Field Crops Research 119 (2010) 106–113
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Effects of plant density and nitrogen and potassium fertilization on cotton yield and uptake of major nutrients in two fields with varying fertility Hezhong Dong ∗ , Xiangqiang Kong, Weijiang Li, Wei Tang, Dongmei Zhang Cotton Research Center, Shandong Academy of Agricultural Sciences, Shandong Key Lab for Cotton Culture and Physiology, Jinan 250100, Shandong, China
a r t i c l e
i n f o
Article history: Received 27 April 2010 Received in revised form 29 June 2010 Accepted 29 June 2010 Keywords: Cotton Plant density Nitrogen fertilizer Potassium fertilizer Yield Nutrient uptake Nutrient use efficiency
a b s t r a c t As the most important cultural practices for cotton production, the single effects of plant density and [nitrogen (N) and potassium (K)] fertilization on yield and yield components are well documented but their combined effects on Bt cotton are poorly understood. Using a split–split plot design with four replications, we conducted a two-year field experiment in two fields, one with lower fertility and the other with higher fertility, in the Yellow River Valley of China. The aim was to evaluate both the individual and combined effects of plant density and nitrogen and potassium fertilization on yield, yield components and uptake of major nutrients. The main plots were assigned to plant density (4.5 and 7.5 plants/m2 ), while nitrogen (0 and 240 kg N/ha) and potassium fertilization (0 and 150 kg K/ha) were assigned to the sub- and sub–subplots. Lint yield was improved with high plant density (7.5 plants/m2 ) in the lower fertility field, particularly without N and K application, but not in the higher fertility field. Nitrogen or K application also increased lint yield, and a combination of high plant density, N and K application further improved lint yield in the lower fertility field, while only K application increased lint yield in the higher fertility field. Lint percentage was not affected by any of the variables studied. Thus, the yield increase due to plant density, fertilization or their combinations was attributed to increases in boll number or boll weight. The ratio of seed cotton to stalk (RSS) was linearly correlated with harvest index, and thus can be a simple indicator of dry matter allocation to reproductive structures. Increased yield due to plant density and fertilization was mainly attributed to the enhanced biological yield in the lower fertility field, while the yield increase due to K fertilization was mainly due to increased RSS in the higher fertility field. The plants used approximately equal N and P to produce 100 kg lint in both fields, but the uptake of K to produce 100 kg lint in the higher fertility field was about 21% more than that in the lower fertility field. Ratios of N:P:K were 1:0.159:0.604 in the lower fertility field and 1:0.159:0.734 in higher fertility field. This study suggests that K fertilization was extremely important for maintaining high yield, although luxury consumption occurred in the higher fertility field; N was applied more than required in the highly fertile field, and increased plant density would be beneficial to cotton yield in the lower fertility field. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Cotton (Gossypium hirsutum L.) is a major cash crop grown primarily for fiber and oil seed in the world. The cotton plant is unique because it is a perennial with an indeterminate growth habit (Oosterhuis, 2001). Associated with this complex growth habit is an extreme sensitivity to adverse environmental conditions and field managements. Cotton yield can only be increased through proper crop management practices of which fertilizer levels and plant population are most important (Ali et al., 2007a). Optimizing plant density and fertilizer levels are, therefore, the target for improved management.
∗ Corresponding author. Tel.: +86 531 83179255; fax: +86 531 88960327. E-mail address:
[email protected] (H. Dong). 0378-4290/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2010.06.019
Nitrogen (N) is the nutrient that is required most consistently and in larger amounts than other nutrients for cotton production (Hou et al., 2007). It is an essential element for canopy area development and photosynthesis (Wullschleger and Oosterhuis, 1990). In addition, N increased boll number and yield of cotton (Bondada et al., 1996; Boquet et al., 1993). Thus, N nutrition, unequivocally, is one of the most pivotal facets of cotton production (Bondada and Oosterhuis, 2001). In recent years, increasing nitrogen (N) fertilizer costs and increased focus on greenhouse gas emissions have prompted greater attention to the efficient use of N fertilizers (Rochester et al., 2007). These issues and the need to optimize fertilizer inputs to meet crop requirements have also increasingly been identified as priorities in feedback from cotton growers and consultants. Recent studies in Australia have shown that about 50 kg N/ha excess N fertilizers were applied to cotton fields averaged over all sites monitored, and 15–25% of the N fertilizer inputs can be safely reduced without yield reduction (Rochester et al., 2009). Studies
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Table 1 Background data of productivity and soil fertility of the two experimental fields studied in 2008 and 2009. Year
Soil fertility level
Lint yield of preceding crop (kg/ha)
pH
Organic matter (g/kg)
Available N (mg/kg)
Available P (mg/kg)
Available K (mg/kg)
2008
Lower Higher
1405 2003
8.11 8.06
10.5 12.5
49.4 68.5
17.9 20.5
105 138
2009
Lower Higher
1398 1955
8.13 8.08
10.7 12.8
48.1 69.7
18.5 20.8
109 133
The preceding crops for both fields were cotton (cv. SCRC 28). Data were collected from 0 to 20 cm soil samples in early spring before irrigation each year.
