Saturday, 15 July 2006

Models Estimating Fertilizer Requirements to Increase Available Soil Phosphorus.

Gerardo Rubio, Maria Julia Cabello, and Flavio Gutierrez Boem. Univ Buenos Aires Fac. Agronomia, Av. San Martin 4453, 1417 Buenos Aires, Argentina

The Pampean Region (Argentina) is one the most productive agricultural areas of the world. Nitrogen (N) and Phosphorus (P) are the main nutrients limiting productivity and application of NP fertilizers is a usual practice in the local farming. For the major crops, there are consistent local estimations of the soil phosphorus critical levels. It has been reported that critical levels range between 15 and 20 ppm extractable P (Bray I) for wheat and maize and between 9 to 13 ppm for soybeans. If available soil phosphorus is below the critical level, phosphorus fertilizers must be added to the soil to eliminate phosphorus deficiencies and obtain maximum yields. Information on the amount of fertilizer phosphorus needed to increase available soil phosphorus to reach a certain value is scarce in the Pampean Region. The objective of the research reported here was to develop models to estimate the amount of fertilizer required to increase the soil available phosphorus to reach the sufficiency levels in soils of the Pampean region. The experimental approach included 73 representative agricultural soils of the Pampean Region. Soil samples were taken from the upper level of the soil. Several simple soil properties related to the phosphorus dynamics in the soil were measured in each soil. Average initial available P was 12.8 mg kg-1, total C 22.3 mg kg-1, clay 21% and pH 6.05. After the addition of 5 doses of fertilizer P (0, 8, 16, 32 y 64 mg P per kg soil), experimental units (150 g soil placed in plastic containers) were incubated for 45 days to evaluate the increment in available phosphorus. Obtained results allowed us to calculate the parameter b (increase in available P per unit of P added to the soil). The coefficient was adjusted by a simple linear model y = bx, where y is the increment in available phosphorus after 45 days of incubation (difference in available phosphorus between each dose and the treatment without phosphorus addition), b is the slope, and x is the phosphorus dose. Average of b (all soils included) was 0.66, what means that around two thirds of the applied phosphorus appeared as available phosphorus 45 days after fertilization. Initial soil phosphorus was the main single factor regulating b (r2 0.3). Parameter b showed a direct relation with initial soil phosphorus what means that poor P soils had a lower increase in available P after the addition of fertilizer phosphorus than rich P soils. The two variable model that better fitted the data included the variable initial soil phosphorus and the variable (%silt+%clay)( r2 0.48). Obtained results can be used to determine the quantity of fertilizer needed to increase soil available phosphorus to the critical level. Parameter b represents the increase in available phosphorus after the addition of 1 mg P per kg of soil. To convert this value to a measure farmers are familiar with (i.e. kg phosphorus per hectare), it is necessary to estimate two variables, soil depth and bulk density. Soil depth must coincide with the depth used to estimate the phosphorus critical level. In most models of the Pampean region this value is 0-20 cm. In this region, soil bulk density usually ranges between 1.1 and 1.3 g cm3. For example, if soil depth= 0-20 cm and bulk density = 1.2 ton m-3, a dose of 1 mg P per kg soil (as in parameter b) is equivalent to 2.4 kg P ha-1. Since most of the widespread agricultural soils were included, obtained models can be used to define the amount of fertilizer P needed to reach the phosphorus critical levels in the Pampean region. In such sense, simple soil parameters can be used to predict fertilizers requirement.

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