5. Discussion
5.1. Soil colour as quality indicator
Although farmers described their own soils as sandy and clayey, they mainly used soil colour as a criterion for soil quality. They related soil colour to soil fertility (organic matter) and consequently to quality. The use of colour as a major descriptor of soils has also been reported by Saito et al. [38] for different ethnic groups in northern Laos. Rice farmers in Laos preferred black soils over red, white and yellow soils, and preferred clayey and loamy soils over sandy and stony soils. Darker soils were considered to contain high levels of organic matter, to have a high water-holding capacity, to be inhabited by earthworms and to produce high rice yields. Such soils were commonly considered more fertile than soils of other colours. This was also the case in our study: black soil (terra preta) was considered the most fertile because of its high clay and organic matter content, whereas white soil (terra branca) was considered the least fertile because of its low clay and organic matter content. A comparison of these local farmers’ responses with the formal values presented in Table 2 supports farmers’ understanding of soil quality. Similar findings [3], [7], [12], [14] and [17] show that soil colour is the most widely used indicator for classifying soils in other parts of the world and for different crops as well.