Gypsum as an Ameliorant for the Subsoil Acidity Syndrome
Soil acidity particularly in the subsoil is a major limiting factor in crop productivity in many parts of the world due to aluminum (Al) toxicity and calcium (Ca) deficiency. This syndrome in the topsoil is easily treated with lime but because of the variable charge nature of many acid subsoils lime does not move down the profile and thus is ineffective. After early observations that gypsum could offset some of the deleterious effects of the subsoil acidity syndrome, a number of experiments in which deep incorporation of lime and phosphogypsum applied to the surface were compared, were laid out in Georgia on a variety of highly weathered soils (Ultisols and Alfisols). Crops studied included alfalfa, corn, soybeans, cotton and peaches. Highly statistically significant and economically profitable yield responses were obtained for all crops. A comparison of phosphogypsum with mined gypsum indicated that as far as crop response and soil reactions were concerned, there were no differences in crop growth but the fluoride content of the phosphogypsum complexed aluminum in the soils. Gypsum (a sparingly soluble salt) applied to the topsoil slowly moves down the profile and in so doing increases labile Ca levels and decreases Al in the subsoil of all soils studied. These effects are intermediate between the control (untreated) and situations where lime had been thoroughly mixed throughout the profile. This amelioration brought about by the gypsum treatments was sufficient to encourage roots to penetrate and proliferate in the subsoil where previous conditions were often so hostile as to prevent root growth. As a result of this improved root penetration, the crops were able to extract subsoil water previously beyond their reach and were consequently able to yield much better, having partially overcome drought stress. In very sandy soils, gypsum application can have potential deleterious effects in terms of leaching of magnesium (Mg) beyond the root zone. However this can be easily overcome by applying corrective Mg applications to the topsoil once the gypsum has moved down. The longevity of the gypsum effect on medium to heavy textured soils from a single application of 10 t/ha is in excess of 6 years and in some soils may be in excess of 8 years. Not only does gypsum treatment offset subsoil acidity but in addition, there is clear evidence that it has an ameliorative effect on subsoil hardpans making them less of an impediment to root penetration. The ameliorative effects of gypsum on subsoil acidity stem from one or more of the following mechanisms: (a) increased levels of subsoil Ca, (b) complex formation between Al and sulfate (SO4) and fluoride (F) which detoxifies the Al, (c) ligand exchange of SO4 for hydroxyls (OH) on sesquioxide surfaces resulting in the so-called “self liming” effect, (d) precipitation of basic aluminum sulfate minerals which renders the labile Al insoluble and (e) salt sorption in which SO4 is specifically adsorbed which causes the removal of some Al from solution.