, in turn, brings about a reduction in seed number. However, when soil water availability improves during subsequent R stages, an increase in seed mass relative to that obtained from soya beans that was not subjected to water stress during the R1 to R3 stages is likely (Carter & Hopper as cited by Essa, 1979:14; Korte et al., 1983b; Eck, Mathers & Musick, 1987; Smiciklas et al., 1992). Eck, Mathers and Musick (1987), Smiciklas et al. (1992) and Smit (2000:55) report that grain yield in soya beans is most affected by a lack of water when water stress occurs during the R4 to R5 stages.

TABLE 2.3: Effects of soil water availability on growth and yield of soya beans during the different reproductive (R) stages of growth after Ritchie et al. (1994)

Effect of water deficit

Stage R1

Water stress results in flower abortion.

R2

Water stress results in flower abortion.

R3

Water stress results in abortion of young pods. Stress from R1 to R3 does not reduce yields greatly because new flowers and pods can still be produced to compensate for those that were aborted.

R4

Water stress results in a reduction of the number of pods per plant. This is the beginning of a critical period, because flowering nears completion. New pods and seeds are prone to abortion.

R5

Water stress reduces seed size and mass. This is still part of the critical stage, because yield can be reduced by up to 75 percent.

R6

Water stress reduces pods per plant, seeds per pod and seed size.

R7

Water stress at this stage has little effect on yield, because the seeds have already accumulated a sizable portion of their mature dry weight.

R8

Water stress no longer has an effect on grain yield.

14

2.4 Planting density and soil water availability interaction effects on the growth and yield of soya beans

Research investigating the interactions between water and planting density in soya beans shows that optimum planting density increases as soil water availability improves (Taylor, 1980; Graterol, Elmore & Eisenhauer, 1996; Elmore, 1998). As a result, research aimed at determining optimum planting density in soya beans needs to take soil water availability into account. Elmore (1998) reports an optimum planting density of 11 plants m-2 under dryland conditions and 35 plants m-2 under irrigated conditions (Figure 2.1).

-2

Grain yield (g m )

600

400

rainfed

200

irrigation

0 11

35

59

82

-2

Plants per m

FIGURE 2.1: Effect of planting density and soil water availability on the grain yield of soya beans after Elmore (1998)

Taylor (1980) tested the hypothesis that wide-row soya beans yield as much, or more, than narrow-row soya beans during years of low seasonal water supply. When water

15

supply was high, narrow-row soya beans yielded more than wide-row plantings. During years with low seasonal water supply, differences in yield among the row spacing treatments were not statistically significant, even though plants growing in wide rows had larger leaf areas and set more pods than those growing in narrow rows.

Alessi and Power (1982) re-examined Taylor’s hypothesis by broadening the soil water availability spectrum. Over a period of four seasons, narrowly spaced soya beans had the lowest water use efficiency in three of the four years. Compared to widely spaced soya beans, planting the crop in narrow rows increased water use early in the season and left less water available for pod fill. However, Alessi and Power (1982) pointed out that planting density and row spacing had little effect on the grain yield of soya beans under conditions of modest water stress.

2.5 Southern African perspectives on planting density and soil water availability effects in soya beans

According to De Beer (2002) and Smit (2000:5; 2003) South African farmers commonly use a planting density of 30 plants m-2 when growing soya beans under rainfed conditions.

Grain yields obtained under these conditions typically vary

between 1 and 2 tons ha-1. Irrigated soya beans in South Africa typically produce a grain yield of about 3 tons of grain ha-1 (Smit, 2000:5; De Beer, 2002). Smit (2003) recommends that a planting density of 40 plants m-2 be used when growing the crop under irrigated conditions. Results reported by Seed Co. (SA) in Zimbabwe, shown in Table 2.4, tend to support this recommendation. However the yield response to increases in planting density from 16 to 40 plants m-2 was limited.

