Improved cassava varieties increase the risk of soil nutrient mining: an ex-ante analysis for western Kenya and Uganda Fermont A.M.a , Obiero H.M.b , van Asten P.J.A.a , Baguma Y.c , Okwuosa E.b a b
IITA-UGANDA, P.O. BOX 7878, Kampala, Uganda,
[email protected] Tel/fax: +256-414-285-060/079; KARI-Kakamega, P.O. BOX 169, Kakamega, Kenya; c NARO/NAARI, P.O. BOX 7084, Kampala, Uganda
Abstract Cassava production in Uganda and western Kenya has been hit hard by the cassava mosaic disease (CMD) epidemic. In response, CMD resistant cassava varieties are currently released on a wide scale. The new varieties yield up to 3 times more than the local varieties. These high yield levels will put major pressure on soil nutrient stocks. Using a local variety, an average farmer will harvest about 10 t ha−1 fresh roots, thereby removing 26 kg N, 3 kg P and 19 kg K per hectare. Using a good CMD-resistant variety, the same farmer can harvest a 30 t ha−1 , thereby removing 83 kg N, 10 kg P and 47 kg K per hectare. If stems are used for planting material and/or firewood, then removal increases to 216 kg of N, 22 kg of P and 102 kg of K per ha for CMD-resistant varieties. Soils in western Kenya and Uganda are predominantly Ferrasols, Acrisols and Nitisols; old weathered soils with small nutrient stocks. Without the use of fertilizers, the rapid depletion of soil nutrient stocks seems unavoidable with the new varieties. This will eventually result in yield decline of cassava and rotational crops. The question arises if traditional cropping systems are suitable for cultivating crops with high nutrient demand. However, production levels of banana, the other important food crop in Uganda, have been sustained for over half a century in several parts of the country, despite K requirements (142 kg ha−1 yr−1 ) of good yielding bananas (25 t ha−1 yr−1 ) being similar to that of good-yielding cassava varieties. But, in contrast to cassava fields, traditional banana fields maintain their soil fertility through large amounts of organic inputs, on the expense of annual cropped fields and grassland. Due to the position of cassava in the farming system, it is unlikely that soil management strategies in banana can be successfully adopted by cassava farmers. However, rotating the improved cassava varieties with fertilized cash crops and introducing promiscuous leguminous inter- and relay crops in cassava fields are potential management options to improve the sustainability of the system. Nonetheless, the development of K deficits will remain a serious concern. The high yield levels of the new cassava varieties have already triggered its promotion as a cash crop. Provided that there is a good (industrial) market outlet, farmers can be motivated to use targeted organic & inorganic fertilizer to prevent soil fertility depletion
Key words: Cassava, soil fertility, nutrient removal, East Africa
Introduction The cassava mosaic disease (CMD) epidemic that started in Uganda in the early nineties has presently reached most of eastern and central Africa with devastating effects on cassava production (Otim-Nape et al., 2000). IITA and its national partners are successfully
developing and releasing CMD resistant cassava varieties to counteract its impact. The latest introductions have a high yield potential characterized by multiple resistance to the major biotic stresses, drought tolerance, earliness and higher dry matter content. Yield levels up to 50 t/ha in advanced yield trials have been obtained in Uganda and western Kenya (Obiero, 2004;
A. Bationo (eds.), Advances in Integrated Soil Fertility Management in Sub-Saharan Africa: Challenges and Opportunities, 511–519. © 2007 Springer.
