ABSTRACT The phosphorus (P) rates recommended for corrective fertilization-P of soils from southern Brazil may be insufficient to reach the critical level for optimal plant growth. This study aimed to quantify the fertilizer-P rates for total correction fertilization with varying soil buffering capacity in the states of Rio Grande do Sul (RS) and Santa Catarina (SC). Soil samples from 0.00-0.10 and 0.10-0.20 m layers were collected from 41 locations distributed in both states. Twelve P rates were applied to each soil, varying between 0 and 100 % of the maximum adsorption capacity (P-max), and incubated for 20 days. After incubation, the extractable P was determined by Mehlich-1. Based on the relationship between applied rates and extracted P, the P buffer capacity (trP_M1) of the soils was quantified, relating it to soil properties. The trP_M1 values, that is, amounts of P2O5 required to increase 1 mg dm-3 of P extracted by Mehlich-1, varied between 2.4 and 34.5 kg ha-1 of P2O5. A multiple explanatory equation for the variable P was generated, in which only P-max, clay content, and initial P availability have a significant effect. The P buffer capacity was significantly higher in the soils with the highest clay content, and there was a reduction in trP_M1 for soils with higher initial P availability. Considering 270 soil samples with low P, the P rate to reach the sufficient levels may be 2-folds higher than the values currently indicated for the RS and SC states, especially for soils with more than 40 % of clay. Phosphorus rates for corrective fertilization must be based on the soil clay content and in P initial availability. The fertilizer-P in clayey soils must be increased.

RESUMO: O estudo objetivou comparar dois métodos de avaliação de acidez potencial (H+Al) e de estimativa da necessidade de calagem (NC) para solos da região Oeste de Santa Catarina (SC). Quatorze solos foram incubados com 7 doses de calcário durante 120 dias, obtendo-se ao final os valores reais de H+Al a pH 7,0 e a NC dos solos. Esses valores serviram de referência para avaliar a exatidão dos métodos de estimativa do H+Al: tampão SMP e acetato de cálcio a pH 7,0 e a exatidão dos métodos de estimativa da NC: índice SMP e saturação por bases (V%). Em média, o H+Al foi subestimado em 6 e 40% pelos métodos SMP e acetato de cálcio, respectivamente, sendo mais problemático nos solos de maior tamponamento. Já a NC foi subestimada em 20 e 30% pelo Índice SMP e V%, respectivamente. O índice SMP estimou melhor a NC que o método V%, principalmente em solos de maior tamponamento.

ABSTRACT: The objective of this study was to compare two methods of evaluation of potencial acidity (H+Al) and two methods to estimate liming requirement (LR) for soils from the west region of Santa Catarina State, Brazil. Fourteen soils were incubated with 7 lime rates for 120 days, obtaining the real values of H+Al at pH 7.0 and LR. These values were used as reference to evaluate the accuracy of two H+Al estimating methods: SMP buffer and calcium acetate at pH 7.0, and to evaluate the accuracy of two LR methods: base saturation (BS) and SMP index. On average, H+Al was underestimated in 6 and 40% by SMP and calcium acetate methods, respectively, especially in soils with high buffer capacity. The LR was underestimated in 20 and 30% by SMP index and BS, respectively. The SMP Index showed a better LR estimative than BS, especially in soils with high buffer capacity.

O Estado de Santa Catarina tem algumas atividades agropecuárias, como a suinocultura, que são responsáveis pela adição de grandes quantidades fósforo (P) nos solos; porém, o Estado necessita de um método para avaliar o risco ambiental desses solos com elevado teor de P. Uma maneira de avaliar o risco ambiental da adubação fosfatada, orgânica ou mineral é determinar o teor máximo de P disponível por Mehlich-1 que um solo pode apresentar sem que haja alto risco de transferência de P do solo para as águas superficiais, chamado Limite Crítico Ambiental de P (LCA-P). Contudo, o extrator Mehlich-1 é sensível ao teor de argila do solo, devendo esse fator ser considerado no estabelecimento do LCA-P. O objetivo deste trabalho foi determinar o LCA-P pelo extrator Mehlich-1 para solos com diferentes teores de argila do Estado de Santa Catarina. Usou-se solo do horizonte B de um Latossolo Bruno com 800 g kg-1 de argila, que foi misturado com areia em quantidades para se obterem solos artificiais com 200, 400, 600 e 800 g kg-1 de argila. Esses solos artificiais foram incubados por 30 dias com umidade de 80 % da capacidade de campo para estabilização das propriedades físico-químicas e posteriormente incubados novamente por mais 30 dias com calcário para elevar o pH(H2O) até 6,0. Nos solos, foram realizadas curvas de adsorção de P e determinadas a adsorção máxima de P (Pmax) pelo modelo de Langmuir para cada textura avaliada. Com base nos valores de Pmax, foram adicionadas aos solos sete doses de P incubados novamente por 20 dias, com quatro repetições. Após a incubação, foram definidos os teores de P disponível (P-Mehlich-1) e P dissolvido em água (P-água). O teor de P por Mehlich-1 a partir do qual o P-água passa a aumentar drasticamente foi obtido por equações segmentadas e, para manter-se uma margem de segurança ambiental, consideraram-se 80 % desse valor como o LCA-P. O LCA-P foi dependente do teor de argila do solo; pelos dados, obteve-se o modelo LCA-P (mg dm-3) = 40 + Argila, em que a argila do solo é expressa em porcentagem. O modelo foi testado em 82 amostras diversificadas de solos do Estado de Santa Catarina e foi capaz de separar amostras com alto e baixo risco ambiental.

The State of Santa Catarina, Brazil, has agricultural and livestock activities, such as pig farming, that are responsible for adding large amounts of phosphorus (P) to soils. However, a method is required to evaluate the environmental risk of these high soil P levels. One possible method for evaluating the environmental risk of P fertilization, whether organic or mineral, is to establish threshold levels of soil available P, measured by Mehlich-1 extractions, below which there is not a high risk of P transfer from the soil to surface waters. However, the Mehlich-1 extractant is sensitive to soil clay content, and that factor should be considered when establishing such P-thresholds. The objective of this study was to determine P-thresholds using the Mehlich-1 extractant for soils with different clay contents in the State of Santa Catarina, Brazil. Soil from the B-horizon of an Oxisol with 800 g kg-1 clay was mixed with different amounts of sand to prepare artificial soils with 200, 400, 600, and 800 g kg-1 clay. The artificial soils were incubated for 30 days with moisture content at 80 % of field capacity to stabilize their physicochemical properties, followed by additional incubation for 30 days after liming to raise the pH(H2O) to 6.0. Soil P sorption curves were produced, and the maximum sorption (Pmax) was determined using the Langmuir model for each soil texture evaluated. Based on the Pmax values, seven rates of P were added to four replicates of each soil, and incubated for 20 days more. Following incubation, available P contents (P-Mehlich-1) and P dissolved in the soil solution (P-water) were determined. A change-point value (the P-Mehlich-1 value above which P-water starts increasing sharply) was calculated through the use of segmented equations. The maximum level of P that a soil might safely adsorb (P-threshold) was defined as 80 % of the change-point value to maintain a margin for environmental safety. The P-threshold value, in mg dm-3, was dependent on the soil clay content according to the model P-threshold = 40 + Clay, where the soil clay content is expressed as a percentage. The model was tested in 82 diverse soil samples from the State of Santa Catarina and was able to distinguish samples with high and low environmental risk.