Influence of various water management and agronomic packages on the chemical changes and on the growth of rice in acid sulphate soils

In potential acid sulphate soils acidity may arise from any combination of reclamation and drainage lowering the groundwater table in adjacent areas, and unusually dry seasons affecting the regional groundwater table. In the long run, natural processes of deacidification will finally make these soils productive. But this natural process is very slow and may take decades for any significant improvement. Thus natural reclamation does not provide the solution to the immediate need of increasing food production at a sufficiently high rate. Developing economic means of enhancing the process of reclamation of these soils is therefore considered very important. Feasible management packages for the development of acid sulphate soils are determined by the soils, the climate, the hydrology and the economic opportunities.The causes of low productivity of acid sulphate soils include soil acidity, salinity, aluminium toxicity, iron toxicity and low nutrient tatus. Different reclamation and improvement methods for low land rice re reviewed in Chapter 2. A simple low-cost lysimeter drum (60 cm diameter, 90 cm depth) was developed to collect undisturbed soil cores for studying the chemical changes and the growth of rice in acid sulphate soils. Forty undisturbed soil cores from Aparri and Sinacaban soils in the Philippines were subjected to different water management practices, liming rates and mulch application for two leaching periods and three rice crops. There was a great difference in chemical properties and grain yields of the two soils. In Aparri soil, there was no marked effect of drainage, liming and mulch practices on the chemical properties of the soil solution during the two leaching periods and during three rice crops. Also the grain yields of three crops in this soil were not influenced by different treatments.In Sinacaban soil, the effects of drainage, liming and agronomic practices on chemical properties of soil solutions were very pronounced. Deep drainage of the soil created favourable conditions for pyrite oxidation which resulted in high acidity and high Fe 2+and Al 3+. Shallow drainage did not cause severe oxidation in the subsoil layers and produced less acidity and Fe 2+, Al 3+. Shallow drainage in combination with mulch application (5 cm of rice straw) minimized the water loss from the soil, and hindered oxygen movement to the subsoil. Thisresulted in less acidity development and less Fe 2+, Al 3+. When the soil was continuously submerged, there was no pyrite oxidation, and no Fe 2+and Al 3+production.The influence of different treatments on grain yields was very pronounced. Deep drainage treatments gave the poorest yield in both the second and third crops. Highest yields were obtained in the surface flushing treatments, the next highest yields with shallow drainage. Mulching increased grain yield by 67 and 30 percent for the second and third crops respectively. In deep drainage treatments, only high amounts of lime, 2.5 tons/ha produced a yield exceeding the equivalent of about 1 ton/ha. In this chapter, the concept of average mineral stress index (AMSI) was introduced to correlate grain yield and toxic elements in acid sulphate soils (pH, Al, Fe).During the dry season, oxidation and acidification occur when groundwater falls below the sulphic layers in the subsoil.In Chapter 3, a study on the evaporation and acidification process in an acid sulphate soil was carried out in forteen undisturbed soil columns of 20 cm in diameter and 70 cm length from an acid sulphate soil in Mijdrecht, the Netherlands. These columns were subjected to two groundwater levels: 40 cm and 65 cm below the soil surface, 5 different durations of evaporation and 2 agronomic practices. The average total acidity over 14 layers in the profile did not show much variation among treatments. The presence of a peat layer on the surface reduced the rate of acidification, presumably mainly by reducing. The evaporation rate and perhaps by hampering the oxygen movement in the profile. In treatments with low groundwater table, the average pH decreased sharply at increasing evaporation.When acid sulphate soils develop, dramatic changes in the chemistry of surface waters take place and these are exported from the reclaimed area as drainage waters. Acid sulphate floodwaters generated over large areas of acid sulphate soils may adversely affect crops growing on adjacent, better soils.In Chapter 4, a study on the one-time and cumulative effects of acid sulphate floodwater on the growth of rice and on the chemical changes in three acid sulphate soils were investigated in two successive pot experiments. Artificial acid floodwater were made with pH ranges from 3.5 to 5.8, Al 3+from 0 to 300 mg/l and Fe 2+from 0 to 500 mg/l.In both experiments, pH of acid floodwater alone, as low as 3.5, did not affect the chemical changes of three acid sulphate soils, whereas the presence of Al 3+and Fe 2+in acid sulphate floodwater produced low pH and enhanced the solubility of Fe 2+.pH of acid sulphate floodwater alone as low as 3.5 showed no effect of the growth of rice at on early stage of growth. Dry matter yields were negatively correlated with the applied Fe 2+and Al 3+.A single factor, total acidity, being the sum of H +, Al 3+and Fe 2+in equivalents, was introduced as a factor determining the plant dry matter yield in cases that none of these reached the limiting toxicity values.In acid sulphate soils, rice responded favourably to rock phosphates. Phosphorus is the main limiting macro-nutrient for crop production in these soils, but the use of superphosphate is a luxury in many developing countries because of their limiting foreign exchange. Therefore, the effectiveness of rock phosphates for lowland rice on acid sulphate soils under various management practices was studied in pot experiments on surface horizons of two acid sulphate soils under three application methods and two rates. Details of this experiment is given in Chapter 5. Rice response to phosphate sources varied between soil. In Malinao, the highest rice yield was obtained when rock phosphate was applied and incubated three weeks before transplanting at field capacity.In Chapter 6 a computer simulation model was developed to calculate the time course of acidity production from pyrite oxidation in relation to the changes of groundwater table, evaporation, entrance of oxygen and different chemical reactions. The model is based on a multi compartment model in which the soil profile is divided into a number of layers, is written in CSMP and contains three main parts: INITIAL, DYNAMIC and TERMINAL. For each compartment of a clay loam and a silty clay soil, the compartment thickness, its water content, pyrite content, cation exchange capacity, total cations, adsorbed cations and oxygen concentration are given as inputs. Three evaporative demands of 4, 6 and 9 mm.d -1were used as a boundary condition at the soil surface. These substantial differences in evaporative demand lead to increasing waterloss. Results show that the oxidation process and the associated acidification of the soil are only marginally affected by the different evaporative demands, although the oxidation rate was very much influenced during the early part of the evaporation process.Results from this thesis shows that minimizing the oxidation of pyrite by application of mulch during the dry season and maintaining higher groundwater tables can avoid the acidic hazards. Research along this line should be directed toward the feasible application of this package to the field. In addition, other experiments should be carried out to study the following:- varietal screening for short duration, acid-tolerant, salt-tolerant, iron-tolerant cultivars;- studies on fertilizer application should aim at optimizing the use of phosphate, preferably rock phosphate;- development of quantitative models to predict the effects of drainage at various depths, irrigation and other management practices on the physical and chemical properties of acid sulphate soils.In order to validate these models, basis data on physical and chemical processes under various boundary conditions are needed.