512 NAARI, 2000). These yield levels have raised concerns about the impact of the new varieties on soil fertility depletion. Soil fertility depletion has been described as one of the most important constraint to food security in subSaharan Africa. Nutrients are commonly not replaced to the degree that they are removed in crop harvesting and other losses, resulting in highly negative nutrient balances (Hilhorst and Muchena, 2000; Stoorvogel and Smaling, 1990). Soils in western Kenya and Uganda are predominantly Ferrasols, Acrisols and Nitisols; old weathered soils that contain predominantly kaolinite and are virtually free of weathering minerals (Braun et al., 1997; Andriesse and van der Pouw, 1985; Jaetzold and Schmidt, 1983). Their fertility (available N and P) is closely related to soil organic matter content and, therefore, to the presence or absence of fallow periods (Foster, 1981; Jones, 1972). Cassava has numerous traits that offer comparative advantages in marginal environments where farmers often lack the resources to improve their lands through purchased inputs. Its tolerance to poor, acidic soils, high levels of exchangeable aluminum, low concentrations of P in the soil solution and drought periods provides it with the ability to grow and produce reasonable yields in places where other crops do not produce well (Howeler, 2002; Fresco, 1986; Cock and Howeler, 1978). As a result, cassava is often produced on areas with soil problems, while the better soils are devoted to ‘more profitable’ crops (Fresco, 1993). In this article we will carry out an ex-ante analysis to evaluate the potential impact of improved varieties on soil fertility depletion in Uganda and western Kenya by means of 1) estimating nutrient removal in four scenarios and analyzing soil fertility management in cassava cropping systems and 2) comparing these results to soil nutrient management of traditional banana cropping systems that have nutrient requirements comparable to good yielding CMD-resistant cassava varieties.
The importance of cassava in Uganda and Kenya Although cassava (Manihot esculenta) was already introduced in Africa in the 16th century, it took until the early 19th century before the crop was grown on a large scale in eastern Africa (Hillocks, 2002). Currently it is one of the main staple foods in the region. It’s adaptability to relatively marginal soils and erratic rainfall conditions, its high productivity per unit of land and labour, the certainty of obtaining some yield even
Improved cassava varieties increase the risk under the most adverse conditions, and the possibility of maintaining continuity of supply throughout the year make this crop a basic component of farming systems in eastern Africa (Fresco, 1986; Nweke et al., 1994). In terms of total production, cassava is the second most important staple food in Uganda, while it is the fifth most important staple food in Kenya. According to the FAO, total annual production levels and area under cassava in 2002 range from 610,000 tons and 80,000 ha in Kenya to 6,300,000 tons and 390,000 ha for Uganda (FAO, 2004). Cassava is grown throughout Uganda and Kenya, but the main production zones are the northern and eastern parts of Uganda and the western provinces and the coastal zone of Kenya.
Actual and potential yield levels According to the FAO (2004) actual fresh yield levels for cassava in Uganda are between 12 and 13.5 t/ha. Current farmer yield levels are on average 4 t/ha higher than before the start of the CMD epidemic and 7 t/ha higher than during the main years of the CMD epidemic (1994–1997). This is likely to be due to: (i) the introduction of improved, CMD resistant varieties, whose proportion increased from 0% in 1996 to 35% in 2003, (ii) possibly to the adoption of local varieties with some degree of resistance or tolerance to CMD, and (iii) to a recovery of yield levels of local varieties once the height of the CMD epidemic has past (Legg et al., 2004). These data correspond well to the average yield level of 10.6 t ha−1 that the COSCA study for Uganda determined in 1990 (Nweke et al., 1999). FAO data for Kenya do not really show the impact of the CMD epidemic on production as fresh yield levels remain constant between 8.5 and 9.5 t ha−1 for the past decade (FAO, 2004). Annual reports for the main cassava districts of western Kenya, however, show that yield levels dropped from 7–10 t ha−1 to 4–8 t ha−1 during the height of the cassava epidemic in the late nineties (Anonymous, 1998; 1999). Potential cassava yields of improved varieties are much higher than the reported actual yields in Kenya and Uganda (Table 1 and 2). Average fresh yield levels obtained in on-station and on-farm trials depend on agro-ecology and climate and ranged between 9 and 41 t ha−1 and 12 and 25 t ha−1 for Uganda and Kenya respectively. Maximum yield in these trials ranged between 18 and 59 t ha−1 in both countries. Tables 3 and 4 show that average yield levels of local varieties are 0 to 20 t ha−1 lower than those of CMD-resistant
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Table 1. Fresh cassava yield1 (t ha−1 ) of improved varieties (n = number of varieties evaluated) in Uganda
Trial On-farm, 6 districts 1993–19952 Advanced yield trial, 1996/73 – Namulonge – Serere Advanced yield, 5 loc., 1997/84 Uniform yield, 4 loc., 1997/84 Multi-locational trials, 1998/995 Uniform yield trials, 1999/20006 – Namulonge – Masafu – Kumi – Serere On-farm, 6 districts, 2001/20027 – Soroti – Katakwi – Kumi – Pallisa – Apac – Lira Yield assessment, Serere, 2002/38 1. 2. 3. 4. 5. 6. 7. 8.
