The topics covered include: the effect of cassava production on soil fertility (i.e. nutrient absorption, distribution and removal in harvested products; nutrient losses by runoff and erosion; long-term effect of cassava production on soil productivity); diagnosis of nutritional disorders (i.e. observation of deficiency and toxicity symptoms; soil analysis; plant tissue analysis; greenhouse and field experiments); correcting nutritional disorders (chemical fertilizers; N; P; K; Ca and Mg; S; micronutrients; soil amendments; acid soils; saline-alkaline soils; animal manures and compost; green manures, cover crops and mulch; inoculation with mycorrhizas).

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Summary Greenhouse and field experiments were conducted on the effect of VA mycorrhiza (V AM) on the growth of cassava, various tropical grass and legume speCies, as well as beans, coffee and tea. A large number of V AM fungal species were evaluated for effectivity in increasing cassava growth and P uptake in acid low-P soils. The effectivity of VAM species and isolates was highly variable and dependent on soil pH and fertilizer applications, as well as on soil temperature and humidity. Two species, Glomus manihotis and Entrophospora colombiana were found to be most effective for a range of crops and pastures, at low pH and at a wide range of N, P, and K levels.At very low P levels nearly all crops and pasture species were highly mycorrhizal dependent, but at higher soil P levels cassava and several pasture legumes were more dependent than grass species.Mycorrhizal inoculation significantly increased cassava and bean yields in those soils with low or ineffective indigenous mycorrhizal populations. In these soils cassava root yields increased on the average 20-25% by V AM inoculation, both at the experiment station and in farmers' fields. V AM inoculation of various pasture legumes and grasses, in combination with rock phosphate applications, increased their early growth and establishment. Agronomic practices such as fertilization, crop rotations, intercropping and pesticide applications were found to affect both the total V AM population as well as its species composition.While there is no doubt about the importance of VA mycorrhizain enhancing P uptake and growth of many tropical crops and pastures grown on low-P soils, much more research is required to elucidate the complicated soil-plant-V AM interactions and to increase yields through improved mycorrhizal efficiency.

Soil conditions associated with an oranging disease which occurs in flooded rice (Oryza sativa) grown on the Oxisols of the Llanos Orientales of Colombia were studied by measuring changes in pH, Eh, and Fe and Mn levels in soil solutions after flooding. It was found that rates of reduction and the build up of Fe in the soil solution vary greatly for different Llanos soils, between virgin and previously‐cropped soils, and between air‐dried and field‐moist soils. Levels of Fe in solution could be reduced by various water management treatments and by the addition of large amounts of lime or MnO2. It was observed that oranging symptoms occur as solution Fe concentration increases and that large plants were more seriously affected than small plants. Roots of affected plants were heavily coated with oxidized Fe and few active white roots formed after the increase in Fe concentration. The disease appears to be due to indirect Fe toxicity, i.e., an Fe‐induced deficiency of mainly P, K, Ca, and Mg. Due to the relatively high level of Fe in solution, the roots become coated with iron‐oxide reducing the root's capacity to absorb enough plant nutrients from an already deficient environment.Under certain conditions plants may also suffer from direct Fe‐toxicity, i.e., excessive Fe absorption by the plant, resulting in bronzing symptoms. A combination of direct and indirect Fe‐toxicity may produce plants with a wide range of intensities of oranging symptoms and brown streaks on the leaves.

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