Soil moisture days and crop yield

In terms of crop production, the most important variable that requires monitoring is soil moisture, which is determined by the balance of water in the soil between rainfall and evaporation.50 We use the example of experiments that measured relationships between good soil moisture days and crop yields at two sites in Kenya51 (one in a low rainfall area and the other in a higher rainfall zone), conducted over a three-year period. The site with less soil moisture days produced fewer crops than the site that sustained higher soil moisture days that produced more crops over the three years.52 This finding suggests that rather than relating total rainfall to crop yields, it would be more effective to relate soil moisture days to crop yields.

In another study, conducted in southern Tanganyika involving nine trial plots distributed across different agro-ecological zones, forage production was greatly influenced by rainfall and soil moisture reserves, as opposed to grazing practices.5’ Soil moisture days of between 100 and 120 days per annum have been shown to produce significant amounts of crops54—the challenge remains the scarcity of research on soil moisture field capacity.5’ Soil moisture days are independent of land-use intensifications (see scenario В in Figure 5.1), but they are associated with soil fertility,56 which we discuss in the following subsection.

Effects of land use on soil fertility

An experiment in Tanganyika in 1954 investigated the efficacy of inorganic and organic fertilizers on crop yields over a period of four years. It assessed the effects of residual phosphate on soya, groundnuts, sorghum and maize production (Table 5.1 and Figure 5.4). Over the four years, the yields by different crops showed fluctuating responses. Among the four crops, maize and groundnuts showed superior responses to applications of phosphates during some of the years, whilst soya beans showed the least response. Evans37 pointed out that the crops were grown on two different types of soils—however, this failed to be reported. G.H. Gethin Jones38 reported that there are no absolute scales of soil fertility; instead, soil fertility is relative to the types of crops under given climatic conditions. This is contrary to the perceived universal decline in soil fertility according to environmental crisis hypothesis (see scenario D in Figure 5.1).

Effects of phosphate fertilizer on two grain crops (maize and millet) and two legume crops (groundnuts and soya beans) over four years

Figure 5.4 Effects of phosphate fertilizer on two grain crops (maize and millet) and two legume crops (groundnuts and soya beans) over four years.

The indigenous African farmers augmented soil fertility by making use of fallow periods and rotational cropping. Organic fertilizers were more accessible to the peasant farmers than costly inorganic fertilizers.59 They applied cattle manure regularly in order to sustain crop yields.40 Such responses are likely to be represented by scenario C in Figure 5.1. However, it is the inorganic fertilizers that were popularized but their disad- vantages were often not explained to farmers. First, inorganic fertilizers have optimum levels above which soil nutrients will be promoted no further. In fact, repeated applications over longer periods might harm the crops. Second, legumes for example, do not require heavy use of fertilizers. Therefore, contrary to the environmental crisis hypothesis, inorganic fertilizers are of limited use in promoting soil nutrients. We now compare these outcomes with a second series of experiments.

In a Tanganyika study between 1955 and 1960, R.C. Grimes and R.T. Clarke41 investigated six fertilizer application treatments (see Table 5.1) comprising mixtures of super-phosphates and cattle manure. Cattle manure promoted better grass yields than inorganic fertilizers.42 The combinations with super-phosphate produced the highest yield in 1956 across the six treatments (Figure 5.5). Conversely, the residual effects of the farm manure—the first crops grown were preceded by a year of applications, and the second were planted two years after the manure application—showed that the soil fertility had declining tendencies after a peak (representing scenario C in Figure 5.1).

The findings did not disclose why soil fertility is generally low. In this regard, Sir Bernard Keen, Director of the East African Agriculture and Forestry Organization (EAAFRO) suggested in his annual address in 1954, that

Responses of maize grain in cwt/acre (lcwt = 0.05 ton UK) to fertilizer applications

Figure 5.5 Responses of maize grain in cwt/acre (lcwt = 0.05 ton UK) to fertilizer applications.

the low fertility of tropical soils could not be attributed to indigenous land use. The low soil fertility according to him was ‘simply because the higher average temperatures cause a rapid oxidation of vegetation organic matter.’ He advised the authorities and researchers that they should not attempt to ‘force tropical agriculture into a system that is alien to it, but to develop one that fits the environmental factors.’43 Unfortunately, colonial officials did not acknowledge this fact, and continued to attribute the loss of soil fertility to African systems of land use.

Overall, none of the agronomic experiments that we reviewed fully verify the equilibrium model (that is, the implied environmental crisis hypothesis). There was always the possibility of alternative explanations of environmental degradation. Since the experiments were not conducted on African farms, any conclusions that relate the outcomes to land use by African peasants would be invalid. Unlike the agronomic experiments, all the range science experiments were conducted on grazing lands used by the African herders.

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