n 8 19 21 21 19 16 10 9 9 9 5 5 6 5 4 4 5
Minimum
Maximum
Mean
7.2 (Masindi ’94)
23.1 (Soroti ’94)
15.2
13.1 (94/NA00044) 13.8 (Nase 2) 5.9 (95/SE-0348) 6.8 (TMS 83350) 9.0 (MH95/0080) 9.7 (MM96/0549) 4.2 (95/SE-00087) 9.3 (MM96/1425) 16.7 (95/SE-00087)
35.2 (94/NA-00172) 51.2 (94/SE-00061) 30.1 (95/SE-00044) 29.1 (94/SE-00088) 30.7 (Nase 12) 26.8 (Nase 2) 19.0 (I92/2327) 19.5 (I92/2327) 51.3 (I92/2327)
18.8 31.2 18.9 18.6 19.5 19.4 9.4 12.7 33.0
28.2 (I91/2327) 22.5 (MM92/00057) 23.9 (TME 14) 7.9 (I92/2327) 19.2 (MM92/00067) 15.6 (MM92/00057)
59.0 (TME 204) 50.0 (MM92/00067) 32.4 (MM92/00057) 24.2 (Nase 3) 34.7 (I92/2327) 21.2 (MM92/00067)
40.8 34.6 26.4 14.5 25.8 18.0
29.3 (TME 14)
44.0 (I92-0427)
34.2
All data refer to sole cropped cassava planted at 10.000 plants per ha. Bua et al., 1997. NAARI, 1997. NAARI, 1998. Ssemakula, 2000. NAARI, 2000. NARO, 2003. Unpublished data IITA.
Table 2. Fresh cassava yield1 (t ha−1 ) of improved varieties (n = number of varieties evaluated) in western Kenya
Trial
N
Minimum
Maximum
Mean
Advanced yield, 3 loc., 2000/011 On-farm, 7 loc., 2000/011 Advanced yield, Oyani, 2001/021 Advanced yield, 2 loc., 2002/031 Agronomy trials, Alupe, 2003/42
21 16 31 17 3
15.3 (MM96/4510) 18.4 (SS4) 2.9 (I92/0323) 15.5 MM96/2480) 13.5 (Nase 3)
43.5 (MM97/2283) 51.1 (MH95/0183) 32.5 (MM96/7023) 24.0 (MM96/3665) 18.2 (MM96/4884)
25.1 27.9 11.8 18.5 15.8
1. Obiero, 2004. 2. Unpublished data A. Fermont, H. Obiero & E. Okwuosa.
varieties. Maximum yield levels of the improved varieties can be up to 35 t ha−1 more than the maximum yield levels obtained with local varieties.