16

TABLE 2.4: Effect of planting density on soya beans grain yield (after Seed Co., SA)

Planting density (plants m-2)

Grain yield (g m-2)

8

305.0

16

332.0

24

339.0

32

342.0

40

348.0

Mean

333.2

2.6 Conclusions

Despite the substantial increase in local soya bean production over the past three to four decades, production of soya beans in South Africa still does not meet the local demand for the crop. Tailoring planting density to suit the expected level of soil water availability is one of the ways in which local soya bean production can be improved. Generally, grain yield of soya beans increases as planting density is raised until an optimum is reached. In terms of grain yield, the optimum planting density in soya beans increases as growing conditions improve, mainly as a result of an increase in the number of pods being produced per unit area. Soil water availability is one of the important factors determining growing conditions.

However, several factors

affect the relationship between soil water availability and planting density in soya beans. Among these are the ability of soya bean plants to adapt their growth and development to the space that is available, the ability of the crop to recover from

17

water stress, especially when stress occurs during the vegetative stages, and differences among cultivars in terms of the way the crop responds to intra-specific competition or the lack there of.

Generally, research results indicate that the

agronomic practice of optimizing planting density in soya beans to suit the prevailing growing conditions can bring about improvements in grain yield of the order of 5 % to 15 % (Duncan, 1986; Ethredge, Ashley & Woodruff, 1989; Board, Kamal & Harville, 1992; Egli, 1994).

18

CHAPTER THREE

3 MATERIAL AND METHODS 3.1 Introduction

A factorial experiment was conducted with the objective of investigating the interaction of planting density and soil water availability on the grain yield of soya beans. The hypothesis of the study was that there is a positive relationship between planting density and soil water content and grain yield.

The experiment was

conducted during the summer of 2002/03 and repeated during the summer of 2003/04.

3.2 Description of the study site

The field experiment was conducted at the Research Farm of the Tshwane University of Technology (TUT). The Farm is located at Honingnestkranz, near Bon Accord, north of the City of Pretoria, and forms part of the City of Tshwane in the Gauteng Province of South Africa. The Farm is situated at an altitude of 1173 m above sea level and its approximate coordinates are 250 37' S and 280 12' E. Maps showing the location of the TUT Research Farm are presented in Figures 3.1 to 3.4.

19

LIMPOPO

FIGURE 3.1: Map of South Africa showing the location of Gauteng Province

20

City of Tshwane

GAUTENG

FIGURE 3.2: Map of Gauteng Province showing the location of the City of Tshwane

21

City of Tshwane

Honingnestkranz

FIGURE 3.3: Map of the City of Tshwane showing the location of Honingnestkranz

22

Honingnestkranz TUT Research Farm

FIGURE 3.4: Map of Honingnestkranz showing the location of the TUT Research Farm

The Roodeplaat Agricultural Research Station is the weather station that is most representative of the climatic conditions experienced at the TUT Research Farm. It is located approximately 30 km north-east of the TUT Research Farm. The weather station at Bon Accord is located closer to the TUT Research Farm than the Roodeplaat Agricultural Research Station, but Bon Accord only records rainfall. The TUT Research Farm is situated between Bon Accord and the Roodeplaat Agricultural Research Station and all three sites form part of the same climatic zone (ARC-ISCW, 2007). This is borne out, for example, by the mean annual rainfall statistics, which are 648 mm annum-1 at Bon Accord and 641 mm annum-1 at the Roodeplaat Agricultural Research Station.

23

The Roodeplaat Agricultural Research Station is situated at an altitude of 1247 m, a latitude of 250 59’ S and a longitude of 280 36’ E. Important variables describing the climate at the Roodeplaat Agricultural Research Station are summarised in Table 3.1.

