The sociological study on farmers’ perceptions of drought in Goima (Slegers, 2006; Slegers, 2008b) revealed that farmers have site-specific knowledge about how soil characteristics influence crop performance during dry and wet conditions (cf. Schechambo et al., 1999).
Farmers believe that water adds fertility to the land; when there is no moisture in the soil, the land is “empty”. A common statement is: “If only it rains, we will harvest”. However, farmers know that a soil cannot give good production when it is infertile, no matter the amount of rainfall. This was clearly explained by some farmers who stated that in a wet year the major limiting factor is soil fertility, while the major limiting factor in a dry year is soil moisture.
The first big rainfall event of the 2005/06 season was very intense. Within a period of twelve hours, 120 mm of rainfall with a highest intensity of 41 mm hour-1 was recorded.
After this downpour, farmers with heavy soils were not yet able to cultivate their land, while farmers on the light soils had started working their land that same day. The big rain was followed by a four-week period of very little rainfall. Some fields needed re-sowing after big rains had resumed, while plants on other fields had survived this dry spell. Again, on other fields, it was just the start of the season, like on our Dark soil.
Farmers mentioned that heavy soils contain more clay and organic matter than light soils, and are generally considered as most fertile, which also appeared from the texture and chemical analyses. The QUEFTS and kPNE data showed that the Dark soil not only had the highest nutrient contents, but also that on these soils N, P and K were in the most balanced situation. The crops grown on this soil least suffer from nutrient deficiency. Farmers also mentioned that the Dark soil was the most fertile soil.
The analyses showed that all three soil types are sandy soils and that the Dark soil had highest clay, silt and organic C contents and water holding capacity. Farmers considered crops on the Dark soil and on heavy soils in general, to be most susceptible to drought because such soil is hard when it is dry. Farmers felt that this limits water infiltration early in the season. A farmer who cultivates a heavy soil generally cannot start cultivation immediately after the first big rain. Their land needs a lot of water before it is workable, which reduces the length of the cropping season on these soils. This can be problematic in years when the rains stop early. Tillage with animal traction or tractor were options mentioned by farmers to overcome these constraints for the Dark soil as it speeds up the work and allows water to infiltrate easier and deeper compared to hand hoe tillage. The data
showed that the Dark soil had the highest potential and actual moisture availability compared to the other soils and that soil moisture after the first big rainfall event was sufficient to start sowing, contrary to farmers’ practices. The effect of late sowing was also visible (Figure 6.1) as moisture had reached PWP before maturation.
The Pale soil was described as sandy and infertile and, therefore, unsuitable for growing maize. Maize plants would show stunted growth, even when rainfall is not limiting plant growth. However, farmers consider crops grown on Pale soils to be the least susceptible in a dry year. Water infiltrates easily and deeply into the sandy or light soils, which makes it very easy for farmers to start cultivation immediately after the first big rain.
It is important for farmers not to miss any rain because rainfall is erratic. The analyses of the Pale soil samples showed that PWP was lowest, which implies that the Pale soil requires the least amount of rainfall before farmers can start cultivation. Barron et al. (2003) found that crops on light-coloured sandy soils least suffered during dry spell periods compared to soils that were more rich in clay. Chemical characteristics as well as crop performance were not as poor as would be expected from farmers’ description of the soil, which later appeared to be the effect of a kraal that used to be located on part of that land. This can explain the high levels of P and K.
Farmers in general judged the Red soil’s susceptibility to drought to lie in between that of the Pale and the Dark soils, depending on whether the soil is heavy or light. The darker red the soil, the heavier it is. Heavy Red soils mostly resemble the Dark soils’
characteristics, while light Red soils mostly resemble the Pale soils in terms of their characteristics. The Red soil used in our study was characterized by farmers as a light soil, exhausted due to continuous cultivation, sheet erosion and sand depositions. The owner of the land had predicted that maize on his land will not perform well; it stunts before the plant starts to develop, resulting in poor cob formation. Indeed the data showed that the Red soil was poorest in terms of nutrient status, had a compacted topsoil, and the highest evaporation (E) and loss (L), resulting in the lowest transpiration (T).