Nutrient removal in cassava cropping systems The amount of nutrients removed with cassava harvest is highly dependent on growth rate and yield, which in turn depend on climate, soil fertility conditions and variety. However, Howeler (2002), who analyzed data
from 15 cassava trials reported in literature, found that there was a good relation between dry matter yields and removal of nitrogen (N), phosphorus (P) and potassium (K). Working with his data, removal of N, P and K (kg ha−1 ) were expressed as a function of dry matter root yield and as a function of total dry matter yield (roots, stems and leaves). Figure 1 present the functions for nutrient removal by roots, while Table 5 gives equations and R2 for nutrient removal by both root and total dry matter yield. Using these data, an estimation of nutrient removal in four different fresh root yields
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Improved cassava varieties increase the risk
Table 3. Mean fresh cassava yield (t ha−1 ) of local varieties in researcher trials in Uganda
Trial
Name of local variety
On-farm trials, 6 districts, 1993–19951 Advanced yield, Namulonge, 1999/20002 On-farm trials, 5 districts, 2001/20023 Agronomy (2), Namulonge, 2003/44
Various Ogwok various - Njule - Bao - Nyaraboke
1. 2. 3. 4.
Mean yield 6.7 9.9 12.2 15.1 8.4 7.6
Bua et al., 1997. NAARI, 2000. NARO, 2003. Unpublished data A. Fermont & Y. Baguma.
Table 4. Mean fresh cassava yield (t/ha) of local varieties in researcher trials in western Kenya
Trial
Name of local variety
Mean yield
On-farm trials, 7 locations, 2000/011 Agronomy trials (2), Alupe, 2003/42
Serere & Adhiambo lera
10.2
- Matuja - Mwitamajera - Gachaga
13.1 6.7 7.0
1. Obiero, 2004. 2. Unpublished data A. Fermont,. H. Obiero & E. Okwuosa.
scenarios is made. The four scenarios are: (1) local variety (10t/ha), (2) improved variety – moderate yield level (20 t ha−1 ), (3) improved variety – good yield level (30 t/ha) and (4) improved variety – very good yield level (40 t ha−1 ). Dry matter content and harvest index of local and improved varieties is taken as 35% and 37%, and 39% and 60% respectively (Fermont, Baguma, Obiero and Okwuosa, unpublished). Table 6 gives a summary of the amount (kg) of N, P and K removed per hectare when (a) only roots removed, and when (b) all roots, stems and leaves removed. Nutrient removal ranges from 26 to 111 kg N ha−1 , 3 to 13 kg P ha−1 and 19 to 74 kg K ha−1 if only the roots are removed. When all biomass is removed from the field, nutrient removal increases to 120 to 274 kg N/ha, 11 to 29 kg P ha−1 and 52 to 156 kg K ha−1 . Most farmers in Uganda and western Kenya use available stem material for either planting material or fire wood (Wortmann & Kaizzi, 1998). Only part of the leaves remains in the field. Actual nutrient removal in cassava fields will therefore be closer to scenario ‘b’. Nutrient removal in scenario ‘1b’ (local variety; all biomass removed) is comparable to the nutrient removal of an average double maize crop of 1.8 t ha−1 per harvest with
all stover removed in western Kenya (van den Bosch et al., 1998). Like elsewhere in Africa, cassava in Uganda and western Kenya is commonly grown in intercropping systems. In Uganda, only 5% of all cassava is grown as a sole crop. In approximately three quarters of the intercropping systems, cassava is the major intercrop, while in 20% it is the minor crop. Cassava-cereal systems (maize, sorghum, millet) are by far the most common in Uganda and western Kenya, but cassava-legume systems (phaseolus beans, peas, groundnut) are also widespread (Nweke et al., 1999).Yield levels of cassava in an intercropping system depend, amongst others, on planting density, type of intercrop, cassava variety and relative planting time (Leihner, 1983). Considering that the average yield level of cassava fields in the COSCA study for Uganda was 10.6 t ha−1 and 95% of those fields was intercropped, nutrient removal by cassava in intercropped systems will be very similar to scenario 1 (i.e. local variety, 10 t ha−1 ). The inclusion of an intercrop in the system will increase nutrient removal through harvest of the intercrop. Cassava-legume systems may benefit from N supplied by the legume, but will still have a higher removal of K and P than sole cropped cassava. Phaseolus beans are the most common legume intercrop, but, unfortunately, their effect on the N balance is small or even negative.