TABLE 3.1: Summary of climatic variables recorded at the Roodeplaat Agricultural Research Station over a period of 45 years (after ARC-ISCW, 2005)

Month

Rainfall

Mean

Mean

Mean

Mean

Sunshine

Wind

(mm)

daily

daily

daily

no of

hours

speed

Tmax 0

January

117.1

Tmin 0

Tmean 0

frost

( C)

( C)

( C)

days

29.6

16.8

23.1

0.0

-1

(hr day )

-1

(km day )

Monthly

Eto

Class-A pan

(mm day-1)

evaporation. (mm)

8.6

96.6

238.1

4.9

February

85.4

29.1

16.5

22.8

0.0

8.4

92.6

202.1

4.5

March

71.9

28.1

14.6

21.4

0.0

8.0

85.8

193.3

3.9

April

44.9

25.6

10.7

18.1

0.0

8.2

84.5

150.7

3.0

May

16.9

23.1

5.8

14.5

0.9

8.9

84.0

129.9

2.2

June

6.7

20.5

2.3

11.4

5.9

8.8

85.6

110.5

1.8

July

3.0

20.8

2.0

11.4

6.5

9.1

89.0

141.5

1.9

August

5.4

23.5

4.4

14.0

2.4

9.3

100.0

170.1

2.7

17.8

26.9

8.9

17.9

0.2

9.2

109.0

219.8

3.6

September October

66.8

28.2

12.8

20.5

0.0

8.7

118.4

246.4

4.4

November

102.3

28.2

14.7

21.4

0.0

8.3

114.2

232.8

4.7

December

102.8

28.8

16.0

22.4

0.0

8.4

106.0

244.3

4.9

Total

641.0

2297.5

At the Roodeplaat Agricultural Research Station, most of the rain (591 mm or 92 % of the annual total) falls during the period October to April. Three-quarter of the total annual rainfall (480 mm) occurs during the five-month period November to March inclusive, and this coincides with the growing season of most summer crops, including soya beans.

The mean daily minimum (Tmin) and maximum (Tmax)

temperatures during this five-month period range between 15 0C and 30 0C, which is considered suitable for the production of soya beans (Farias, 1994; Smit, 2000:16). Mean total reference crop evapotranspiration for the five-month period from 24

November to March inclusive, calculated by means of the Penman-Monteith method (ARC-ISCW, 2005), amounts to 692 mm. The mean total class-A pan evaporation for the five-month period November to March inclusive amounts to 1111 mm. Assuming a planting date of 1 November and a growing season that lasts 140 days, and using the crop factors published by Green (1985:33), which are based on American Class A pan evaporation (Green, 1985:6, 33), total evapotranspiration (mm) of soya beans growing at Roodeplaat, was estimated to be 715 mm.

Estimates of mean daily

evapotranspiration by soya beans during the growing season at Roodeplaat are presented in Figure 3.5.

Daily evapotranpiration (mm)

8 7 6 5 4 3 2 1 0 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20

Weeks after planting

FIGURE 3.5: Estimated mean daily evapotranspiration (mm) of soya beans planted on 1 November at Roodeplaat using the procedures of Green (1985)

The estimates presented in Figure 3.5 indicate that evapotranspiration of soya beans planted on 1 November at Roodeplaat peaks during weeks 11 to 13, attaining a value of 7.13 mm day-1 or about 50 mm week-1.

25

The land at the experimental site was nearly flat, sloping gently in a southern direction with a gradient of 1%. The soil at the site was classified as being of the Hutton form and Ventersdorp family (Soil Classification Working Group, 1991:138139). Generally, the texture of the soil was clayey, changing from sandy clay in the A horizon to clay in the B2 horizon and clay loam in the C horizon. The soil was about 2 m deep and moderately well drained. The dry bulk density of the soil was 1 581 kg m-3 in the top 200 mm of the A horizon and declined with soil depth to 1 486 kg m-3 in the B horizon and 1 440 kg m-3 in the C horizon. Following long periods of dry weather the topsoil developed surface cracks typical of soils with vertic properties, but no evidence of slickensides was found in the soil profile. A complete description of the soil profile is presented in Appendix Table A1 and the analytical data appear in Appendix Table A2.