in other countries also suggested that N fertilizer can be used at a moderately lower rate and more efficiently than they have been traditionally used, although the optimum N rates and N use efficiency are affected by a number of factors like yield potential, soil fertility and field management (Boquet, 2005; Clawson et al., 2008; Hou et al., 2007; Janat, 2008; Kumbhar et al., 2008). Potassium plays a vital role in cotton growth and metabolism, although it is not a constituent of any known plant components (Read et al., 2006). As an enzyme activator, potassium has been implicated in over 60 enzymatic reactions, which are involved in many processes in the plant such as photosynthesis, respiration, carbohydrate metabolism, translocation and protein synthesis (Dong et al., 2004; Pettigrew, 2008). Potassium also plays an important role in the maintenance of osmotic potential and water uptake during fiber development, and a shortage will result in poorer fiber quality and lowered yields (Oosterhuis, 2001). Although potassium is one of the major plant nutrients underpinning crop yield and quality, use of K fertilizer for cotton production has not been popular in the Yellow River Valley, one of the largest cotton growing regions in China, before Bt (Bacillus thuringiensis) transgenic cotton was introduced in 2000. This was mainly because, on the one hand, unit yield was relatively lower and total quantity of K required was relatively small; on the other hand, organic manure was popularly applied before planting each year, which maintained a moderate level of available K in soil (Dong et al., 2004). However, cotton yields have steadily increased during the last ten years through commercial use of high-yielding Bt cotton cultivars, and organic manure was less applied to cotton field due to industrialization of China. Bt-transgenic cotton cultivars seemed to be more sensitive to K+ deficiency than conventional cultivars (Zhang et al., 2007). Wide use of Bt-transgenic cotton and yield increase has prompted greater attention to K fertilization in China. The third important factor that influences cotton yield is plant density. Maximum yield is achieved at optimum plant density which depends upon cropping system, environmental condition and cultivar (Bridge et al., 1973; El-Shinnawy and Ghaly, 1985; Halemani and Hallikeri, 2002), but it is usually relatively stable across a wide range of population densities (Jones and Wells, 1998) through manipulation of boll occurrence and boll weight (Bednarz et al., 2000). Maximum yields were obtained in the Mississippi Delta within a population range of 7.0–12.1 plants/m2 (Bridge et al., 1973). Maximum lint yields in Texas occurred within ranges of 7.9–15.5 plants/m2 (Fowler and Ray, 1977). Bednarz et al. (2000) reported that cotton lint yield was relatively stable across a wide range of plant densities, and yield stability was achieved through manipulation of boll numbers and boll weight. Still other studies conducted in India, China and Egypt showed that the optimal plant population depended on environment, varieties and planting date (El-Shinnawy and Ghaly, 1985; Halemani and Hallikeri, 2002; Jadhao et al., 1993). From our recent studies, we recommended a plant density range of 4.5–7.5 plants/m2 in non-saline fields and 7.5–9.0 plants/m2 in saline fields of the Yellow River valley (Dong et al., 2006, 2010). Plant density, N or K fertilization greatly affects cotton plant growth and yield, and their single effects have been well docu-
mented. However, the combined effects of the three factors were seldom studied, particularly with high-yielding Bt-transgenic cotton cultivars. Our objectives were to investigate the effects of plant density and N and K fertilization on lint yield, yield components, and harvest index in two cotton fields with varying soil fertility and yield potential. In light of recent trends of decreasing N and increasing K fertilization rates for cotton in this area, we also compared the nutrient uptake and ratios of N, P and K between lower and higher fertility fields. We hypothesized that (i) additional N or increased plant density may increase biological yield, but not the lint yield in the highly fertile field; (ii) applied K increases partitioning to reproductive structures relative to vegetative growth, especially in the highly fertile field; (iii) harvest index can be estimated from the ratio of seed cotton to stalk; and (iv) it requires more K than P and N per unit lint yield in higher yielding (-fertility) than lower yielding (-fertility) conditions. 