Goro Uehara

Nutrient mobility in variable- and permanent-charge soils

Variable-charge (v-c) and permanent-charge (p-c) soils differ fundamentally with regard to many nutrient-cycling processes. Variable-charge soils are more common in the tropics than in temperature zones because their formation requires desilication, which proceeds fastest in warm, moist climates. The dynamics of nutrient mobility tend to be more complex in v-c than in p-c soils. For example, theory predicts that, as pH of v-c soils decreases, cation exchange capacity (CEC) also decreases and anion exchange capacity (AEC) increases. If AEC exceeds CEC, cations such as ammonium and potassium will be more mobile than anions such as nitrate; this is the reverse of the situation in p-c soils, on which most of our knowledge of nutrient cycling is based. Variable-charge surfaces sorb phosphorus, creating plant nutritional problems throughout large areas of the humid tropics. Desilication, the same process that creates v-c surfaces, results also in stable aggregation, creating soils that retain water, yet drain rapidly and resist erosion. The Soil Taxonomy system incorporates information on mineralogy, texture, and organic matter content, and therefore provides insights into patterns of charge chemistry and nutrient cycling across a wide range of soil types.

Charcoal ash and volatile matter effects on soil properties and plant growth in an acid ultisol.

There is a growing interest in converting organic wastes to charcoal for use as a sustainable soil amendment with a potential to improve soil productivity and sequester C. Three consecutive greenhouse experiments were conducted to investigate the effects of charcoal with different ash and volatile matter (VM) contents on soil properties and maize (Zea mays) growth and to evaluate the effect of time on charcoal performance. Five charcoal amendments (high-VM corncob, low-VM corncob, Kiawe, Binchotan, and a gasification charcoal of Leucaena leucocephala) applied at a 2.5% (wt/wt) rate were compared with a zero-charcoal control with and without fertilization. Only the gasification charcoal significantly increased maize growth without fertilization. The low-VM corncob charcoal with fertilization significantly increased maize growth by 164% compared with the fertilized control in the first planting cycle. Maize growth in the high-VM corncob charcoal supplemented with fertilizer treatment was significantly lower than that of the fertilizer-alone treatment in the first planting cycle. The negative effect of the high-VM charcoal on the fertilizer was caused by bioavailable carbon in the charcoal, which increased soil microbial activity and could have caused N immobilization. Both the beneficial and detrimental effects of charcoal did not persist beyond the first planting cycle, suggesting that charcoal impacts are temporary. Whereas charcoal ash and VM content seem to be important parameters for predicting charcoal behavior in the short-term, more research is needed to examine a broader spectrum of feedstocks exposed to varying thermal treatments.

Technology Transfer in the Tropics

It is generally acknowledged that the lot of resource-poor farmers in underdeveloped countries can be improved only by better technology transfer. There is, however, less agreement on how this transfer is to be effected under field conditions. This article reviews a sophisticated modelling system which provides an agricultural decision support system which has the all-important merit of allowing the individual farmer to exercise a choice.