The field capacity was determined using the procedure described by Hensley and de Jager (1982:44) and Boedt and Laker (1985:20). This involved the construction of an earthen ridge around a 3 m x 3 m plot, followed by ponding the soil to thoroughly wet the profile. Once all the water had infiltrated the soil surface was covered with a black plastic sheet to prevent evaporation of water from the surface. Field capacity of the rooting depth of the soil, which was 1030 mm, was determined gravimetrically after allowing for 72 hrs of drainage as recommended by Boedt & Laker (1985:20). Gravimetric sampling to determine the water content of the rooting depth at field capacity was done at six different positions in the plot. The field capacity values that were obtained ranged between 324 mm and 339 mm and the average value was 336 mm. The profile available water capacity (PAWC) of the rooting depth was estimated using two different models. A PAWC value of 118 mm was obtained when using the

26

model developed by Laker (1982: 157) and a value of 94 mm when using the model of Boedt and Laker (1985: 313).

3.3 Experimental procedures

3.3.1 General procedures

In both seasons the experiment employed a split plot design (Little & Hills, 1978:87100) using level of water as main plots and planting density as splits. The experiment involved three levels of water and five planting densities. Treatments were replicated three times. A separate set of plots was used to monitor plant growth by means of destructive sampling.

In both seasons, the upper 1030 mm of the soil profile in the high (HW) and intermediate (IW) water treatments was raised to field capacity three days before planting. Once the plants had emerged, these two treatments received water every week. Irrigation scheduling aimed at supplying the HW-treatment with at least 50 mm water per week, either from rainfall, irrigation or both, and the IW-treatment with at least 25 mm, being half the amount supplied to the HW treatment. The decision to supply at least 50 mm water per week to the HW-treatment was made to ensure that the crop demand for water was met at all times. Since peak demand was estimated to amount to 50 mm per week (Figure 3.5), this amount was supplied to the HWtreatment on a weekly basis from about four weeks after planting until the crop approached physiological maturity. Limiting soil water extraction to a maximum of 50 mm per cycle was deemed adequate to prevent soil water availability from being a

27

limiting factor in crop growth in the HW-treatment, because this amount represented about half of the PAWC of the rooting depth. The implication of the irrigation scheduling practice applied in the HW-treatment was that during both the early and the late parts of the growing season the supply of water to the HW-treatment probably exceeded water use by the crop, but since the soil was deep and moderately well drained, it was assumed that excess water would be removed from the rooting zone by deep percolation without having a negative effect on the development of the crop. The low water treatment (LW) consisted of planting in soil that had not been raised to field capacity. Following establishment, which involved several light irrigations, because conditions during emergence were dry, especially during the 2003/04 season, the crop was left to subsist on rainfall and stored soil water.

Irrigation was applied by means of a hosepipe to which a water meter with accuracy of 1 litre was attached (Figure 3.6). During the growing season rainfall was measured daily using a plastic rain gauge that was positioned on the edge of the experimental site at a height of 1.20 m above the soil surface.

28

FIGURE 3.6: A water meter with accuracy of 1 litre was used to measure the amount of irrigation water applied to the plots

The profile water content of the upper 1030 mm of the soil profile was determined at the time of planting and again when the crop had reached physiological maturity. This enabled the estimation of the net contribution of stored soil water to total consumptive water use in the three water treatments, but water lost through deep percolation could not be accounted for.

Soil water samples were taken

gravimetrically at depth intervals of 200 mm by means of a soil auger. Dry soil bulk density was determined for each depth interval enabling the conversion of gravimetric water content to volumetric water content.

In the 2002/03, experiment the soil profile water content at planting was determined only in six of the plots in which the soil was raised to field capacity (IW and HW treatments) and three of the plots that received no pre-plant irrigation (LW treatment).

29

The mean of the six profile water contents of the plots that were charged before planting was used to represent the profile water content in all the plots of the HW and IW treatments and the mean of the three soil water contents in the plots that did not receive pre-plant irrigation was used to represent the profile water content in all the plots of the LW treatment. At physiological maturity the profile water content was determined in all 45 plots separately.

In the 2003/04 experiment, profile water

content was determined in each plot individually both at planting and at physiological maturity.