2. Materials and methods 2.1. Experimental site and cultivar An experiment was conducted in two cotton fields (1 km apart) at the Experimental Station of Shandong Cotton Research Center, Linqing (115◦ 42 E, 36◦ 61 N), Shandong, China, from 2008 to 2009. The two fields were sandy loamy, but differed greatly in soil fertility and productivity according to soil analysis and cotton yield for the latest three years (Table 1). One field was more fertile and productive than another, and thus the fields were designated as higher and lower fertility fields, respectively. The experiment was conducted in different parts of the same field for two years to avoid residual effects of N or K fertilization from the previous year’s experiment. A high-yielding commercial Bt (B. thuringiensis) transgenic cotton cultivar, SCRC 28 was used for both years. Acid-delinted seeds (percentage germination ≥80%) of each cultivar were treated with imidacloprid (Gaucho FS600, Bayer CropScience, Monheim, Germany) by the Luyi Cottonseed Company Ltd., Jinan, Shandong. The climate of the study area is temperate and monsoonal. The rainfall is variable with greater distribution in July and August. Cotton is usually planted in mid-April and harvested at the end of October with a typical plant density of 4.5 plants/m2 . It is usually fertilized with 240 kg N and 33–40 kg P/ha, but without potassium application. 2.2. Experiment design A split–split plot design with four replications was used for the study. The main plot was plant density (4.5 and 7.5 plants/m2 ), while nitrogen (0 and 240 kg N/ha) and potassium (0 and 150 kg K/ha) fertilization constituted the sub- and sub–subplots. Each sub–subplot contained six rows of cotton, 10 m long with an inter-row spacing of 0.80 m. The sources of N and K were urea (50% N) and potassium sulfate (41.5% K), respectively. All the K and half of the N were applied basally before planting; the other half of N was top-dressed at
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early flowering. All plots received a basal rate of 33 kg P as calcium superphosphate (5.2% P) according to local practice. 2.3. Field management For both fields, the soils were subjected to irrigation (1200 m3 /ha) 20 d before sowing each year. Soils were then plowed and harrowed when their mellowness was considered physically acceptable. Cotton was sown on 24 April 2008 and 19 April 2009 in both fields. By passing an animal-drawn plough at 80 cm rowdistance, seeding furrows (35 mm deep and 50 mm wide) were directly formed. With manual hill-drop planting method, four to six seeds per hill were dropped into the prepared furrow at a hill–hill distance in the same row of 27.7 and 16.7 cm for traditional (4.5 plants/m2 ) and high plant density (7.5 plants/m2 ) treatment, respectively. The seeds were quickly covered with moist soil from both sides of the furrow and then mulched with plastic film (0.008 mm) along the rows. Seedlings were freed from mulching by cutting film above hills at full emergence, and thinned to 4.5 plants/m2 or 7.5 plants/m2 by leaving one vigorous plant per hill at the two-leaf stage. Vegetative branches and growth terminals on the main stems in each plot were totally removed by hand at the peak squaring and peak bollsetting stages, according to local practice. Throughout the growing season, plots were irrigated once in late June of 2008, but not in 2009 due to the sufficient rainfall. Other management practices, including insect and weed control were conducted according to local agronomic practices unless otherwise indicated. 2.4. Data collection Data were collected for lint yield, yield components, stalk yield and nutrient (N, P and K) uptake by plants for both years, and biological yield and harvest index in 2009.
leaves) were recorded. The ratio of seed cotton to stalk (RSS) was determined. Plant debris was estimated by collecting all recognizable plant material within 1-m2 frames placed between rows at weekly intervals. Biological yield (stalk plus seed cotton and plant debris) and harvest index (seed cotton yield/biological yield) were then determined. For determination of nutrient uptake, samples of each plant part, including debris were milled with a Wiley mill and screened through a 0.5 mm sieve. Total nitrogen concentration was determined by the micro-Kjeldahl method (Bremner and Mulvaney, 1982). About 0.2 g of milled samples were digested for 1 h in concentrated H2 SO4 plus 1/4 catalyst tablet. After cooling, the digest was made alkaline with 40% NaOH solution and the NH3 distilled was collected in 10 ml boric acid containing mixed indicator. Total N was determined by titrating the distillate against 0.01 M HCl. Phosphorus contents were determined colorimetrically using a spectrophotometer. An atomic adsorption spectrophotometer (TAS-990, Beijing, China) was used to determine K concentration. Concentrations of all nutrients were expressed on a dry weight basis and the nutrient uptake and accumulation were calculated as the product of concentration and dry weight. 2.5. Statistical analysis An analysis of variance was performed using DPS Data Processing System (Tang and Feng, 1997). The initial combined data showed interactions with year or fields. Thus, all the data are presented separately for each field and year (Steel and Torrie, 1980). Means were separated using the least significant difference (LSD) test at the 5% probability level. Use of difference between treatments implies statistical difference (P = 0.05) while no difference implies no statistical difference. 3. Results
