ABSTRACT
Three field experiments to investigate the partial nutrient balance (N and K) in cassava/soybean intercrop system were conducted at University of Nigeria, Nsukka in a derived Savannah location of South Eastern Nigeria. Effect of fertilizer rate, residual fertilizer and soybean residue management on growth and yield of soybean and cassava and soil nutrient reserve were studied. Soybean growth and yield were significantly affected by their varieties, fertilizer rate and cropping system but not by cassava variety. Application of fertilizer at 50 Kg ha-1
fertilizer rate gave the highest soybean total dry matter (898.24 Kg ha-1), highest grain yield (156.91 Kg
ha-1) and highest fresh cassava tuber yield (30.7 t ha-1) at 12 months after planting (MAP). Intercropping cassava variety (NR 8230) with medium maturing variety of soybean, (TGX 1894-3E) gave the highest grain yield of soybean (133.60 Kg ha-1), highest fresh tuber yield of cassava (30.8 t.ha-
1) at 12 MAP, highest N and K balances (+44.06 and +72.70 Kg.ha-1), highest LER of 2.71 and ATER
of 2.10 at 50 Kg K.ha-1 fertilizer rate. The effect of residual fertilizer on soybean plant height, litter weight and shoot dry weight was highest at 45 Kg N.ha-1 and 50 Kg K.ha-1 fertilizer rate, while soybean grain yield of 204.1 Kg.ha-1 was highest at residual 50 Kg K.ha-1 fertilizer rate. Also, cassava leaf-N (4.05%) and cassava stem-K (2.8%) were highest at residual 45 Kg N.ha-1and 50 Kg K.ha-1, while cassava tuber yield at 12 MAP was highest (24.27 t.ha-1) at residual. 50 Kg K.ha-1 Soil nitrogen increased from 0.04%N to 0.34%N and soil- pH from 4.2 to 6.6 at sole soybean. In sole cassava, soil-N
decreased from 0.04% initial soil-N to 0.03%N but soil-N under 3 year natural fallow was unchanged. Residual fertilizer affected nitrogen balance of sole cassava which was negative, while intercrop had positive nitrogen balance at all residual fertilizer rates. Potassium balance was positive with intercrop at all residual fertilizer rates except at no fertilizer. Incorporation of soybean residue into cassava/soybean intercrop system increased cassava tuber yield and soil nutrient status and resulted in positive N and K balances. Analysing of nutrient stock: balance (NSB) ratio revealed that fertilizer rate of 50 Kg K ha-1 in the first year of cassava/soybean intercrop followed with incorporation of residue in the subsequent year (without further application of fertilizer) gave the highest dynamic reserve of 310.6 Kg N ha-1 and
311.64 Kg K ha-1. Under sole soybean, production can be sustained for 15 years (NSB ratio 14.58),
while under intercropped system production can be sustained for 9 years for nitrogen and 9 years for potassium (N balance of +62.64 Kg ha-1 and K balance of +87.66 Kg ha-1). Sole cassava without fertilization and no residue incorporation can be sustained for one year with NSB: ratio of 0.88.
INTRODUCTION
Nutrient monitoring (NUTMON) is an integrated methodology that targets different factors in the process of managing soil nutrients and other natural resources in agriculture (Vlaaming et al., 2001). With the NUTMON methodology, farmers and researchers analyse the environmental and financial sustainability of farming systems. A quantitative analysis generates important indicators such as nutrient flows, nutrient balances, cash flow, gross margins and farm income. Both the quantitative and qualitative analysis are then used to improve or design new technologies that tackle soil fertility management problems and can help improve the financial performance of the farm. Manipulation of nutrient ‘stock and flows’ of farm lands is vital in the derived Savannah zone. Soil organic matter (SOM) in the farms, which accounts for the major cation exchange capacity and nitrogen content of the soil are not often replenished as they decline from crop cycle to crop cycle. The soil therefore becomes rapidly degraded and highly weathered especially where inorganic fertilizers become the major methods for soil nutrient replenishment. According to FAO (2003), agricultural intensification without adequate restoration of soil fertility threatens the sustainability of agriculture. A nutrient input-output balance analysis will therefore help in predicting the sustainability.
Guillard et al. (1995) reported that traditional farming system is a concept that will be developed. This concept is mostly based on ecological principles of constant utilization of all tropic levels. Some approaches to the concept have been reported by Defoer et al. (2000) as integrated nutrient management (INM) and integrated soil fertility management (ISFM). Both concepts included intercropping and biological nitrogen fixation (BNF) as part of input data. By the application of these concepts in any agricultural system, the system should maintain sustainable mineral availability in the soil and results in a balanced nutrient management system (BNMS), (IITA, 1997).