Soil fertility management in cassava cropping systems Table 7 shows the soil fertility practices used in cassava fields on a village level in Uganda. Soil fertility management in western Kenya is very similar to Uganda. Intercropping with legumes is used in all villages, while only 15 and 5% of the villages use manure and chemical
Fermont A.M. et al.
515 180 N uptake in roots
N uptake in roots (kg/ha)
160 140 120 100 80 60 40 20 0 0
5
10
15
20
25
30
15 20 Root DM Yield (t/ha)
25
30
25
30
Root DM Yield (t/ha) 30 P uptake in roots
P uptake in roots (kg/ha)
25 20 15 10 5 0 0
5
10
400 K uptake in roots K uptake in roots (kg/ha)
350 300 250 200 150 100 50 0 0
5
10
15
20
Root DM Yield (t/ha) Figure 1. Relationship between N, P and K uptake in roots (kg ha−1 ) and root dry matter yield (t ha−1 ).
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Improved cassava varieties increase the risk
Table 5. Equations and R2 ’s for nutrient uptake (kg/ha) for root dry matter and total yield
Roots N P K Total yield N P K
y = 7.49 x y = 0.59 x 1.16 y = 12.52 e 0.12x
R2 = 0.67 R2 = 0.87 R2 = 0.77
y = 19.77 x 0.82 y = 1.17 x y = 27.79 e 0.07x
R2 = 0.71 R2 = 0.86 R2 = 0.76
fertilizer respectively. As discussed before N contribution of legumes is highly variable and may even be negative, especially if phaseolus beans are used as an intercrop. Residue incorporation will be mainly fresh leaves and litter fall collected on the soil surface during the growing season as most stems will be removed. Carsky and Toukourou (2004) found that litter fall could return quite considerable amounts of N (35–71 kg ha−1 ) and moderate amounts of K (9–25 kg ha−1 ) to the soil. Many farmers cultivate local bitter varieties. These varieties can remain in the field for 2–3 years without rotting. Farmers allow these fields to develop into a ‘natural’fallow (i.e. no more weeding) after 8–12 months. This practice allows the soil to rest and during this time some nutrients (especially N) can accumulate in the topsoil. Burning will make a large amount of the P and K contained in the vegetation available to the subsequent cassava, but most N will be lost. Total amounts of nutrients added the root zone depend on the duration of the fallow. Fallows periods in east Africa have reduced due to increasing population pressure. This is especially the case in western Kenya (Mango, 1999). In Uganda, half of the cassava farming systems still allows a fallow period of 2–4 years (Nweke et al., 1999). However, even with such
fallow durations, there is little chance that P and K nutrient stocks are sufficiently being replenished. In addition, grazing is likely to be too scattered and too much affected by losses to be able to make a significant contribution to restoring soil fertility. Cassava is often grown on soils with low fertility. In western Kenya, farmers intentially plant cassava on the poorest soils, because they believe that nutrient requirements of cassava are lower than for other crops (Ojiem and Odendo, 1997). This is in line with results from Mango (1999), who observed that when soil degradation becomes widespread, cassava becomes increasingly important in cropping systems in western Kenya. In addition, the COSCA studies (Nweke et al., 1999) have shown that approximately 50% of all cassava in Uganda is cropped in continuous systems that have no fallow to (partially) restore soil fertility levels.