2.4.1. Yield and yield components For each year, plants from the central four rows of each plot were manually harvested three times. Seed cotton (moisture ≤11%) was ginned on a 10-saw, hand-fed laboratory gin, and lint yield (kg/ha) as well as lint percentage (lint/seed cotton, w/w) was determined after ginning. Total number of bolls on a unit ground-area basis and boll weight (moisture ≤11%) were determined from 20 plants in the central two rows in each plot. 2.4.2. Biological yield, harvest index and nutrient uptake Ten uniform plants per row of each plot were tagged at squaring for nutrient uptake analysis and determination of stalk and biological yields. These were manually uprooted at maturity, partitioned into roots, stems, branches, leaves, seeds, lint and carpels. After drying at 70 ◦ C to a constant weight, they were weighed, and the yields of seed cotton and stalk (root, stem, branches, carpels, and remnant
The monthly rainfall, average air temperature and sunshine duration during the growing season (2008–2009), and the average for the last thirty years (1978–2007) are presented in Table 2. Total rainfall in 2009 was 206 and 175 mm more than 2008 and the thirty-year average. 3.1. Yield and yield components in lower fertility field Lint yield, boll number and boll weight were affected by plant density, N fertilization, K fertilization, or their combinations in the lower fertility field, but the lint percentage was not (Table 3). Increased plant density improved lint yield in the lower fertility field. Without N and K fertilization, high plant density (7.5 plants/m2 ) had 4.4 and 5.3% more lint than low plant density (4.5 plants/m2 ) in 2008 and 2009, respectively. Lint yield was
Table 2 Monthly weather summary during the cotton growing season from 2008 to 2009 at Linqing, Shandong, China. Month
Average temperature (◦ C)
Precipitation (mm) a
Sunshine duration (h)
2008
2009
30 years
2008
2009
30 years
April May June July August September October
15.5 21.6 24.6 26.4 25.6 21.0 15.7
15.7 20.8 26.9 26.3 24.8 19.8 16.8
14.6 20.2 25.5 26.6 25.4 20.7 14.5
48.1 53.6 44.7 134.6 110.0 34.4 24.2
45.8 101.3 86.5 178.2 136.9 91.1 16.1
24.0 40.1 64.9 150.9 128.5 48.0 34.5
216.7 254.0 279.0 175.1 221.9 173.3 186.8
250.0 260.1 282.8 202.5 155.3 162.4 223.5
235.6 271.5 252.5 216.7 225.7 216.6 205.3
Average/total
21.5
21.6
21.1
449.6
655.9
490.9
1506.8
1536.6
1623.9
a
Represent values of thirty years’ (1978–2007) average.
2008
2009
30 years
H. Dong et al. / Field Crops Research 119 (2010) 106–113
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Table 3 Main and interaction effects of plant density and N and K application on yield and yield components in the lower fertility field in 2008 and 2009. Treatment
2008
2009 No. of bolls/m2
Boll weight (g)
Lint percentage (%)
Lint yield (kg/hm2 )
No. of bolls/m2
Boll weight (g)
Lint percentage (%)
Plant density (PD) (plants/m2 ) 4.5 1377b* 7.5 1453a
70.1b 82.6a
4.77a 4.24b
41.2a 41.4a
1402b 1458a
68.9b 70.9a
4.92b 5.02a
41.3a 40.9a
N fertilization (kg/ha) 0 1379b 240 1452a
73.6b 79.2a
4.55a 4.46a
41.3a 41.3a
1379b 1481a
67.1b 72.8a
4.98a 4.96a
41.3a 40.9a
K fertilization (kg/ha) 0 1389b 150 1441a
76.1a 76.6a
4.43b 4.59a
41.5a 41.1a
1390b 1470a
69.7a 70.2a
4.83b 5.11a
41.2a 41.0a
PD × N × K interaction 4.5 × 0 × 0 1326d 4.5 × 0 × 150 1357cd 4.5 × 240 × 0 1412c 4.5 × 240 × 150 1415b 7.5 × 0 × 0 1385cd 7.5 × 0 × 150 1445bc 7.5 × 240 × 0 1433bc 7.5 × 240 × 150 1545a
67.3d 66.9d 72.5c 73.8c 80.3b 79.7b 84.3a 86.0a
4.75b 4.94a 4.69b 4.71b 4.17de 4.36c 4.10e 4.34cd
41.5a 41.1a 41.5a 40.7a 41.4a 41.4a 41.4a 41.3a
1324d 1378cd 1417c 1488b 1394cd 1422bc 1425bc 1593a
66.1de 65.7e 71.8b 72.4b 68.7c 67.7cd 72.5b 74.8a
4.80c 5.06ab 4.80c 5.02ab 4.89bc 5.17a 4.83c 5.20a
41.7a 41.4a 41.1a 41.0a 41.4a 40.6a 40.7a 41.0a
Source of variance PD N K PD × N PD × K N×K PD × N × K
0.0017 0.0034 0.0562 0.6560 0.7721 0.0026 0.4658
0.0017 0.1014 0.0008 0.2953 0.1185 0.3740 0.1286
0.5933 0.7943 0.2773 0.7943 0.3729 0.7476 0.8298
0.0491 0.0021 0.0000 0.9921 0.0314 0.0005 0.0021
0.0323 0.0000 0.2115 0.1474 0.3241 0.0069 0.0716
0.0176 0.6995 0.0001 0.9865 0.2873 0.8045 0.3908
0.0577 0.0542 0.0591 0.1672 0.6150 0.5194 0.0783
Lint yield (kg/hm2 )
*
0.0097 0.0132 0.0075 0.9569 0.0447 0.6700 0.0222
For each factor, means within the same column followed by different letters differ significantly according to the least significant difference (LSD) test (P < 0.05).
