Mix de plantas de cobertura reduzem o acesso ao potássio não trocável em solos arenosos

Abstract

Potassium (K) deficiency can be common in Brazilian soils, especially in sandy-textured ones, requiring management strategies to improve nutrient use efficiency. Cover crops play a central role in this process, as they not only absorb K from the exchangeable and non-exchangeable fractions but also promote its cycling through straw decomposition, increasing the surface availability of the nutrient and reducing leaching losses. Potassium in the soil is found in different fractions (solution, exchangeable, non-exchangeable, and structural), which, to varying degrees, regulate its availability to plants. This study evaluated the impact of different cover crop associations on the availability and utilization of exchangeable (Ke) and non-exchangeable (Kne) potassium, as well as the rhizospheric effect of these plants on the bioavailability of this nutrient. Two experiments were conducted. Experiment 1 was carried out in the field during the 2023/24 season in Presidente Bernardes (SP), under a randomized block design with five treatments, consisting of different cover crop compositions: fallow, Brachiaria, Brachiaria + millet, millet + velvet bean, and a mixed system. The cover crops were grown during the cotton off-season, fertilized, and subsequently desiccated for direct sowing of the crop. Soil samples were collected at depths of 0–10, 10–20, 20–40, 40–60, and 60–80 cm at two time points: before sowing and 90 days after emergence (DAE). The cover crops were sampled before cotton sowing, and cotton plants were sampled at the same time as the second soil collection (90 DAE). Exchangeable and non-exchangeable K contents, K balance, and the contribution of Kne to the total K absorbed by cotton were determined. Brachiaria showed the highest dry matter production and contributed to higher Ke levels in the upper soil layers compared to fallow. Regarding K balance, all cover crop systems utilized reserves of Kne, with millet + velvet bean showing the highest efficiency. Experiment 2 was conducted in a greenhouse at UNOESTE, under a randomized block design with eight replicates, evaluating four cover crop systems (Mix, Brachiaria, Millet, and Velvet bean) with and without K fertilization (equivalent to 150 kg ha⁻¹ KCl). Plants were cultivated using the planar rhizosphere technique, obtained by growing them in pots that allowed root mat formation. After 45 days, the soil was sliced into millimetric layers (0–1, 1–2, 2–3, 3–4, 4–5, and 5–10 mm) for exchangeable and non-exchangeable K analysis, while roots and shoots were dried to determine dry matter. Brachiaria, both with and without K application, provided the highest exchangeable K (Ke) contents, while millet only showed increased Ke with K fertilization. The Mix treatment resulted in the lowest Ke values, with or without fertilization, especially in the layers closest to the plant rhizosphere. For non-exchangeable K (Kne), the Mix treatment showed the highest values across all depths, whereas Brachiaria and millet exhibited lower concentrations.