Estimates of the consumptive water use by soya beans in the different water treatments, inclusive of any water that was lost through deep percolation, were the sum of rainfall and irrigation water received during the growing season and the net contribution of stored water in the upper 1030 mm of the soil profile.

Total

consumptive water use data for the 2003/04 season were analysed statistically, because all the necessary measurements were made in each plot individually. As indicated, in the 2002/03 experiment the profile water content at planting was not determined for each plot separately, and for this reason the consumptive water use data obtained in that experiment were not analysed statistically.

Five planting densities were established. Square planting was employed and plant spacing ranged from 45 cm x 45 cm to 11 cm x 11 cm, generating planting densities that ranged between 5 plants m-2 and 80 plants m-2 (Table 3.2).

30

TABLE 3.2: Plant spacing and planting densities used in the 2002/03 and 2003/04 soya bean experiments

Spacing (cm)

Equivalent planting density (plants m-2)

45 x 45

5

32 x 32

10

22 x 22

20

16 x 16

40

11 x 11

80

3.3.2 Specific procedures that applied to the 2002/03 experiment

The 2002/03 experiment was conducted on land that was previously planted to Teff grass (Eragrostis tef). Soil preparation started by incorporating the sod and stover of the previous crop using a disc-plough, and was followed by seedbed preparation using an S-tine implement with a rotocart. The plots assigned to the HW and IW treatments were converted to level basins using spades and rakes. Levelling occurred during successive irrigations aimed at charging the soils in these plots to field capacity. A view of these level basins is presented in Figure 3.7.

31

FIGURE 3.7: View of the level basins used to maintain the high water (HW) and intermediate water (IW) treatment

The gross plot size was 5 m x 5 m = 25 m2. The net plots obtained by removing the guard rows ranged in size from 24.56 m2 to 24.89 m2 depending on the spacing of the treatment.

Samples of the topsoil (0 - 20 cm) were taken in each plot a week before planting and again on the day after harvesting. In each case the samples from the different plots were bulked and homogenised before taking a single composite sample for analysis, which was conducted by the ARC-Institute for Soil, Climate and Water.

Selected chemical

properties of the topsoil before planting and after harvesting the experiment are presented in Table 3.3.

32

TABLE 3.3: Chemical properties of the topsoil at the site of the 2002/03 experiment before the experiment was planted and after it was harvested

Time of

Total N

K

Ca

Mg

Na

sampling

S-

pHwater

P

value (%)

Before planting

0.094

After harvest

0.099

--------------------(cmol(+) kg-1)---------------------1.35 9.65 4.32 0.21 15.83 1.39

9.56

4.22

0.29

15.43

(mg kg-1) 6.94

93

6.95

109

At the time of planting, the results of the soil analysis were not yet available. The Tshwane Nutrition Project, which provided the funding for the conduct of the experiment, prescribed an organic approach to plant production. For this reason, it was decided to apply Promis, which is a composted breeding litter certified organic by ECOCERT S.A. and supplied by National Plant Foods CC in Rustenburg. On average, Promis contains 4 % N, 1.5 % P, 1.5 % K and 3 % Ca by mass (Hogen, 2002). Promis was applied broadcast at a rate of 5 tons ha-1 (12.6 kg per plot), and worked into the surface soil (top 150 mm) using spades. The application rate used was aimed at meeting the phosphorus requirements of a soya bean crop that produced a grain yield of 4.5 t ha-1. The choice of this particular target yield was based on the results of the national soya bean cultivar trial conducted during 2001/02 on the TUT Research Farm (De Beer & Pretorius, 2002), in which grain yields of 4.5 tons ha-1 were achieved. To achieve a grain yield of 4.5 t ha-1, the FSSA (1999:187) recommends the application of 60 kg P ha-1 on soils that are low in P ( F

