Possible Solutions to Remedy the Detrimental Effects of Soil Acidity on Tropical Agriculture
Arable tropical soils, which mainly consist of Oxisols and Ultisols but also include regions of Inceptisols and Alfisols, cover approximately one billion hectares; this area represents 33 percent of the total potentially arable land of this world which does not require irrigation. Most of these soils have optimum conditions for crop production, including low population density, excellent physical conditions, and a favorable climate (Van Wambeke, 1976). The main limitation of agricultural productivity and the reason that these tropical soils are underutilized for farming is soil acidity and its related factors. Improving the quality and yield of crops in these regions would do much to relieve the food pressures imposed upon us by the world's growing population. As stated by I.M. Rao, et al.:
It is particularly critical to realize the agricultural potential of Ultisols and Oxisols which remain idle in huge areas of the tropics...(but) we cannot repeat previous attempts to settle farmers in these areas before we have collected sufficient data and facts upon which we can honestly base our recommendations which guarantee a decent living from agricultural enterprise (Rao et al., 1993).
Therefore, it is of the utmost importance for us to find a feasible and cost-effective way to alleviate the problems of soil acidity on tropical agriculture.
Before the effects of soil acidity on tropical agriculture is examined, it is necessary to have a good background on what soil acidity is and how it occurs. Soils become acid because of vigorous leaching, coupled with the inputs of acids (substances capable of releasing positive hydrogen atoms (H+). There are many types of acids which infiltrate the soil, including water, carbonic acid (formed from CO2), nitric acid, sulfuric acid, and organic acids released by roots and soil microbes. The cumulative effect of acidification occurs because of the cation exchange of soils. Each H+ in the soil competes with other cations such as calcium (Ca), magnesium (Mg), potassium (K), or sodium (Na) to be bonded to the negative exchange surfaces of the soil colloids. As H+ ions displace these other cations, the anions (conjugate bases of the acids) and displaced cations which would compete for the attachment to the colloids are leached from the soil (Singer and Munns, 1996). Thus, an important characteristic of soils with respect to acidity is their cation exchange capacity (CEC), which governs the rate at which H+ ions and the nutrient cations are exchanged. A large CEC is indicative of a soil which has a high nutrient holding capacity and is an effective buffer from abrupt fluctuations in pH. Another important aspect of acidic soils is the accumulation of dissolved and exchangeable aluminum (Al) and manganese (Mn) as pH drops. These two elements are toxic to plants in that they injure the roots, which stunts their growth...