Organic materials and soil productivity [antikvár]

- 1 - 1. INTRODUCTION Most of the industrialized countries are in the temperate zones of the world where a favourable balance exists between input and decomposition of organic materials in the soil. The conditions there cause a relatively high level of soil organic matter. This balance is influenced by climatic conditions and the inorganic constituents of the soil, in which the amount and type of clay minerals play an Important role. The positive effect of mineral fertilizers depends upon well regulated humus husbandry. The plant growth caused by the use of mineral fertilizers Increases not only the economically important parts of the plants, but also the growth of roots and crop residues which both remain on the fields and are ploughed into the soil. Furthermore, in many farms animal excreta is used to maintain the level of soil organic matter in arable land. There are some interactions between certain clay minerals and fractions of soil organic matter which act as favourable factors for soil productivity and therefore for plant production. This is not always the case in developing countries. SOIL CONSTITUENTS AS FACTORS OF PLANT PRODUCTION Plants are the basis of food for human beings and animals. If the energy from the sun, the content of carbon dioxide in the air, the amount and distribution of rainfall, the supply and the balance of inorganic nutrients are optimum, the growth of plants and therefore the 3aeld of harvests depends upon the soil physical and mechanical properties as well as upon the chemical composition of the inorganic and organic soil constituents. The properties of the inorganic part of soil are mainly influenced by the chemical composition of the parent material and weathering. The smaller particles, especially the colloid phase, determine the value of a soil for plant production more than the others. The mechanical properties of a soil, which Influence gas exchange, water permeability, rooting of the plants, biochemical activity etc. , are mainly caused by interactions between inorganic and organic soil constituents, mainly in the colloid phase. In spite of the fact that the organic part of soil constituents is on average only from 0.1 to 5% of the dry matter of arable land, it Influences the production capacity of soil to nearly the same degree as the Inorganic. The organic part is formed mainly by biochemical transformations of residues of dead organisms living in and on the soil caused by microbial activity. The dynamics of these processes follow environmental conditions where climatic factors play the main role and it makes a difference if climatic conditions allow biochemical reactions to occur the whole year round or are stopped for a while in wintertime when temperatures are around 0°C or below. The main initial materials of soil organic matter are compounds of constituents of plants and micro-organisms or their metabolic products. Therefore, the more or less stable products of these processes, the humic substances, have in principle similar compositions. However, deviation in their composition occurs because of: different conditions during the humlfication of the organic materials, such as climate with its different temperatures or precipitation patterns; the actual composition of the Inorganic soil components with its different amounts of nutrients and particle sizes; the degradabillty of organic material in the soil and the biological activity, which is determined by the first three factors mentioned. The characterization of the humic systems developed under the different environmental conditions by determination of physical and chemical data Is therefore an important task for evaluation of the contribution of soil organic matter to the productivity potential of soils in different regions of the world.