Model

20

370464.6258

18523.2313

28.27

< 0.0001

Error

24

15724.9040

655.2043

Corrected total

44

386189.5298

R-square

Coefficient of variation

Root MSE

Yield g m-2 Mean

0.959282

8.175388

25.59696

313.0978

Source

DF

Type I SS

Mean square

F value

Pr > F

Water

2

1054.6991

527.3496

0.80

0.4589

Density

4

363790.7231

909476808

138.81

< 0.0001

Block

2

148.9231

74.4616

0.11

0.8931

Water*Block

4

2811.2996

702.8249

1.07

0.3918

Water*Density

8

2658.9809

332.3726

0.51

0.8389

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

1054.699111

527.349556

0.75

0.5288

102

TABLE C2: Analysis of variance for number of soya bean pods plant-1, 2002/03 experiment

Source

DF

Sum of square

Mean square

F value

Pr > F

Model

20

220988.9333

11049.4467

22.32

< 0.0001

Error

24

11880.2667

495.0111

Corrected total

44

232869.2000

R-square

Coefficient of variation

Root MSE

Pods plant-1 Mean

0.948983

18.66514

22.24885

119.2000

Source

DF

Type I SS

Mean square

F value

Pr > F

Water

2

3910.5333

1955.2667

3.95

0.0329

Density

4

212798.9778

53199.7444

107.47

< 0.0001

Block

2

370.5333

185.2667

0.37

0.6917

Water*Block

4

1400.5333

350.1333

0.71

0.5948

Water*Density

8

2508.3556

313.5444

0.63

0.7420

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

3910.533333

1955.266667

5.58

0.0695

TABLE C3: Analysis of variance for number of soya bean pods m-2, 2002/03 experiment

Source

DF

Sum of square

Mean square

F value

Pr > F

Model

20

44738411.03

2236920.55

16.64

< 0.0001

Error

24

3226439.31

134434.97

Corrected total

44

47964850.34

R-square

Coefficient of variation

Root MSE

Pods m -2 Mean

0.932733

16.98557

366.6537

2158.619

Source

DF

Type I SS

Mean square

F value

Pr > F

Water

2

1129579.26

564789.63

4.20

0.0273

Density

4

42024393.02

10506098.26

78.15

< 0.0001

Block

2

116625.93

58312.96

0.43

0.6530

Water*Block

4

241104.15

60276.04

0.45

0.7725

Water*Density

8

1226708.67

153338.58

1.14

0.3732

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

1129579.261

564789.630

9.37

0.0309

103

TABLE C4: Analysis of variance for number of soya bean seeds plant-1, 2002/03 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

1056429.244

52821.462

19.62

< 0.0001

Error

24

64627.333

2692.806

Corrected total

44

1121056.578

R-square

Coefficient of variation

Root MSE

Seeds plant-1 Mean

0.942351

20.33572

51.89225

255.1778

Source

DF

Type I SS

Mean square

F value

Pr > F

Water

2

22314.444

11157.222

4.14

0.0285

Density

4

1012132.356

253033.089

93.97

F

Water

2

22314.44444

11157..22222

7.10

0.0483

TABLE C5: Analysis of variance for number of soya bean seed m-2, 2002/03 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

187030598.0

9351529.9

12.47

< 0.0001

Error

24

17997240.9

749885.0

Corrected total

44

205027838.9

R-square

Coefficient of variation

Root MSE

Seeds m-2 Mean

0.912221

18.94802

865.9590

4570.181

Source

DF

Type I SS

Mean square

F value

Pr > F

Water

2

5943450.1

2971725.0

3.96

0.0326

Density

4

175217599.7

43804399.9

58.41

< 0.0001

Block

2

640172.4

320086.2

0.43

0.6574

Water*Block

4

1017300.3

254325.1

0.34

0.8488

Water*Density

8

4212075.5

526509.4

0.70

0.6867

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

5943450.090

2971725.045

11.68

0.0214

104

TABLE C6: Analysis of variance for number of soya bean seeds pod-1, 2002/03 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

