INTERIM EVALUATION OF PHOSPHOGYPSUM AS AN AMELIORANT FOR SOIL ACIDITY IN SUGARCANE
Results are reported of a 3x3x3 factorial trial in which phosphogypsum (PG), dolomitic limestone (DL) and single superphosphate (SS)were applied to sugarcane. The trial was situated on a Kranskop form soil with a moderately toxic aluminium (AI) content. Based on the combined yield responses from the plant and first ratoon crops, treatments with SS were superior to those with PG, and little response was obtained from DL. The high P in-furrow treatment (90 kg P/ha) gave a highly significant response of 3 t/ha sucrose compared with 2,1 t/ha from the best PG treatment (5 t/ ha). Leaf P and Ca also increased with both PG and SS application. Chemical changes in the soil following PG application resulted in considerable leaching of Mg below the 400 mm soil depth. Improved chemical changes included increased pH and Ca levels in the subsoil together with a reduction in exchangeable AI. The SS/PG interaction, evidence from third leaf P analyses, and lack of any marked response to liming coupled with good root development to depth indicated that yield response to PG was due mainly to its P content. Enhanced Ca content and diminished acid saturation in the subsoil may be beneficial.
CALCIUM DEFICIENCY IN CLAYEY B HORIZONS OF SAVANNA OXISOLS
The restriction of crop root growth in Cerrado (savanna) Oxisols often reduces yields because the volume of soil explored by the roots is insufficient to supply enough water during dry periods that occur in the rainy season. A recent soil survey of the 580 000 hectares of the Federal District in Central Brazil shows a number of profiles that have clayey B21 horizons within 50 centimeters of the surface with very low levels of exchangeable calcium + magnesium (0.2 milliequivalents per 100 grams or less) and no exchangeable aluminum. The hypothesis that root penetration in such subsoils is limited by lack of calcium, rather than by aluminum toxicity, is supported by the results of simple and inexpensive tests showing that the addition of 0.10 to 0.15 meq/100 g calcium as chloride, phosphate, or carbonate effectively normalized seedling root growth, which, in the absence of such additions, stopped within a few days. The addition of magnesium carbonate, however, did not improve root growth. To test whether the leaching of calcium salts might ameliorate the calcium deficiency in the subsoil, samples were taken from an experiment that had received about 2500 kg/ha calcium as limestone and ordinary superphosphate at installation 2 years previous. Compared with root growth in samples from adjacent virgin soil, these soils showed improved root growth to as deep as 75 cm, thus suggesting that fertilizer management can have a beneficial effect on subsoil rooting in such soils.
Cation Exchange Capacity Variations with pH in Soil Clays
Soil clays containing pedogenically chloritized 2:1 layer silicates, after brief contact at 25C with 2% Na₂CO₃ (pH 11), developed a proportionately large increment of CEC which was retained after thorough washing with neutral salts (KCl, NaCl) to reduce the pH value below 7. A soil clay from Natal, S. Africa showed the largest increase—from 2.4 meq/100 g at pH 5 to 8.0 meq/100 g after alkaline and KCl treatments. This CEC hysteresis or delta value largely disappeared when washing with a NaOAc buffer of pH 5 followed the alkaline treatment. Similar effects were absent from specimen kaolinite, montmorillonite, and vermiculite. The presence of aluminous, pedogenic chlorite was shown, by X-ray and analytical investigation, to be a common property of the soil clays exhibiting pH dependent CEC. This finding is supported by separate studies on synthetic aluminous chlorite, which showed variable CEC properties essentially identical to those of these soil clays. Release of the initially blocked isomorphous substitutional negative charge by deprotonation of the positive hydroxy alumina in aluminated soils, clays and allophane is the indicated mechanism accounting for pH dependent, hysteretic CEC charge of the mineral portion of soils.