Evaluation of phosphorus uptake from Minjingu Phosphate rock, growth and nodulation of agroforestry tree species in an acid soil from Kenya. In: Assessment of soil phosphorus status and management of phosphatic fertilizer
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Most acid soils of the tropics and subtropics show a widespread phosphorus (P) deficiency and high P sorption capacity, and therefore require substantial P inputs for optimum plant growth and production of food and fiber. Manufactured water-soluble P fertilizers such as superphosphates are commonly recommended to correct P deficiency, but most developing countries import these fertilizers, which are often in limited supply for resource-poor farmers. In addition, sustainable intensification of agricultural production in these regions necessitates the addition of P inputs not only to increase crop production but also to improve soil P status to avoid further soil degradation. Therefore, it is imperative to explore alternative P inputs. In this context, the use of phosphate rocks is particularly attractive under some conditions to develop an effective and economic phosphate management program. Geological deposits of phosphate-bearing minerals are distributed worldwide. Phosphate rock (PR) deposits have been found in many developing countries of Asia, Africa and Latin America. However, few large deposits are commercially mined for use as raw materials for manufacturing P fertilizers. The use of locally available resources is an important factor in the development of a sustainable agriculture. Their use for direct application as a finely-ground product, though recommended, is not practiced in these regions due to the lack of adequate information on the reactivity of the PRs (potential for direct application) as well as their agronomic effectiveness (P supply to soils and crops). It is well known that the PRs show different reactivity as a result of their extremely variable chemical and mineralogical composition. In addition to P, they also contain a wide range of chemical elements, some of which are beneficial (nutrient supply), while others have long term harmful effects. The agronomic effectiveness (capacity of P supply to crops) of PRs depends not only on inherent factors, but also on the soil conditions and plants/crop genotypes utilized. In terms of improving the soil P status, they have both immediate and residual effects, which can be better measured in a crop rotation or within a cropping systems context. To optimize their utilization as alternative P sources, it is necessary to develop and utilize new technologies to enhance the agronomic effectiveness of less reactive PRs. In some cases, this is done through chemical processing such as partial acidulation and the preparation of compacted granulated formulations of mixtures of PR and superphosphate. There is also the potential to utilize biological approaches such as selected plant genotypes, which possess better natural or improved mechanisms in the rhizosphere for P acquisition from PRs; incorporation of organic residues and agro-wastes together with PRs; inoculation with improved micro-organisms, etc. In addition to conventional techniques, the radioisotope 32P was employed as a tracer in several studies at different stages of the project to obtain quantitative and precise information on the dynamics of P in the soil when amended with PRs and water-soluble P fertilizers (32P isotopic exchange kinetics) and the value of management practices employed to enhance the agronomic effectiveness of PRs in greenhouse and field studies (32 P isotope dilution techniques such as L and A values). The utilization of these techniques required trained technical staff with skills and expertise and laboratory facilities for handling and measuring the radioisotope. The required support in terms of research, training and analytical services was provided by the FAO/IAEA Agriculture and Biotechnology Laboratory, Seibersdorf, Austria, and the “Centre d’Etudes Nucleaires”, Cadarache, France.