Abstract
In a field experiment on legume-maize cropping sequence, the effect of nitrogen and phosphate fertiliser application on nutrient budgeting after a two year cropping cycle was studied. The results of this work showed that partial nutrient balances can give an indication about the sustainability of a system unless a full stock of all the nutrients is known. It is also clear from these results that there would unlikely be a carryover of N to the next season, probably due to leaching of N from the top soil. Positive partial P balances would, on the other hand, be of residual value. However, it should be noted that the positive partial P balances were only observed where maize yield was low. The long-term perspectives of a crop production system cannot be assessed solely on the basis of partial nutrient balances. The nutrient stock: balance ratio may be a better indicator of sustainability, giving a more accurate indication of how long farming can continue in the same way, given the available nutrients. These points towards taking a fresh look at the existing recommended application rates for maize crops.
Highlights
Soil fertility is an important form of renewable natural capital
The partial nutrient flows for the weed fallow followed by maize system on the different soil types are shown on Table 3
The results generally show that maize after velvet bean hay system was the most unsustainable, having negative partial N and P balances (N balances on all sites and P balances on the light clay soils) after the two years, whilst the green manure system was the most sustainable, having less negative partial N balances and all positive partial P balances
Summary
Soil fertility is an important form of renewable natural capital. Plant nutrients in the soil, are among the least resilient components of sustainability (Bationo, 2007). The maintenance of soil fertility involves the return to the soil of the nutrients removed from it by harvest, runoff, erosion, leaching, and other loss pathways. The two most widespread limiting nutrients to food production in Africa are N and P, in that order (Khosla et al, 2002). Nutrient capital can be defined as the stocks of N and P, and other essential elements in the soil, which become available to plant roots in a given period. The nutrient capital can be expressed as kg ha-1 of N or P within the rooting depth of plants
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