A system for quantitative evaluation of the fertility of tropical soils (QUEFTS)

A system is described for a quantitative evaluation of the native fertility of tropical soils, using calculated yields of unfertilized maize as a yardstick. The system is applicable to well drained, deep soils, that have a pH(H20) in the range 4.5–7.0, and values for organic carbon, P-Olsen and exchangeable potassium below 70 g/kg, 30 mg/kg and 30 mmol/kg, respectively (0–20 cm). Soil fertility is interpreted as the capacity of a soil to provide plants with nitrogen, phosphorus and potassium, but the methodology allows for including other nutrients. The procedure consists of four successive steps. First the potential supplies of nitrogen, phosphorus and potassium are calculated, applying relationships between chemical properties of the 0–20 cm soil layer and the maximum quantity of those nutrients that can be taken up by maize, if no other nutrients and no other growth factors are yield-limiting. In the second step the actual uptake of each nutrient is calculated as a function of the potential supply of that nutrient, taking into account the potential supplies of the other two nutrients. Step 3 comprises the establishment of three yield ranges, as depending on the actual uptakes of nitrogen, phosphorus, and potassium, respectively. Next, these yield ranges are combined in pairs, and the yields estimated for pairs of nutrients are averaged to obtain an ultimate yield estimate (Step 4). The relationships used in Steps 1 and 3 were derived from empirical data of field trials in Suriname and in two strongly different agro-ecological zones in Kenya. The equations developed for Steps 2 and 4 were mainly based on theoretical considerations. The equations used to calculate the potential supplies of nutrients (Step 1) should be applied only to soils with the indicated properties. The other equations are more generally applicable. Examples are given to elucidate the procedure. QUEFTS may be a very useful tool in quantitative land evaluation. Its principles may be applied to other crops, soils, nutrients and agro-ecological regions than those involved in this study.