increased 6.5% in 2008 and 7.0% in 2009 by single application of N at 4.5 plants/m2 . Single application of K at 4.5 plants/m2 , and N or K at 7.5 plants/m2 did not significantly alter the lint yield. However, a combination of N and K significantly increased lint yield relative to its single application regardless of plant density. Averaged across the two years, N and K fertilization at 4.5 and 7.5 plants/m2 provided 6.8% and 18.4% more lint than NK-free treatment at 7.5 plants/m2 . Nitrogen combined with K fertilization at 7.5 plants/m2 was the best treatment combination for yield formation among the treatment combinations. Plant density affected boll number and boll weight regardless of N or K fertilization. High plant density increased boll number by 17.8 and 2.9%, but decreased boll weight by 11.1 and 2.0% in 2008 and 2009, respectively. Nitrogen fertilization increased boll number and K fertilization improved boll weight; a combination of N and K increased both the boll number and boll weight regardless of plant density. Averaged across the two years, the boll number and boll weight were increased 8.2% by N and 4.7% by K fertilization at 4.5 plants/m2 ; they were increased 5.3% by N and 5.2% by K fertilization at 7.5 plants/m2 .
in 2008, but decreased it on average by 3.4% in 2009, particularly at high plant density (6.2%). In contrast to N, K fertilization increased lint yield regardless of plant density. Lint yield was increased 3.2% in 2008 and 6.5% in 2009 by K fertilization across plant density and N fertilization. A combination of N and K did not improve yield relative to single NK-free control and single K fertilization either year. High plant density increased the number bolls per unit area, but reduced boll weight relative to low plant density. Potassium application increased both the boll number and boll weight in 2008, but neither K nor N had an effect in 2009. 3.3. Harvest index and ratio of seed cotton to stalk Linear correlation analysis showed a highly significant (R2 = 0.9603, n = 32) positive correlation between harvest index and the ratio of seed cotton to stalk, suggesting RSS could well indicate
3.2. Yield and yield components in the higher fertility field The effects of plant density, N and K fertilization, and their interaction on yield and yield components in the higher fertility field were much more complicated than in lower fertility field, although lint percentage was also not affected by either factor or their combinations. Lint yield was affected by N × K in 2008, by N and K × plant density in 2009 and by K for both years (Table 3). The number of bolls was affected by plant density, N and K fertilization and their interactions in 2009. Lint weight was affected by K and the three-factor interaction in 2008 and by plant density for both years. Increased plant density did not improve lint yield in higher fertility fields (Table 3). Nitrogen fertilization did not affect lint yield
Fig. 1. Correlation between harvest index and ratio of seed cotton to stalk (RSS). Values for both harvest index and ratio of seed cotton to stalk were obtained from a random sample of 3 plants per plot in both higher and lower fertility fields in 2009 (n = 32).
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Table 4 Main and interaction effects of plant density and N and K application on yield and yield components in the higher fertility field in 2008 and 2009. Treatment
2008
2009 2
2
Lint yield (kg/hm ) No. of bolls/m
Boll weight (g) Lint percentage (%) Lint yield (kg/hm2 ) No. of bolls/m2 Boll weight (g) Lint percentage (%)
Plant density (PD) (plants/m2 ) 4.5 2135a* 7.5 2145a
97.8b 99.8a
5.11a 5.05b
42.8a 42.6a
1966a 1931a
78.9b 95.0a
6.15a 5.06b
40.5a 40.1a
N fertilization (kg/ha) 0 2145a 240 2135a
99.1a 98.4a
5.10a 5.06a
42.9a 42.5a
1982a 1914b
87.0a 86.9a
5.69a 5.55a
40.5a 40.0a
K fertilization (kg/ha) 0 2107b 150 2175a
98.1b 99.5a
5.04b 5.12a
42.6a 42.7a
1887b 2009a
85.9a 88.0a
5.71a 5.53a
40.1a 40.5a
PD × N × K interaction 4.5 × 0 × 0 2089c 4.5 × 0 × 150 2194a 4.5 × 240 × 0 2113bc 4.5 × 240 × 150 2146abc 7.5 × 0 × 0 2097bc 7.5 × 0 × 150 2201a 7.5 × 240 × 0 2125bc 7.5 × 240 × 150 2158ab
98.0bc 98.2bc 96.7c 98.1bc 98.3bc 102.2a 99.4b 99.4b
5.05b 5.24a 5.05b 5.09b 5.03b 5.05b 5.01b 5.10b
42.2b 42.6a 43.2a 43.0a 42.4a 42.6a 42.6a 42.6a
1920b 2018a 1897b 2028a 1926b 2064a 1806c 1926b
79.5b 78.9b 78.4b 78.9b 91.9a 97.7a 93.8a 96.5a
6.03a 6.36a 5.95a 6.28a 5.22b 5.13b 4.92b 5.07b
40.1a 40.3a 40.7a 41.0a 40.5a 41.2a 39.1a 39.4a
Source of variance PD N K PD × N PD × K N×K PD × N × K
0.0113 0.0660 0.0024 0.8078 0.1095 0.0675 0.0043
0.0146 0.2373 0.0009 0.1197 0.1035 0.1909 0.0084
0.3923 0.0537 0.3945 0.0620 0.9172 0.0589 0.4647
0.0732 0.0069 0.0000 0.0097 0.5764 0.7686 0.3500
0.0117 0.9628 0.2021 0.8091 0.1925 0.7393 0.5102
0.0124 0.2749 0.0801 0.6356 0.1311 0.5269 0.5155
0.0587 0.0535 0.1009 0.0525 0.5809 0.7388 0.5431
*
0.5004 0.4989 0.0000 0.8629 0.9738 0.0013 0.9563
For each factor, means within the same column followed by different letters differ significantly according to the least significant difference (LSD) test (P < 0.05).
