Introduction to soil fertility

Soil fertility is a concept based on the amounts of essential nutrients available to the plants. The plant nutrients involved are classified as macronutrients such as nitrogen (N.), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S); or as micronutrients such as i iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), and molybdenum (Mb). The amount of each of these nutrients is not the only factor in determining whether there are nutrient shortages. Soil pH (acidity) is also important because under certain pH conditions, nutrients form insoluble compounds which are unavailable to be taken up by plant roots. A shortage of any of these elements in the soil can slow crop development and reduce yields, something which extentionists should consider when working with farmers to improve crop production.

In practice, soil fertility is determined by collecting a soil sample and submitting it for chemical analysis. Since a small sample will be used to infer the characteristics of an entire field, care should be taken to ensure that the sample represents as accurately as possible the conditions present in the plot.

Sampling may be done with a soil probe or a shovel, being careful to exclude the surface litter layer and sampling at a depth of approximately 1"-8" below the surface. Subsamples should be taken from random positions throughout the entire field (10-20 per hectare) and mixed thoroughly in a bucket or bag. From this mix, a 1-2 lb. sample is air dried, labelled, and submitted to a laboratory for analysis. A highly variable field should be subdivided into fairly uniform sections which should each be treated separately, as they may have very different soil characteristics. Samples may be analyzed with portable field test kits, if available, but it is also advisable to familiarize farmers with the laboratory facilities available to them. In Honduras, for example, soil analyses are performed at the laboratories of Recursos Naturales in Tegucigalpa, Standard Fruit Company in La Ceiba, the Escuela de Agricultura Panamericana in El Zamorano Valley and FHIA in La Lima, Cortes. The most common soil analyses performed are those for nitrogen, phosphorous, potassium and pH, which generally are most important in limiting crop yields. If crops do not appear to respond to changes in these factors then perhaps a deficiency of another nutrient is involved and a mere detailed analysis can be carried out.