Continuous crop expansion has led to a growing demand for phosphate fertilizers. A sound knowledge of the dynamics of phosphorus, and its interaction with iron oxides and organic matter, can be useful to develop effective strategies for sustainable management, especially in a scenario of increasing shortage of mineral phosphate resources. In this paper, we review the relationship of phosphate to iron oxides and organic matter, and its effect on phosphorus availability. Crops typically obtain phosphate from weathered minerals and dissolved fertilizers. However, the amount of phosphorus present in the soil solution depends on the extent to which it is adsorbed or desorbed by iron oxides, which may be influenced by interactions with organic matter. Therefore, systems for fertilizer recomendation based on methodologies considering interactions between soil components such as oxides and organic matter, and the phosphorus sorption capacity resulting from such interactions (e.g. residual P analysis), may be more reliable to ensure efficient, rational use of phosphate. A contínua expansão da produção agrícola tem levado a uma crescente demanda de fertilizantes fosfatados. O conhecimento da dinâmica do fósforo no solo e suas interações com óxidos de ferro e matéria orgânica podem ser uteis no desenvolvimento de estratégias eficientes para o manejo sustentável, especialmente em um cenário de crescente escassez de fontes de minerais fosfatados. Nesta revisão bibliográfica foi abordado a relação do fósforo com óxidos de ferro e matéria orgânica, e seu efeito na disponibilidade de fósforo. As culturas, normalmente, obtém fosfato de minerais intemperizados ou fertilizantes dissolvidos. No entanto, a quantidade de fósforo presente na solução do solo depende das reações de adsorção e dessorção por óxidos de ferro, as quais podem ser influenciadas por interações com a matéria orgânica. Portanto, os sistemas de recomendação de fertilizantes com base em metodologias que consideram as interações entre componentes do solo, tais como óxidos e matéria orgânica, e a capacidade de adsorção de fósforo, resultantes de tais interações (por exemplo, análise de P remanescente), pode ser mais confiável para garantir o uso eficiente e racional de fertilizantes fosfatados.

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ABSTRACT. The low phosphorus availability in tropical and subtropical soils, normally related to adsorption of phosphate to the minerals surfaces, can be attenuated when organic matter (OM) accumulates in the soils. Herein, we report the results of long-term experiments (18-32 years) aimed at quantifying the maximum phosphorus adsorption capacity (MPAC) and its determinant mineralogical variables in Brazilian soils and at assessing the effect of no-tillage (NT) in mitigating the phosphorus adsorption of soils. The MPAC of soils ranged from 297 to 4,561 mg kg -1 in the 0.00-0.10 m layer and from 285 to 4,961 mg kg -1 in the 0.10-0.20 m layer. The MPAC was correlated with the concentrations of iron oxides, goethite and ferrihydrite, gibbsite/(gibbsite+kaolinite) ratio and the specific surface area. The OM increased in the 0.00-0.10 m layer of NT soils, which was not reflected on the decrease of MPAC for the no-tillage soils.Keywords: iron oxides, gibbsite, organic carbon, phosphate.Mineralogia e adsorção de fósforo em solos sob preparo convencional e plantio direto nas regiões sul e centro-oeste do Brasil RESUMO. A baixa disponibilidade de fósforo em solos tropicais e subtropicais, normalmente relacionada à adsorção de fosfato pelas superfícies minerais, pode ser atenuada quando matéria orgânica (MO) é acumulada nos solos. Em experimentos de longa duração (18-32 anos) em solos brasileiros, o estudo objetivou quantificar a capacidade máxima de adsorção de fósforo (CMAP) e as variáveis mineralógicas determinantes, bem como verificar o efeito do plantio direto em mitigar a adsorção de fósforo nos solos. A CMAP dos solos variou de 297 a 4.561 mg kg -1 na camada 0,00-0,10 m e de 285 a 4.961 mg kg -1 na camada 0,10-0,20 m. A CMAP apresentou relação com a concentração de óxidos de ferro, de goethita e de ferrihydrita, com a razão gibbsita/(gibbsita+caulinita) e com a área superficial específica do solo. O conteúdo de MO aumentou na camada 0,00-0,10 m dos solos em plantio direto e não afetou a CMAP em comparação aos solos em sistema de preparo convencional.Palavras-chave: óxidos de ferro, gibbsita, carbono orgânico, fosfato.

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Replacing conventional tillage (CT) with no-tillage (NT) management alters the pedoenvironment and the rate of topsoil processes, with possible effects on dissolution processes associated with iron oxides and therefore soil mineralogy. This study aimed to determine the effect of NT on the content and distribution of types of iron oxides in a Rhodic Paleudult in southern Brazil. Soil samples were collected at eight depths within the 0.00-0.80 m layer under CT and NT in a long-term experiment (21 years). Mineralogical identification was conducted by X-ray diffraction (XRD), and the Fe content related to specific types of iron oxides determined by selective dissolution and diffuse-reflectance spectroscopy. Kaolinite, quartz, goethite, hematite, and maghemite were identified in the clay fraction. In the NT-managed soil, there was a decrease in the content of crystalline iron oxides and an increase in the content of poorly crystalline iron oxides with increasing proximity to the soil surface. These results suggest that iron oxides are rearranged in this soil by reductive dissolution of the crystalline types and neoformation of metastable ferrihydrite in topsoil layers, which should be assessed further in laboratory studies.

Soil mineralogy and management can have substantial effects on phosphorus (P) diffusion in soil and hence on root morphology. The aim of this study was to assess these effects in soils differing widely in P-sorbing components. Samples of a Rhodic Paleudult (RP), a Rhodic Hapludox (RH), and a Humic Hapludox (HH) under conventional tillage (CT) or no-tillage (NT) were placed in Petri dishes to assess P diffusion. Performance and root morphology in corn grown on soil samples subjected to three different P fertilization treatments (no fertilizer, surface, and deep fertilizer application) were examined. Phosphorus diffusion was slower in the HH than in the other soils by effect of its high P adsorption capacity because of its mineralogy. Soil management had no effect on P diffusion. Fertilization with P only affected plant-related variables in RH: surface fertilization increased root biomass, and deep fertilization shoot biomass and proportion of thick roots. Soil management affected plants growing on HH and RP: the plants grown under NT were taller (RP) and developed more fine roots (HH) than those under CT. Fertilization with P had little effect on plant growth in the goethite-rich soil (HH) or the one with high content in available P (RP). Our results suggest that management system in these soils has therefore greater influence on fertility than fertilization treatment.