Soil fertility management in banana cropping systems Cassava is the dominant food crop in eastern and northern Uganda, but banana is so in central and southwest Uganda (Gold et al., 1999). Fresh yield levels of banana systems are similar to what one can expect in a good cassava field (option 3). Like cassava, banana is also a huge consumer of K (Lahav, 1996). At a fruit yield of 25 t ha−1 yr−1 , banana removes on average some 52 kg N, 5 kg P, and 142 kg K ha−1 yr−1 . Banana yields are particularly good (25 t ha−1 yr−1 or more) in areas in southwest Uganda (van Asten et al., 2004), where banana fields of 50 years and older are common. This suggests that banana farmers have developed management practices that lead to sustainable production and the maintenance of sufficient soil nutrient stocks. The question is whether banana management strategies can
Table 6. Removal of N, P and K (kg ha−1 ) at four levels of cassava root yield at A) only roots removed and B) all roots, stems and leaves removed
Scenario
I. Local variety 10 t/ha1 II. Impr. variety 20 t/ha2 III. Impr. variety 30 t/ha2 IV. Impr. variety 40 t/ha2
A. Only roots removed
B. Roots, stems and leaves removed
N
P
K
N
P
K
26.2 55.4 83.1 110.9
2.5 6.0 9.6 13.4
19.1 30.4 47.4 73.9
119.5 155.1 216.3 273.9
10.5 14.4 21.6 28.9
52.1 65.9 101.5 156.2
1. Dry matter content of local varieties is estimated at an average 35% and harvest index at an average 39. 2. Dry matter content of improved varieties is estimated at an average 37% and harvest index at an average 60.
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Table 7. Soil fertility practices in cassava fields in Uganda (% of villages)1
Practice Intercropping with legumes Incorporation of residues Burning Grazing Mulching Manure Chemical fertilizer
Near fields
Distant fields
100 59 54 36 15 15 5
95 56 56 28 13 3 3
1. Own analysis from original data set of Nweke et al., 1999.
be transferred to cassava fields, in order to maintain soil fertility. In Uganda, a substantial proportion of the bananas are grown near the homestead (Rufino, 2003). Plots near the homestead generally receive more organic household residues and are often more mulched than plots further away. In a study by Briggs and Twomlow (2002) in southwest Uganda, most farmers (90%) indicated that they considered the closest homestead plot and the banana plantation to be the most important fields on which to apply their manure. Wortmann and Kaizzi (1998), Bekunda and Woomer (1996), Baijukya et al. (1998) and Bosch et al. (1996) all found that that homestead/banana plots contain more nutrients (particularly P and K) when compared to annual cropped fields and plots further away from the homestead. Baijukya et al. (1998) quantified nutrient balances for banana cropping systems in northwest Tanzania. They observed neutral to positive nutrient balances in banana fields when farmers possessed cattle (on average 5 heads per farm), due to the nutrient flow from grassland areas to banana fields. However, few farmers in their study used mulch and crop residues. This is in contrast with findings by Bekunda and Woomer (1996), who observed that Ugandan banana farmers applied a wide range of additional resources to bananas, including field crop residues (81%), burned residues (3%), onfarm manures (31%), compost (16%), external organic (17%) and chemical (4%) inputs. The majority of farmers also plants intercrops in (part) of the banana fields. Phaseolus beans are the most common intercrop and crop residues are normally left or returned to the field (Bananuka and Rubaihayo, 1994). Wortmann and Kaizzi (1998) quantified nutrient balances of different farm units in central Uganda and found that all nutrient balances were negative except for banana fields, were N and P nutrient balances were marginally positive. Nutrient balances were particularly negative
in annual cropped fields, due to the fact that fertilizer and nutrients in any form (e.g. crop residues) were applied to banana only.
