Phosphorus Cycles in Terrestrial and Aquatic Ecosystems

The objective of this study is to develop a model that predicts the optimum phosphorus (P) fertilizer rate for soybeans in the Cerrado region of Brazil. The model combines extractable soil P, clay content, and other selected soil properties to determine the fertilizer rate that maximizes net income. The study used seven soils from five long-term study sites, which were initially very low in extractable P, ranging from 0.6 to 6.0 g m-3 among the four extractants. The soil with the least clay content (12%) had the highest native extractable P level, which was expected due to its characteristics of low P adsorption maximum and low surface area. The yield response per unit of soil P extracted with M1P, M3P, and B1P was greater with increasing clay content, indicating that a selected yield, such as 90% of the maximum, would be reached at a much lower soil P level as clay content increases. The immediate residual effect of P fertilization estimated as about 15% immediate recovery for the soil with only 12% clay, increasing to over 50% recovery for the soils with around 60% clay. However, when Mehlich-1 extractable P was used, a quadratic relationship between applied P and soil P recovered just after application and rototilling was observed, whereas this relationship was linear when the other soil P extractants were used. The proposed model was validated using data from greenhouse experiments, and the results showed that the model was able to predict the fertilizer rate that maximizes net income for different given soil test levels and other selected soil properties. The model was also able to consider economic considerations, such as fertilizer cost and crop income, to determine the optimal fertilizer rate. The study concluded that the proposed model can be used to predict the optimum P fertilizer rate for soybeans in the Cerrado region of Brazil, taking into account the effects of clay content and other selected soil properties on phosphorus cycling. The model can be applied to other regions with similar soil properties and can be used to optimize fertilizer application and maximize net income. The mathematical approach used to generate the model is based on a descriptive model developed by Cox et al. (1981), which expresses extractable P level (X) following fertilization (F) with time (T). The model was modified by Lins et al. (1985) to allow for other initial soil P concentrations. The model was further modified to include economic considerations, such as fertilizer cost and crop income, to determine the optimal fertilizer rate. The model was validated using data from greenhouse experiments, and the results showed that the model was able to predict the fertilizer rate that maximizes net income for different given soil test levels and other selected soil properties. The model was also able to consider the effects of clay content and other selected soil properties on phosphorus cycling. The study highlights the importance of considering the effects of clay content and other selected soil properties on phosphorus cycling when developing fertilizer application recommendations. The proposed model can be used to optimize fertilizer application and maximize net income in the Cerrado region of Brazil and can be applied to other regions with similar soil properties.