Local knowledge and natural science

There is another avenue, which reverses the foregoing critique and endorses soil science. While I can give an account of Wola horticultural practices (Sillitoe 2010: 253—329) and report their comments on soils and their behaviour under cultivation — about ‘grease’ levels, and so on, so far as 1 apprehend them — the question remains: what is it about the soils and crops of this region that allows the anomalous cultivation regime to exist? No systematic answer is forthcoming locally. People follow practices that impact on the environment, without articulating a theory of why, which is understandable given the pragmatic character of their soil knowledge, it not being codified but diffuse and communicated piecemeal, even disjointedly from our perspective.

The theory and concepts of natural science allow us to address the question. The implication is not that we can translate Wola conceptions about the environment into scientific discourse. It is entirely foreign to them, with no ideas equivalent to nutrient ions, gas exchange, physical functions, and so on. Nor is the aim to assess the veracity of local ideas against scientific ones, both are relative. It is to further our understanding of environmental interactions within the cultural context. Although taken up with different issues expressed in quite different idioms, both concern the same natural environment ‘out there’ and together can further our understanding of it, people’s place within it and the impact of their activities on it. (For similar attempts, which consider both Indigenous and scientific knowledge of soils in African and MesoAmerican cultural contexts, see Krogh and Paarup-Laursen 1997; Ericksen and Ardon 2003; and Gray and Morant 2003.) Furthermore, combining natural science with ethnographic enquiry responds to post-modern criticism, distancing such work from any pretense about achieving an understanding of a foreign population as it understands itself.

It is necessary to know something about the changes that occur in the nutrient status of soils under cultivation for varying periods of time to understand how the Wola can cultivate sites semi-permanently with no soil amendments. Briefly, the properties of the region’s soils (see Table 7.2)⁸ that limit crop nutrition and production are: (1) low levels of phosphorus availability and high rates of phosphate fixation (reflecting the strong immobilising capacity of volcanic ash-derived minerals; Parfit and Mavo 1975);'’ (2); sub-optimal pH, these acid conditions interfering with the supply of some nutrients, notably reducing total base saturation; (3); depressed cation exchange capacities and lowered availability of exchangeable cations, which is particularly problematic with potassium; and (4); low levels of available nitrogen, which is probable with the high organic matter contents (as high C:N ratios indicate).The physical properties of the soils are, by contrast, generally favourable to crop production, with their high organic matter contents, low bulk densities, and good topsoil aeration and drainage.

The following soil-related processes occur with the establishment of a garden. The burning of cleared vegetation returns nutrient elements to the soil (except for that fraction lost as gas to the atmosphere) via ash that is rapidly broken down further for plant uptake, as documented for swidden regimes elsewhere. It increases pH and gives a critical, though short-lived boost to the availability of several elements. The boost is particularly significant for phosphorus, and also potassium and nitrogen, three major plant nutrients. The increased availability of these limiting nutrients is sufficient to allow the cultivation of a wide variety of crops (Figure 7.8), several of them annuals, including a range of beans (e.g. Lablab niger, Phaseolus vulgaris), green leafy vegetables (e.g. Rorippa sp., Dicliptera papuana), aroids (e.g. Colocasia esculenta, Xanthosoma sagittifolium) and cucurbits (Lagenaria siceraria, Cucumis sativus), plus some longer-term crops such as bananas (Musa sp.) and sugar cane (Saccharum officinarum) (Sillitoe 1983: 29—136). But the increase is short-lived (Figure 7.9).There is a decrease in the variety of crops cultivated after two or three plantings, reflecting a change in soil nutrient status, with nutrient availabilities

Table 7.2 Measures of topsoil chemical fertility compared with site land use status

Notes: values in brackets = standard deviations; n = 110 sites; degrees of freedom =10, 99.

Soil ethnoecology

Figure 7.8 New garden with taro, greens, sugar cane and beans (trained up poles)

Figure 7.9 Mature garden of sweet potato with bananas and some sugar canefalling to pre-burn levels due to soil processes largely, with some removed in harvested crops. Organic matter and nitrogen decline significantly with time under cultivation together with potassium, but it is the latter, together with phosphate (the availability of which is relatively low throughout) that fall to levels below those required by many crops (other nutrients show no significant variation).¹" They decline to new equilibrium points that remain relatively constant, even after years under cultivation.

After the first couple of cropping cycles, a markedly narrower range of crops occurs, with many gardens passing under a virtual monocrop of sweet potato, perhaps with a few longer-term crops, and the occasional patch of pumpkin (Cucurbita maxima), edible pitpit (Setaria palmifolia) and acanth greens (Rungia klossii).The sweet potato (Bourke 1985) occupies a central place in this farming system. It is the staple crop, comprising about 75 per cent of all food consumed by weight (Sillitoe 1983: 239), and makes up by far the largest area under crops. It has the capacity to continue producing tolerable yields under these nutritionally constrained conditions with its relatively low phosphorus requirements and preference for fairly high potassium to nitrogen ratio.¹¹ Contrary to expectations, these changes in soil fertility do not necessarily lead to reduced staple sweet potato yields. Farmers maintain that the soil on some sites improves with use, becoming better for tuber production with time. Measurement of crop yields confirms their assertions, the harvest of tubers from newly cleared garden areas being some 40 per cent below that from established ones. Far from experiencing a decline in yields, as the accepted model of low-input subsistence agriculture predicts, the reverse occurs on some sites and they increase under cultivation.