allocation of dry matter to reproductive parts as a harvest index (Fig. 1). RSS was affected by plant density, N, K, or their interactions regardless of yield level (Table 4). Generally, increased plant density and N application significantly reduced RSS while K application increased it especially in the higher fertility field in 2009. Averaged across fields and years, RSS was reduced 9.1% and 4.5% by high plant density and N fertilization. A combination of high plant density and N application without K (7.5/240/0) further reduced RSS (16.2%) relative to NK-free control at 4.5 plants/m2 (Table 5). 3.4. Nutrient accumulation in cotton plants Total nutrient (N, P and K) uptake per hectare was significantly affected by lint yield in both 2008 and 2009 (Table 6). On per hectare basis, the plants uptake 41.8, 41.6 and 72.1% more N, P and K from the higher fertility field (lint yield 2087 kg/ha) than the lower fertility field (lint yield 1451 kg/ha). The result suggests a requirement of high nutrient uptake for enhanced yield, and lint yield as a function of nutrient uptake. The plants used approximately equal N and P to produce 100 kg lint from lower fertility (12.4–12.9 kg and 1.96–2.05 kg) to higher fertility (12.3–12.7 kg and 1.96–2.01 kg) fields in both years. However, significant difference in K uptake for producing 100 kg lint was observed between both fields. The plants used 20.6% more K to produce 100 kg lint in the higher fertility field compared with the lower fertility field. Ratios of N:P:K also showed that cotton plants used relatively equal N and P from the higher fertility to the lower fertility field, but significantly more K from the former than the latter.
Table 5 Main and interaction effects of plant density and N and K application on ratio of seed cotton to stalk (RSS) in both higher and lower fertility fields in 2008 and 2009. Treatment
Lower fertility field
Higher fertility field
2008
2009
2008
2009
Plant density (PD) (plants/m2 ) 4.5 0.920a* 7.5 0.799b
0.869a 0.768b
0.871a 0.801b
0.822a 0.795b
N fertilization 0 240
0.876a 0.843b
0.827a 0.810b
0.853a 0.821b
0.844a 0.773b
K fertilization 0 150
0.852b 0.867a
0.817a 0.819a
0.821b 0.854a
0.785b 0.834a
PD × N × K interaction 4.5 × 0 × 0 4.5 × 0 × 150 4.5 × 240 × 0 4.5 × 240 × 150 7.5 × 0 × 0 7.5 × 0 × 150 7.5 × 240 × 0 7.5 × 240 × 150
0.918a 0.940a 0.893b 0.930a 0.858c 0.788d 0.740e 0.810d
0.904a 0.850b 0.841b 0.881a 0.793c 0.761d 0.731e 0.786cd
0.875b 0.905a 0.831c 0.872b 0.807c 0.826c 0.770d 0.812c
0.843b 0.876a 0.760d 0.810c 0.812c 0.847b 0.725e 0.797c
Source of variance PD N K PD × N PD × K N×K PD × N × K
0.0007 0.0001 0.0269 0.0022 0.0282 0.0001 0.0005
0.0014 0.0262 0.6593 0.8393 0.0766 0.0000 0.7106
0.0010 0.0013 0.0014 0.1910 0.7701 0.2482 0.6664
0.0048 0.0001 0.0000 0.5070 0.3433 0.0446 0.3992
* For each factor, means within the same column followed by different letters differ significantly according to the least significant difference (LSD) test (P < 0.05).
