Chemical Speciation and Coffee Nutrition in Soil Amended with High Doses of Gypsum.

Soils in the cerrado region in Brazil are being incorporated to the coffee production. In these areas, topography favors mechanization, which is the main reason for this expansion. However, there are chemical limitations to be mitigated, such as low calcium and aluminum toxicity in most of these soils, mainly bellow the A horizon. Need for irrigation is also an important issue, due to the bad rainfall distribution which imposes a long period of drought from May to early October. Technologies for mitigating these chemical constraints in the topsoil are well defined, as well as irrigation techniques. However, even during the most limiting drought season, there is still water deep in the soil profile – about 1.5-2 m deep. Some coffee producers in the south eastern region of Minas Gerais State are using high doses of gypsum at the soil surface, along with soil fertilization up to 60-cm deep, to mitigate both Al toxicity and lack of Ca below 60 cm. This work aimed to quantify ions in the soil solution throughout the soil profile and compare the chemical species with root depth and plant nutrition. After the first year, sulfate, calcium, calcium sulfate and aluminum sulfate were found up to 80-cm deep in the soil, regardless if 7 or 28 ton ha^-1 of gypsum was applied; the root system was able to get water from bellow 80 cm in the soils, keeping the coffee plants healthy during the drought season. Free forms of SO₄^-2, Ca⁺² and Mg⁺² were dominant among the chemical species in the soil solution, regardless of the dose of gypsum and depth in the soil. The ion pair CaSO₄⁰ represented 11,6% of the Ca form in the samples at the plant rows, and less than 1% in the samples between the plant rows; MgSO₄⁰ represented  9,9% at the plant row, and also less than 1% in between the plant rows. The contents of exchangeable Ca and Mg in the soil solution were above the critical level, but exchangeable K remained in the range of the critical level; consequently, K content in the leaves were below the desirable level for coffee crops, thereby reducing the K/Ca and K/Mg ratios in the leaves.