6.4.2. The effects of land management practices in good and bad years
Farmers not only identified natural factors that create spatial differences in crop performance in dry conditions, they also identified anthropogenic factors. Soil fertility is not an intrinsic permanent characteristic of a soil; it changes over time. After a consecutive number of years of cultivation the soil gets exhausted. According to the farmers the soil becomes “tired” and loses its productivity and capacity to retain water. Farmers also recognized the effects of soil erosion on the land. Farmers believe that the soil layer up to a depth of 30 cm is good soil as it has been enriched by decaying grass, leaves and crop residues. Analyses of soil samples showed that the topsoil layers had higher nutrient levels than the subsoil layers.
Soil erosion was generally known as a process whereby the fertile topsoil is washed away and deposited in other places. Eroded areas are less productive and crops on these soils
more easily suffer from water stress. Farmers complained about the loss of fertile topsoil by runoff water, especially on sloping land and land where livestock and people pass regularly.
Farmers agreed that sedimentation can have both positive and negative effects. Where sand is deposited, the land loses productivity, and where fertile soil particles are deposited the land gains productivity. According to the farmers, the Dark soil in our study showed the beneficial result of soil deposition, whereas our Red soil was exhausted and suffered from sheet erosion and sand depositions, both of which are visible in the soil profiles.
Soil fertility can be enhanced when fertilizers or (green) manure is applied, or when land is fallowed. Increased pressure on the land has limited fallow practices and is, according to the farmers, exhausting the land. Chemical fertilizers are not easily available and are too expensive for most farmers. Only one-third of the farmers own livestock.
Transportation and application of manure require transportation means, labour and/or capital, which many farmers lack. In the 2004-05 crop season, less than 10 % of questioned farmers (N=120) had applied manure. Some farmers have experienced that crops ‘burn’ on fields where manure had been applied in bad rainfall years and are reluctant to apply manure, while others felt that their land was still productive.
Ridge cultivation is a common practice, particularly since most farmers work their land with hand hoe. Farmers agreed that ridges prevent the soil from washing away and help to retain water in the depressions, which makes plants to grow stronger and faster. Farmers felt that fewer weeds come up on the ridges, which makes their job of weeding easier. Some farmers believed that the ridges increased the fertility of the soil because it increases the depth of the fertile topsoil layer; however, most farmers believed that the ridges only had beneficial effects on the moisture availability in the soil. The chemical soil data (Table 6.4) showed a nutrient accumulation effect in the ridges, whereas there was no significant difference in the available moisture availability. AM was generally sufficient throughout the season, the difference between plots with and without ridges might, however, show significant differences during an extended dry spell period.
6.4.3. Estimated yields
Farmers’ yield estimates showed low figures. Farmers indeed found it difficult to estimate yield due to the small sizes of the sample plots, which might have resulted in these low figures. Yields estimates through dry matter production ranked the soils in the same order as the QUEFTS calculated yields and with the average yield estimates of the farmers. Average yield estimates were highest for the Dark soil and lowest for the Red soil. However yield estimates of farmers showed a large absolute difference compared to the other estimates, particularly to the QUEFTS calculated yields. QUEFTS takes no other limitations into account for yield production besides nutrient deficiency. Even though no extreme moisture deficiency was visible during the 2005/06 cropping season, Figure 6.1 does show AM close to zero, which is not always readily available. The yield estimates based on the ∑DM accumulation is also rather high. The 40% mass fraction for some plots on the Red and Pale soil was too high because of poor cob formation and growth. Since there was no moisture
stress throughout the cropping season, with the exception of a short period early in the season, the differences in LAI and yield must be ascribed to differences in nutrient availability.
The QUEFTS analyses confirmed outcomes of previous studies in Tanzania that soils are mostly N deficient (Table 6.9), and that P is the one but most deficient nutrient (Vesterager et al., 2008). In general terms, it can be stated that the nutrient levels and the productivity of the different soils in Goima were quite good compared to FAO’s maize production estimates of 905 kg ha-1 (Kaihura et al., 2001).