The concentration of phosphate ions in solution is critical in defining the ability of a soil to adequately meet crop demand. That concentration is regulated by adsorption/desorption and precipitation/dissolution processes, particularly its kinetics of reaction. This work was developed in order to find the kinetic model describing the sorption/desorption of phosphate in soils of the Argentinean Pampas region, to describe the processes associated with these reactions and to define the effect of temperature on them. A bi-linear model that adequately describes the data obtained in the adsorption and desorption experiments is proposed. Thus, it was possible to clearly differentiate two different kinetic mechanisms, each characterized by different reaction rates. Two very labile pools of phosphorus were determined in these soils. The rate at which these processes occur indicates the occurrence of chemical reactions of precipitation in highly soluble compounds and surface adsorption reactions with very low binding energy, as P is quickly released into the soil solution in the desorption process. However, since the amounts of P released in the desorption process are very much less than the adsorbed, is possible to infer that both processes occur at non-equilibrium conditions and there is a lack of reversibility of the reaction P-Soil – P-Solution in the soils studied. The intensity of these processes is differentially affected by temperature and, in general, there is a positive effect of phosphorus fertilization history, although the effect of this is not clearly seen in the amounts of extractable phosphorus with the classic Bray & Kurtz N°1 method.

Los objetivos de este trabajo fueron: i) determinar en qué medida las fuentes fosfatadas y sus mezclas con urea afectan la biodisponibilidad de P, ii) analizar los efectos de interacción urea/fuente fosfatada, y iii) analizar el efecto de la ureas obre el P nativo del suelo.El experimento se condujo en un Hapludol típico Serie Santa Isabel (pH 6,1; 29 g kg-1 de materia orgánica; 12,4 mg kg-1 de P extractable;P total, 446 mg kg-1; MBC 35,8 L kg-1). Se hicieron ensayos de fertilización de trigo con dos fuentes fosfatadas: fosfato monoamónico y fosfato diamónico endosis de 0, 9 y 17 kg P/ha, en mezclas físicas con urea en dosis de 0, 19 y 37 kgN/ha. Se concluyó que la aplicación de urea no causa efectos sobre los tenores de fósforo nativo en la zona de aplicación. Además, la disponibilidad del fósforo aplicado varía con la dosis y fuente fosfatada utilizada: la aplicación de fosfato monoamónico aumenta significativamente la biodisponibilidad del nutriente en este suelo, con respecto al fosfato diamónico. La utilización de mezclas de fertilizantes fosfatados, con bajas dosis de urea, aumenta la biodisponibilidad del nutriente para el trigo, con relación a las aplicaciones fosfatadas puras.

Se evaluó la relación entre los indicadores de disponibilidad de fósforo y la respuesta de los cultivos de trigo, maíz y soja a la inoculación con Penicillium bilaii. Se trabajó en distintos Molisoles con un diseño en bloques con parcelas completamente aleatorizadas y tres repeticiones para uno de los tratamientos: semillas inoculadas con P. bilaii y testigo sin inocular. Las parcelas de trigo y maíz se fertilizaron con dosis entre 8 y 16 kg P ha-1. Para cada tipo de suelo se estimó la energía de retención de fósforo (k de Langmuir), la capacidad buffer máxima (MBC), el diferencial de capacidad buffer (DCB), el índice de adsorción de fósforo (P-SI) y el índice de disponibilidad fosfórica (F-Index). Se evaluó la respuesta relativa en términos porcentuales del rendimiento máximo (RR%) realizando un análisis multivariado de componentes principales y coeficientes de correlación de Pearson. No hubo relación entre la respuesta y la dosis de fertilizante aplicada. La RR% de los cultivos de trigo y soja se asoció positivamente con los sitios de mayor k, mayor MBC y negativamente con FIndex. Para las condiciones de este ensayo, k, MBC y DCB se comportaron como indicadores sensibles de la disponibilidad de fósforo.

Retention of phosphorus in the soil solid phase is a complex process, caused by a combination of physical and chemical mechanisms that determine pools of compounds with different degrees of solubility. The amount of P available in a given pool can be associated with the adsorption maximum proposed by the Langmuir isotherm model, and with the energy with which it is retained. The aims of this work were: i) to evaluate the impact of the fertilization history and soil type on phosphorus adsorption parameters, and ii) to analyze the effect of soil properties, mainly the phosphorus adsorption parameters, on the yield of wheat, soybean and corn crops. In this study we established that the Bray & Kurtz 1 extractant is insufficiently sensitive for assessing changes in the nutrient availability when phosphorous concentrations are relatively low. The amount of phosphorus retained in the labile pool and its retention energy, however, are sensitive indicators of the availability of this nutrient. These indexes explain most of the variations in crop yields, and are determined more by P fertilization management than by soil type.

Under the hypothesis that the stratification of soil properties under directseeding conditions (SD) modifies P sorption parameters, the objectives of this work were: i) to estimate P sorption parameters by using Langmuir isotherms in samples of different depth, extracted from fields under continuous direct seeding; (ii) to analyze the similarities and differences between the parameters obtained; and (iii) to identify the relationship between the edaphic properties of the surface stratum and the estimated parameters. Soil samples of 0 to 5 and 0 to 20 cm were equilibrated with  solutions with increasing concentrations of P. From the relationship between the adsorbed P and the concentration of P in equilibrium, the maximum adsorption capacity (Qmax) and the retention affinity constant (k) were estimated. The higher Qmax and the low k of 0 to 5 cm samples show that there is a distinct behaviour in the first centimeters of depth. In that layer the bioavailability of P and the residuality of the applied fertilizers are regulated by adsorption phenomena responding to the  presence of organic colloids, which are accumulated by the stratification associated with SD.