A good knowledge of possible changes of nutrient stock of a system involves balancing of nutrient input and output in the system (nutrient flow analysis). Nutrient flow analysis using an approach based on “balance” is one of the possible ways to estimate the diversities in the sources and flows of nutrient content of a system (applied fertilizer, fertility in soil, plant mineral uptake and crop residue). It quantifies the whole system management method together with their cost and time. However, nutrient balances involving flows of few nutrient elements are considered “partial balances” because they basically show only a portion of what the farmer takes out of the system and what they put back (Browner and
Powell, 1993). They do not include the farmers’ action while managing the farm in terms of
cost and time rather helps to understand the status of soil fertility. When the nutrients extracted from the soil roughly equals the nutrient brought back, it would be assumed that the system is in equilibrium. A large negative or positive difference is cause for concern and will require some form of correct action. Nutrient accumulation occurs only when more nutrients are added than removed. A negative balance means that the production system is being degraded as the store of available soil nutrient is depleted. Nutrient stock: balance ratio (NSB) which is an indicator for sustainability gives a more accurate indication on the length of time in years farming can continue in the same way, given the available nutrients.
Ledgard, (2001) noted that intercropping promises sustainable plant production Aggarwal et al. (2002) suggested that to select and integrate legumes into various production systems should maintain a non-declining mineral trend from crop cycle to crop cycle. The rate of replaced nutrient within or between cropping cycles must at least equal the rate of removable during the previous cropping. Benefiting effect of succeeding crops after legumes which were recorded by many workers (Touchtors et al., 1982; Thimonier et al., 2000; Alewell et al., 2000; Krajiek, 2001) were traced to the decomposition of litter deposits and sloughed off dead nodules left in the soil (Bohra and Singh, 1990).
The challenge therefore, is to sustain soil fertility over time. It requires judicious system management and integration of crops for biological mineral fixation to achieve favourable nutrient balance. The use of soybean in legume/non legume crop production system has been sparingly exploited with cassava. There is need to identify legume (soybean) cultivars capable of near maximum levels of N fixation and dry matter accumulation in our soils and utilizing them in such intercropping system. Results from work carried out by the International network on soil fertility and fertilizer evaluation for rice (INSFFER) indicate that incorporation
of one crop of Azolla anabena increased rice yield as does the input of 30 Kg urea ha-1
(Watanabe, 1987). In addition, repeated incorporation of Azolla improved soil structure significantly (Lumpkin and Plucknett, 1982). Likewise, inclusion of soybean in cassava-based cropping system improved chemical properties of the soil (Umeh, 2002). Repeated incorporation of soybean residue into the soil over .a period of time should have also the potential to improve soil structure as does Azolla spp. The incorporated plant residue absorb soil nitrate, reduce leaching and stimulates yields of non-nitrogen fixing crop (Ladha and Reddy, 2003). It has been reported that potential contribution of cowpea to a subsequent crop increased with maturity class of cowpea (Abaidoo et al., 1999). Soybean mineral contribution to associate crop may increase also with maturity class of soybean and influence the nitrogen
balance of the soil for both accompanying and subsequent crops. Letting legume crop residue ploughed-in or recycled into the soil should increase soil humus content, stabilize C/N ratio and thereby reduce volatilisation of minerals during decomposition. Low C/N ratio favours high rates of mineralization and volatilization (Ladha and Reddy, 2003)
The increasing world population and limited availability of energy has prompted a recent surge of interest in cassava, not only for traditional use as a human food, but also for specialised starches, animal feedstuff and then industrial uses. Cassava, which has been
reported as the 4th most important energy staple of the tropics, providing food and income for
more than 750 million people annually (FAO, 1992) is still cultivated on small farms, with little technology. Nigeria being one of the leading nations of the world in cassava production has depended largely on use of chemical fertilizer. Leihner (1988) noted that cassava removes
about 90 Kg N ha-1 from soil in a cropping season. Within the derived Savannah ecology,
cassava fertilizer K and N recommendation of 112 Kg K ha-1 and 120 Kg N ha-1 was for optimum productions in pure stands (ARTS, 1994). Since the mid 1990’s it has been suggested that to argument this fertilizer need for the higher yield of cassava and to sustain the growing
human population without further degrading the soil, this integration of biological nitrogen fixation and nutrient stock balance are needed (Aggarwal et al., 2002). Very few studies have pursued this suggestion. The objectives of this research were to
(1) select soybean variety most suitable for intercropping with cassava, and determine the combined effects of fertilizer (N and K) on the growth and yield of the crops.
(2) determine N and K levels in the crop and soil at different cropping systems and predict sustainability using nutrient stock: balance ratio,
(3) determine the cropping system efficiency from land equivalent ratio and Area x
Time equivalent ratio and
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(4) determine the effect of soybean residue management on cassava yield and soil properties.
This material content is developed to serve as a GUIDE for students to conduct academic research
PARTIAL NUTRIENT BALANCE IN CASSAVA (MANIHOT ESCULENTA CRANTZ) AND SOYBEAN (GLYCINE MAX (L) MERRIL) INTERCROP FOR SUSTAINABLE AGRICULTURE IN A DERIVED SAVANNAH LOCATION
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