Discussion and conclusions Local cassava varieties do not seriously deplete soil fertility due to low yields (10 t/ha) and long cropping cycles (2 years). Farmers in Benin, Ghana and Kenya even claim to employ cassava as a sort of fallow to revive soil fertility levels of degraded fields, using either late bulking varieties or high planting densities (Adjei-Nsiah, personal communication; Carsky and Toukourou, 2004; Obiero, 2004). On the other hand, cassava is also known as a ‘scavenger crop’, capable of removing high quantities of nutrients from the soil, thereby increasing the risk of soil exhaustion (Cock and Howeler, 1978). This appears especially true for the high yielding (20–30 t/ha) improved varieties when all biomass is removed from the field. There are four reasons to suspect that the introduction of high yielding, improved varieties will pose a serious threat to the sustainability of cassava production systems in the region. These are: (i) high nutrient removal, (ii) small nutrient stocks of most cassava soils, (iii) reduced fallowing, and (iv) no significant replenishment of nutrient stocks (especially K) through addition of (in) organic inputs. Depending on initial soil conditions, nutrient depletion will result in declining yield levels within a few years after introduction of the new varieties. This risk is higher in the continuous farming systems of western Kenya than in Ugandan systems that still allow some fallowing. Farming systems with a high percentage of cassava are particularly at risk, as crops rotated with cassava will be affected by the decreasing soil fertility levels. This concern is being reinforced by the general trend of soil fertility decline, particularly in western Kenya due to increasing land use pressure and the lack of external inputs (Soule and Shepherd, 2000). To prevent (accelerated) soil fertility decline due to the introduction of high yielding cassava varieties, their introduction should be accompanied by appropriate soil and crop management practices. Banana farmers in Uganda have shown that proper management can sustain high yield and soil fertility levels for several decades, even when no inorganic fertilizers are used. However, cassava takes a very different position in the farming system as banana (i.e., annual crop, mostly
518 grown in distant plots). For that reason, the soil fertility practices used for banana fields will probably not be adopted for cassava fields. Nonetheless, various options are available to maintain soil fertility, depending on the objectives and characteristics of the production system.
Low-input systems In low-input systems without a readily available market for cassava, planting densities of the improved varieties can be reduced to match subsistence requirements and reduce nutrient removal. This can be combined with the introduction of leguminous grain legumes with a positive N balance (groundnut, soyabean, cowpea).Another option is to concentrate cassava production for subsistence needs on a smaller area. This can be combined with an increase in the proportion of the farm under legumes with a positive N balance. The farmer practice of allowing a cassava field to develop into a fallow can be improved by the introduction of improved fallow species. This system could be adapted to continuous cropping systems by the introduction of a relay leguminous cover crop in the second rainy season. The use of legumes as an inter- or relay cover crop allows one to actually use the cassava field as an entry point to improve soil fertility for subsequent crops. In areas where a considerable part of the available farm land is dedicated to cassava, soil fertility management in cassava fields may contribute significantly to the overall fertility and sustainability of the farming systems.
Moderate-high input systems In areas without a stable cassava market, farmers are not likely to use inputs in their cassava fields to maintain soil fertility levels. This implies that one can better manage soil fertility on the farm level through practices that aim at increasing yields of the other crops in the production system, rather than targeting cassava directly. This can be achieved by increasing the percentage of promiscuous legumes in the system and/or targeted fertilizer usage to cash crops. Cassava grown in rotation with fertilized cash crops or intercropped with fertilized cash crops will benefit from the applied nutrients. The use of small applications of P has been shown to increase the N contribution of grain legumes to the soil (Giller, 2001). However, the depletion of soil K stocks will remain a serious concern in these systems
Improved cassava varieties increase the risk as K fertilizer is hardly used on the commonly fertilized crops as maize, sugarcane and legumes.
Intensive cassava systems The high yield levels of the new cassava varieties have already triggered its promotion as a cash crop in Uganda. Provided that there is a good (industrial) market outlet, farmers can be motivated to use targeted organic & inorganic fertilizer dosages to prevent soil fertility depletion and maintain high cassava yields. However, initial results from agronomic trials (unpublished data Fermont, Obiero, Baguma and Okwuosa) seem to indicate that many improved varieties are not responding to fertilizer usage. This is due to the fact that cassava breeding in East Africa was always conducted at non-fertilized plots, creating varieties that are sturdy in a large range of conditions, but that do not give an optimal response to fertilizer usage.
Acknowledgements The authors wish to thank IITA, NARO and KARI for their support of this work and the Dutch Ministry of Foreign Affairs for financing A. Fermont’s position.
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