4. Discussion This study has added new information on the common perception that fertilizer application and plant population are most
important practices for improving cotton yield, yield components, and nutrient uptake. These parameters as well as harvest index
H. Dong et al. / Field Crops Research 119 (2010) 106–113
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Table 6 Nitrogen, P and K uptake by cotton plants at different lint-yield (fertility) levels. Soil fertility level
Lint yield (kg/ha)
Main nutrient uptake (kg/ha)
Nutrient requirement per 100 kg lint (kg)
N:P:K
N
P
K
N
P
K
1415b* 2146a
182.3b 264.4a
29.0b 42.2a
106.7b 198.6a
12.9a 12.3a
2.05a 1.96a
7.55a 9.30b
1:0.159:0.585 1:0.159:0.751
2009 Lower Higher
1488b 2028a
185.1b 256.5a
29.5b 40.7a
115.5b 183.7a
12.4a 12.7b
1.96a 2.01a
7.72b 9.05a
1:0.159:0.623 1:0.159:0.716
Average Lower Higher
1451.5b 2087.0a
183.7b 260.5a
29.2b 41.4a
111.1b 191.2a
12.7a 12.5a
2.01a 1.96a
7.64b 9.21a
1:0.159:0.604 1:0.159:0.734
2008 Lower Higher
*
Means within the same column followed by different letters differ significantly according to the least significant difference (LSD) test (P < 0.05).
were significantly affected by plant density, N and K fertilization, or their interactions. Such effects varied with fertility and yield potential of fields. The present study has also provided new information on correlation between harvest index and ratio of seed cotton to stalk, as well as the importance of potassium fertilization for further yield increases in Bt cotton. The ratio of seed cotton to stalk was significantly correlated with harvest index, and may thus be used as a simple indicator of dry matter partitioning. Cotton plant used more K for producing each unit of seed cotton in higher fertility than in lower fertility conditions, suggesting that more K fertilizer should be applied for increasing yield. 4.1. Yield and yield component Numerous studies have focused on the effects of plant density, N or K fertilization on cotton yield and yield components (Bondada and Oosterhuis, 2001; Ali et al., 2007b; Clement-Bailey and Gwathmey, 2007; Sawan et al., 2008). Still other studies have reported interaction effects of plant density with N or K fertilization (Ali et al., 2007a; Boquet, 2005; Rinehardt et al., 2004). In the present study, we found that the effects of plant density, N or K rate varied with yield levels. In the lower fertility field, high plant density increased boll number, but decreased boll weight. Nitrogen fertilization increased boll number and K fertilization improved boll weight. Thus, increased plant density, single application of N or K, and a combination of N and K significantly increased lint yield. A combination of nitrogen with K fertilization at 7.5 plants/m2 was the best for lint yield and this could be attributed to the increase in both the number and weight of bolls. In contrast to the higher fertility field, increased plant density or application of N did not improve lint yield in the lower fertility field. However, K fertilization increased lint yield regardless of plant density. Our results indicate that increased plant density with moderate N and K fertilization is beneficial to lint yield in the lower fertility field, but typical plant density (∼4.5 plants/m2 ) with moderate K fertilization is beneficial in the higher fertility field. Most importantly, the results suggest that N fertilizers might be applied more than required in fertile field, and N inputs can be safely reduced without decreasing yield as reported in Rochester et al. (2009). Further experiments on both the exact rates and timing of N and K are necessary and important. 4.2. Harvest index As a direct indicator of dry matter partitioning in cotton, harvest index (total biomass divided by seed cotton yield) is positively correlated to lint yield. It is traditionally determined by collecting all the debris at short intervals (5–7 d) throughout the growth season (Hay, 1995). This process is time-consuming and labor-intensive. Although harvest index can be simply determined by collecting
only the above-ground biomass at harvest, it may not correctly indicate dry matter partitioning since the root system is not included (Sadras et al., 1997). Thus, it is necessary to find an alternative indicator that is easily determined. In the present study, linear correlation analysis showed a highly significant positive correlation between harvest index and the ratio of seed cotton to stalk. Since RSS is easier to determine, it may be used as an indicator of dry matter allocation and also as an alternative to harvest index. Previous studies with cotton indicated that reproductive allocation of dry matter at harvest was fairly stable in relation to major environmental factors such as water availability, nitrogen supply, CO2 concentration and plant density (Constable and Hearn, 1981; Orgaz et al., 1992; Kimball and Mauney, 1993; Sadras et al., 1997). Changes in dry matter allocation are more likely to occur when environmental factors affect the duration of reproductive growth (Sadras et al., 1997). However, some more recent studies have shown significant changes induced by environmental factors or agronomic practices (Ali et al., 2009; Clement-Bailey and Gwathmey, 2007; Gwathmey et al., 2009; Makhdum et al., 2007; Darawsheh et al., 2009). Although high plant density and additional N or K can increase total plant biomass per unit ground area, the final reproductive allocation as indicated by RSS or harvest index was usually decreased by increased plant density (Ali et al., 2009), excessive fertilizer N because of