CALCIUM-SULFATE, PHOSPHOGYPSUM AND CALCIUM-CARBONATE IN THE AMELIORATION OF ACID SUBSOILS FOR ROOT-GROWTH

The chemical barrier to root development existing in the subsoils of a cid soils is a subject of increasing interest. In order to better unde rstand the factors involved in the amelioration of subsoil acidity, th e effects of calcium sulphate, phosphogypsum and calcium carbonate on the properties of the solid and liquid phases of subsoil samples and o n the growth and nutrient uptake by maize (Zea mays L.) were evaluated . The soils used were two alic red-yellow latosols, two acric dusky re d latosols and one alic dark-red latosol from the State of Sao Paulo, Brazil. A vertical split-root technique was used in a greenhouse exper iment, with the plants initially grown in a small pot with 130 g ferti le soil, which was introduced in a larger pot containing 2 dm(3) of th e subsoil samples. The treatments consisted of a control (C) and appli cations of calcium carbonate (CC), calcium sulphate (CS) and phosphogy psum (PG) at the rate of 10 mmol(c) Ca2+ dm(-3). CS and PG reduced soi l acidity, but in a much smaller proportion than CC. Calcium carbonate reduced the activity of Al3+ because of the increase in pH. Total alu minum and calcium contents in the soil solution were much higher for t he red-yellow latosols than for the other soils, indicating lower sorp tion of Ca2+ and SO42- in these soils. The activity of Al in the soil solution was decreased in different ways for the five soils, depending on the ionic strength and the formation of the ionic pair AlSO4+ and, in the case of PG, the formation of complexes of Al with F (AlF2+, Al F2+ and AlF3 degrees). The subsoil samples presented severe restrictio ns for maize root growth and all three treatments were equally effecti ve in increasing root development, which could be attributed to the su pply of calcium in one of the acric dusky red latosols and a combined effect of the amendment in reducing the activity of Al and increasing the activity of Ca in the soil solution in the other soils. As a conse quence the three treatments increased in the same manner water, N and K uptake from the subsoil and the dry matter production of maize. It c an be concluded that, for the soils considered in this research, phosp hogypsum is an effective amendment for acid subsoils containing low ca lcium or toxic aluminum contents.