0.53777778

0.02688889

3.14

0.0042

Error

24

0.20533333

0.0085555.6

Corrected total

44

0.74311111

R-square

Coefficient of variation

Root MSE

Seed pod-1 Mean

0.723684

4.273441

0.092496

2.164444

Source

DF

Type I SS

Mean square

F value

Pr > F

Water

2

0.00577778

0.00288889

0.34

0.7168

Density

4

0.16755556

0.04188889

4.90

0.0050

Block

2

0.22711111

0.11355556

13.97

0.0001

Water*Block

4

0.03422222

0.00855556

1.00

0.4269

Water*Density

8

0.10311111

0.01288889

1.51

0.2070

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

0.00577778

0.00288889

0.34

0.7320

TABLE C7: Analysis of variance for 100 seed mass of soya bean, 2002/03 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

6695.14044

334.75702

1.50

0.1720

Error

24

5368.66267

223.69428

Corrected total

44

12063.80311

R-square

Coefficient of variation

Root MSE

100 seeds mass Mean

0.554978

9.485562

14.95641

157.6756

Source

DF

Type I SS

Mean square

F value

Pr > F

Water

2

1355.328444

677.664222

3.03

0.0671

Density

4

1272.280889

318.070222

1.42

0.2570

Block

2

425.659111

212.829556

0.95

0.4003

Water*Block

4

1358.451556

339.612889

1.52

0.2285

Water*Density

8

2283.420444

285.427556

1.28

0.3015

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

1355.328444

677.664222

2.00

0.2506

105

TABLE C8: Analysis of variance for consumptive water use of soya bean, 2003/04 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

8

36893.4

4611.7

22.86

< 0.001

Error

24

4841.8

201.7

Corrected total

44

1338027

R-square

Coefficient of variation

Root MSE

Yield g m -2 Mean

0.931312

10.84198

50.50056

465.7871

Source

DF

Type III SS

Mean square

F value

Pr > F

Block

2

1431.9

716.0

1.56

Water*Block

4

1831.4

457.9

2.27

Water

2

1107761.8

553880.9

1209.73

< 0.001

Density

4

185267.1

46316.8

229.58

< 0.001

Water*Density

8

36893.4

4611.7

22.86

< 0.001

TABLE C9: Analysis of variance for grain yield of soya bean, 2003/04 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

829881.7027

417494.0851

16.27

< 0.0001

Error

24

61207.3525

2550.3064

Corrected total

44

891089.0551

R-square

Coefficient of variation

Root MSE

Yield g m -2 Mean

0.931312

10.84198

50.50056

465.7871

Source

DF

Type III SS

Mean square

F value

Pr > F

Block

2

8251.5301

4125.7650

1.62

0.2192

Water*Block

4

57463.7289

14365.9322

2.63

0.0024

Water

2

143988.2195

71994.1098

28.23

< 0.0001

Density

4

588902.0947

147225.5237

57.73

< 0.0001

Water*Density

8

31276.1295

39039.5162

1.53

0.1981

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

143988.2195

71994.1098

5.01

0.0814

106

TABLE C10: Analysis of variance for number of soya bean pods plant-1, 2003/04 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

87641.64444

4382.08222

33.75

< 0.0001

Error

24

3116.00000

129.83333

Corrected total

44

90757.64444

R-square

Coefficient of variation

Root MSE

Pods plant-1 Mean

0.965667

12.50915

11.39444

91.08889

Source

DF

Type III SS

Mean square

F value

Pr > F

Block

2

863.51111

431.75556

3.33

0.0531

Water*Block

4

391.82222

97.95556

0.75

0.5649

Water

2

1876.31111

938.15556

7.23

0.0035

Density

4

79592.75556

19898.18889

153.26

< 0.0001

Water*Density

8

4917.24444

614.65556

4.73

0.0014

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

1876.31111

938.15556

9.58

0.0298

TABLE C11: Analysis of variance for number of soya bean pods m-2, 2003/04 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

