Interações entre adubos verdes e fosfato natural na ciclagem de nutrientes do solo para produção de milho na agricultura familiar

Abstract

The family farming sector produces the majority of food for human consumption, however, family farmers receive a small proportion of total agricultural investments in Brazil, and therefore lack technical assistance and resources to invest in crop production. In such areas, regular export of soil nutrients, without replenishment, will deplete soil fertility over time, reducing soil organic matter and crop production. The objectives of this study were: to analyze whether the use of low cost green manures combined with different rates of rock phosphate increase soil fertility and microbiological activity, while minimizing potential environmental impacts, so that family farmers continue your vocation, food production. A field experiment was designed to test different green manure species and phosphorus rates within a maize rotation. The experiment was conducted over two years: year 1 (2014/2015) and year 2 (2015/2016), in the southwestern region of the State of Mato Grosso (Brazil), in the settlement Florestan Fernandes. The was randomized complete block design, with a 6 x 3 factorial scheme, four replications, and plots with dimensions of 3 x 3 m, the leguminous plants tested were: velvet bean - MUC (Mucuna cinereum), two varieties of sunnhemp - CJ (Crotalaria juncea) and CO (Crotalaria ochroleuca), and dwarf pigeonpea - G (Cajanus cajan), only in year 2, the legume seeds were inoculated. Plots with a non-legume cover crop, pearl millet - MI (Pennisetum glaucum), and a weedy fallow (VE) were also included. Rock phosphate (i.e., reactive natural phosphate) was applied as a source of phosphorus at 50 and 100 kg P2O5 ha-1 . The green manure plants were cut at full bloom and were evaluated for aboveground dry biomass (DB) production and analyzed for nutrient content. Then, maize was planted in all plots to evaluate yields, root/ DB shoot ratio and an index of sensitivity to water deficit under agroecological management. Microbiological parameters were also quantified including arbuscular mycorrhizal fungi (AMF), phosphate solubilizing microorganisms (PSM), and acid and alkaline phosphatases. The green manure treatments, especially the legumes, produced high amounts of DB (5.9 – 39.3 Mg ha-1 ), and increased the stocks of nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) in the soil. Interactions with rock phosphate did not increase the DB production of the green manures, and did not raise the stocks of N, carbon (C), K, and magnesium (Mg) in soil. The CJ species is the one that reaches higher productivity of DB and accumulation of N, being recommended in consortium with species of C/N>30 ratio, to minimize the risk of soil nitrate contamination. Inoculation of legume seeds does not increase DB yield and does not increase N content in plants. The CO, MI and MUC species have lower C/P ratios, and are indicated as green manures for more availability of P to the soil. CJ and MUC managements increase AMF concentrations in the soil, and the use of CO, G and MI species increases the PSM population in the soil. The variables Ca, P, C in the soil, alkaline phosphatase and PSM, correlate positively with maize yield, and the C/P and N/P ratios variables of the green manure plants correlate negatively. The legume specie CO, show the lowest (year 1) sensitivity index to the water deficit (Ky), being an alternative of management in the current scenario climatic changes. The use of legume CO, as a green manure in interaction with rock phosphate, increases maize yield, which enables it as a viable recommendation for family farmers.