luxurious vegetative growth and lower seed cotton yield (Boquet and Breitenbeck, 2000), and additional K (Clement-Bailey and Gwathmey, 2007; Gwathmey et al., 2009; Makhdum et al., 2007). Our result agreed with previous findings that increased plant density and excessive N application significantly reduced reproductive allocation (Ali et al., 2009; Boquet and Breitenbeck, 2000; Boquet, 2005). In contrast to previous studies (Clement-Bailey and Gwathmey, 2007; Gwathmey et al., 2009; Makhdum et al., 2007), however, K application in the present study increased the RSS, especially in the higher fertility field. Although the underlying mechanism for the enhanced reproductive allocation was not clear, our results reinforced the importance of potassium application in the high-fertility field. Optimal yield can only be obtained under proper coordination of total biomass and RSS by modification of plant density, N and K fertilization. 4.3. Nutrient uptake Although nutrient uptake does not discriminate between soil and fertilizer sources, it does give some insight into whether inadequate, sufficient or excessive amounts of fertilizer were applied (Rochester et al., 2009). Unruh and Silvertooth (1996b) determined that the N–P–K requirement for each 100 kg lint for Upland cotton was 15–2.3–19 kg/ha, whereas Pima cotton required a higher rate of 21–3.3–23 kg/ha. The higher requirement was attributed to Pima’s lower harvest index (Unruh and Silvertooth, 1996a) and
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thus a higher K uptake was needed for it to produce yields equal to Upland cotton (Sabbe and Hodges, 2009). In the present study, the plants absorbed approximately equal N and P to produce 100 kg lint from lower to higher fertility field, but absorbed 20.6% more K to produce 100 kg lint in the higher fertility than the lower fertility field. The ratio of N:P:K further confirmed that the plants absorbed relatively equal N and P from higher fertility to lower fertility fields, but more K from the former than the latter. The finding suggests that K application should be emphasized in high-yielding Bt cotton production. 5. Conclusion Lint yield was improved by the increased plant density in the lower fertility field particularly when no N or K fertilizer was applied, but it was not affected in the higher fertility field. Nitrogen or K also increased lint yield, and a combination of high plant density, N and K application further improved lint yield in the lower fertility field, while only K application increased it in the higher fertility field. Since lint percentage was not affected by the factors studied, the yield increase due to plant density, fertilization or their combinations was realized via increases in boll number or weight. Ratio of seed cotton to stalk was linearly and positively correlated with harvest index and is easy to determine; thus it can be an alternative indicator of dry matter allocation. The yield increase due to plant density and fertilization was mainly attributed to the enhanced biological yield in the lower fertility field, while that due to K fertilization was mainly ascribed to increased RSS in the higher fertility field. Plants used approximately equal N and P to produce 100 kg lint whether in the lower or higher fertility field. However, they used 20.6% more K in the higher than the lower fertility field to produce 100 kg lint. Ratios of N:P:K in both fields (1:0.365:0.728 and 1:0.364:0.884) also showed that the plants used more K in the higher fertility (high-yielding) field. On the basis of these observations, we recommend moderate plant density with low N and moderate K applications in the higher fertility field; for the lower fertility field, a high plant density and moderate N and K fertilization is recommended in the Yellow River Valley and other areas with similar ecologies. Acknowledgements This work was supported by the by the earmarked fund for Modern Agro-industry Technology Research System (Cotton 2007-2010), the Agricultural Research Project (Project No. nyhyzx07-005-02) and the National Natural Science Foundation of China (30971720). We thank Prof. A. Egrinya Eneji of the University of Calabar, Nigeria, for critical reading of the manuscript. References Ali, H., Afzal, M.N., Muhammad, D., 2009. Effect of sowing dates and plant spacing on growth and dry matter partitioning in cotton (Gossypium hirsutum L.). Pak. J. Bot. 41 (5), 2145–2155. Ali, M.A., Mushtaq, Ali., Mueen-ud-Din, Y.K., Yamin, M., 2007a. Effect of nitrogen and plant population levels on seed cotton yield of newly introduced variety CIM-497. J. Agric. Res. 45 (4), 289–298. Ali, M.A., Tatla, Y.H., Aslam, M.J., 2007b. Response of cotton (Gossypium hirsutum L.) to potassium fertilization in arid environment. J. Agric. Res. 45 (3), 191–198. Bednarz, C.W., Bridges, D.C., Brown, S.M., 2000. Analysis of cotton yield stability across population densities. Agron. J. 92, 128–135. Bondada, B.R., Oosterhuis, D.M., 2001. Canopy photosynthesis, specific leaf weight, and yield components of cotton under varying nitrogen supply. J Plant Nutr. 24 (3), 469–477. Bondada, B.R., Oosterhuis, D.M., Norman, R.J., Baker, W.H., 1996. Canopy photosynthesis, growth, yield, and boll 15 N accumulation under nitrogen stress in cotton. Crop Sci. 36, 127–133. Boquet, D.J., Moser, E.B., Breitenbeck, G.A., 1993. Nitrogen effects on boll production of field-grown cotton. Agron. J. 85, 34–39. Boquet, D.J., 2005. Cotton in ultra-narrow row spacing: plant density and nitrogen fertilizer rates. Agron. J. 97, 279–287.
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