Soil is the most important production factor for crops and at the same time is also the most influenced by the farmer. Soils
are very diverse and complex systems full of life. The soil itself can be viewed as a living organism, because it is a habitat for plants, animals and microorganisms, which are all interlinked with each other. The difference between sand, silt and clay is not visible to the naked eye. Still, it is important to distinguish between them, as the properties of the soil is very much dependent on the composition of the different particle sizes. Soils having equal amounts of clay, silt and sand are ideal for agricultural use. Such soil is called loam. Mineral particles contain
nutrients, which are slowly released in the process of weathering. Plant roots and some micro-organisms can actively dissolve nutrients from mineral particles and use them for their growth. The plants need minerals to build up organic matter and for physiological processes. Besides mineral particles, soil contains smaller or larger quantities of organic matter or humus, resulting from the decomposition of biomass. Though in most agricultural soils of the tropics, it makes only a few percent or even less than one percent of the total solid material, it is of tremendous importance for the soil fertility. Organic matter is mainly present in the top layer of the soil, which is subject to continuous transformation processes. The active part of soil organic matter can be further decomposed by soil organisms. The resulting structures can recombine themselves to form very stable humus structures, which can remain in the soil for many years. This long term soil organic matter or humus contributes a lot to improve the soil
structure.Besides mineral particles and soil organic matter, soils also consist of small pores (tiny hollows) filled with air or water. The spatial arrangement of particles and pores is summarized as “soil structure”. Small pores are good in preserving moisture while the larger ones allow a fast infiltration of rain or irrigation water, but also help to drain the soil and ensure aeration. In soils of good standing, mineral particles and soil organic matter form stable crumbles (aggregates). Organic matter works as a kind of glue, sticking together soil particles. Soil organisms such as earthworms, bacteria and fungus support this process. Thus, the soil structure can be improved by supplying organic matter to the soil. But wrong management can also ruin it, e.g. tilling the soil in wet conditions causes compaction. Most people have strong trust in any scientific work. Therefore, when it comes to soil fertility, farmers might also think of getting their soil analyzed in a laboratory. Though chemical soil testing may yield valuable information to specific questions, farmers should not expect too much from it. For example, there are some inherent problems related to analyzing nutrient contents: For the plant, the total content of a certain nutrient in a sample is not always relevant, as the nutrient may be absorbed to minerals so strong that it is not available to the plant roots (e.g. Phosphorus). Therefore, some tests treat the sample with solvents in order to stimulate the fraction of the nutrient available to plants. This might be a realistic simulation for conventional farming. In organically managed soils, however, the higher activity of soil organisms can result in a better availability of the nutrient, thus the result of the test is not fully appropriate. The content of other nutrients such as nitrogen is extremely fluctuating within few days, so that it highly depends on the point of time when the sample is taken. Still, chemical soil analysis can be usefulin some cases, e.g. to analyze the level of acidity of the soil (pH) or to detect deficiency of nutrients such as Potassium (K) or Zinc (Zn). Organic farmers might be especially interested in knowing and monitoring the content of soil organic matter. Chemical soil analysis on pesticide residues is highly complicated as one must know which pesticide to look for, and they are very costly. Physical testing, e.g. related to water retention capacity or soil structure can yield interesting information, but samples must be taken very carefully. Biological analysis, e.g. of the activity of soil organisms must be done in specially equipped laboratories and is rather costly. Altogether, the use of soil analysis on the farm level is limited due to the scientific methods, the availability of suitable laboratories and the costs involved. If soil tests are used, make sure that the relevant aspects are investigated and that the results of the test are critically discussed.