24752252.00

1237612.60

15.18

< 0.0001

Error

24

1956632.00

81526.33

Corrected total

44

267088884.00

R-square

Coefficient of variation

Root MSE

Pods m-2 Mean

0.926742

16.42540

285.5282

1738.333

Source

DF

Type III SS

Mean square

F value

Pr > F

Block

2

170497.60

85248.80

1.05

0.3669

Water*Block

4

384633.07

96158.27

1.18

0.03449

Water

2

233030.93

116515.47

1.43

0.2592

Density

4

23670368.89

5917592.22

72.59

< 0.0001

Water*Density

8

293721.51

36715.19

0.45

0.8782

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

233030.9333

116515.4667

1.21

0.3878

107

TABLE C12: Analysis of variance for number of soya bean seeds plant-1, 2003/04 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

389180.4444

19459.0222

26.17

< 0.0001

Error

24

17847.8667

743.6611

Corrected total

44

407028.3111

R-square

Coefficient of variation

Root MSE

Seeds plant-1 Mean

0.956151

14.56393

27.27015

187.2444

Source

DF

Type III SS

Mean square

F value

Pr > F

Block

2

4608.5778

2304.2889

3.10

0.0635

Water*Block

4

2694.8889

673.7222

0.91

0.4762

Water

2

6730.8444

3365.4222

4.53

0.0215

Density

4

34.9526.9778

87381.7444

117.50

< 0.0001

Water*Density

8

25619.1556

3202.3944

4.31

0.0025

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

6730.844444

3365.422222

5.00

0.0817

TABLE C13: Analysis of variance for number of soya bean seeds m2, 2003/04 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

90294262.7

4514713.1

10.53

< 0.0001

Error

24

10286223.1

428592.6

Corrected total

44

100580485.8

R-square

Coefficient of variation

Root MSE

Seeds m-2 Mean

0.897731

18.66870

654.6699

3506.778

Source

DF

Type III SS

Mean square

F value

Pr > F

Block

2

98155.38

49077.69

0.11

0.8923

Water*Block

4

2724495.56

681123.89

1.59

0.2095

Water

2

779113.64

389556.82

0.91

0.4164

Density

4

84866156.22

21216539.06

49.50

< 0.0001

Water*Density

8

1826341.91

228292.74

0.53

0.8202

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

779113.6444

389556.8222

0.57

0.6047

108

TABLE C14: Analysis of variance for number of soya bean seeds pod-1, 2003/04 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

0.63184551

0.03159228

1.22

0.3193

Error

24

0.62243680

0.02593487

Corrected total

44

1.25428231

R-square

Coefficient of variation

Root MSE

Seed pod-1 Mean

0.503751

7.793998

0.161043

2.066244

Source

DF

Type III SS

Mean square

F value

Pr > F

Block

2

0.09160551

0.04580276

1.77

0.1925

Water*Block

4

0.08222102

0.02055526

0.79

0.5416

Water

2

0.09603258

0.04801629

1.85

0.1787

Density

4

0.29320520

0.07330130

2.83

0.0472

Water*Density

8

0.06878120

0.00859765

0.33

0.9453

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

0.09603258

0.04801629

2.34

0.2128

TABLE C15: Analysis of variance for seed mass (100 seeds), 2003/04 experiment

Source

DF

Sum of squares

Mean squares

F value

Pr > F

Model

20

21488.40000

1074.42000

3.51

0.0020

Error

24

7348.80000

306.20000

Corrected total

44

28837.20000

R-square

Coefficient of variation

Root MSE

100 seed mass Mean

0.745162

7.726776

1749857

226.4667

Source

DF

Type III SS

Mean square

F value

Pr > F

Block

2

1216.93333

608.46667

1.99

0.1590

Water*Block

4

1934.26667

483.56667

1.58

0.2120

Water

2

1106.80000

553.40000

1.81

0.1857

Density

4

15116.75556

3779.18889

12.34

< 0.0001

Water*Density

8

2113.64444

264.20556

0.86

0.5599

Source

DF

Type III SS

Mean square

F value

Pr > F

Water

2

1106.800000

553.400000

1.14

0.4046

109