Acidez do solo e toxicidade do alumínio em sistema plantio direto de longa duração

Abstract

Brazilian soils are, for the most part, naturally acidic. Calcium (Ca) deficiency and toxicity caused by aluminum (Al) and manganese (Mn) are the factors that have most limited crop yield in acidic soils of tropical and subtropical regions. The primary and most obvious symptom of Al toxicity in plants is an inhibition in root elongation and decreased nutrient uptake. This study was carried out with the objective of (i) evaluating the influence of surface liming and plant residues left on the soil surface during the autumn–winter season in the contents and forms of Al in a long-term no-till system; (ii) to verify the benefits of surface liming associated with autumn–winter crop residues in combating soil acidity and alleviating the long-term toxicity of Al in a no-till system; and (iii) to correlate the Al forms present in the soil and the grain yields of corn, soybean, and bean in a long-term no-till system. The experiment was carried out on loamy, kaolinitic, thermic Typic Hapludox, managed for a long period under a no-till system in Ponta Grossa, Parana State, Brazil. A randomized complete block design was used, with three replications in a split-plot arrangement. In the plots, four rates of dolomitic lime were applied on the soil surface in May 2004 (0, 4, 8, and 12 t ha-1), and in the subplots three cover crop residue treatments were applied during the autumn–winter season, without or with nitrogen (N) addition: (i) uncovered soil, (ii) with crop residue (oat or wheat), and (iii) with crop residue (oat or wheat) + 60 kg ha-1 of N. Nitrogen was applied annually in the form of ammonium nitrate in top dressing, at the beginning of tillering of oat or wheat crops. Surface liming corrected soil acidity and increased calcium (Ca) and magnesium (Mg) contents and base saturation up to a 0.20 m depth. Aluminum exchangeable contents were very low with lime application, with non-exchangeable Al forms predominant at all depths evaluated. Liming neutralized Al in an exchangeable form, and a fraction of the different forms of non-exchangeable Al. With liming, there was a great reduction of Al highly complexed by the soil organic matter, being much precipitated in inorganic forms of Al (OH)3. The cumulative grain yield of the crops increased by an average of 23% with the surface lime application over the 10 years of evaluation. The largest increases in grain yield of the crops were obtained with the lime application at 8.3 t ha-1, whose rate was very close to that calculated to raise soil base saturation in the topsoil (0–0.20 m) to 70%. Non-exchangeable Al complexed poorly with soil organic matter as well as exchangeable Al showed a strong correlation with crop yields, revealing that the increase of these Al forms in soil affected drastically crop yields. The alleviation of soil acidity by the surface application of lime under a no-till system proved to be a fundamental practice to guarantee high crop yields by alleviating the Al toxicity in both forms exchangeable and poorly complexed with the soil organic matter.