ABSTRACT: There are several globally accepted methods to chemically characterize P-fertilizers, but not all are suitable to predict the agronomic efficiency of the P sources in terms of plant nutrition. Our aim was to investigate the performance of P extractors for fertilizers, investigating the consistency of different methods for P sources of varying properties and the related plant responses. The experiment was carried out in a greenhouse, using corn as a model plant. Phosphorus values extractable in water, 2 % citric acid, 2 % formic acid, ammonium neutral citrate + water, and acid ammonium citrate were evaluated for eight P fertilizers of varied solubility and correlated with P uptake by corn. The extractors citric acid and formic acid recovered no predictive amounts of P from crystalline apatite sources (Araxá and Patos phosphate rocks, PRs). However, they showed a satisfactory performance for Bayóvar PR and partially acidulated PRs but extracted low amounts of P from soluble P sources such as superphosphates. Neutral ammonium citrate + water extractors could accurately predict the efficiency of soluble P sources but underestimated the performance of Bayóvar (a reactive PR). In contrast, the extractor acid ammonium citrate, AAC, (pH 3) accurately predicted the agronomic efficiency of all P fertilizers. We therefore suggest AAC as an effective predictor of the agronomic effectiveness of any inorganic phosphorus sources.

O K é um nutriente encontrado em baixos teores em solos brasileiros, sendo necessário grande aporte via fertilização para alcançar produtividades satisfatórias das culturas. O Brasil apresenta elevada dependência externa e possui limitadas reservas de minerais solúveis de K, tradicionalmente explorados para a produção de fertilizantes. Por outro lado, é comum a ocorrência no país de minerais silicatados ricos em K não explorados comercialmente. Neste trabalho, objetivou-se caracterizar mineralógica e quimicamente amostras de verdete, separadas em frações granulométricas e avaliar seu potencial de uso com fertilizante potássico.A composição mineral do verdete é à base de glauconita, quartzo e feldspatos. Os teores totais de K2O no verdete variaram de 5,18 a 9,0 dag/kg. O teor de K extraído em água ou em ácido cítrico a 2% foi inferior a 2,4% do K total, indicando baixa reatividade do verdete e limitações para seu uso direto como fonte de K. O fracionamento físico e a sedimentação em água são processos ineficientes para promover a concentração de K nas diferentes frações do verdete. Os teores totais de K em algumas amostras de verdete são consideráveis, podendo viabilizar o uso desta rocha como matéria prima para produção de fertilizantes potássicos mais reativos.

Potassium is a nutrient found at low levels in Brazilian soils, requiring large inputs of fertilizers to achieve satisfactory crop yields. Brazil has high external dependence and limited reserves of soluble K mineral, which is traditionally exploited for the production of fertilizers. On the other hand, it is common the occurrence in the country of potassium-rich silicate minerals which are not commercially exploited. This study aimed to characterize mineralogically and chemically samples of verdete rock separated into size fractions and evaluate its potential as potassium fertilizer. The mineral composition of verdete rock is based on glauconite, quartz and feldspar. The total K2O content in verdete rock ranged from 5.18 to 9.0 dag/kg. The K content extracted in water or 2% citric acid was 2.4% below the total of K, indicating low reactivity of verdete rock and limitations for direct use as K source. The processes of physical fractionation and sedimentation in water are inefficient to promote the concentration of K in the different verdete rock fractions. The total K content in some samples are considerable and may enable the use of this rock as raw material for production of more reactive potassium fertilizers.