Soil fertility refers to the ability of a soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent yields of high quality. A fertile soil has the following properties:-
- It is rich in nutrients necessary for basic plant nourishment. This includes nitrogen, phosphorus and potassium.
- It consists of adequate minerals such as boron, chlorine, cobalt, copper, iron, manganese, magnesium, molybdenum, sulphur and zinc. These minerals promote plant nutrition.
- It contains soil organic matter that improves the structure of the soil. This enables the soil to retain more moisture.
- The soil pH is in the range 6.0 to 6.8.
- It has a good soil structure which results in well-drained soil.
- It consists of a variety of micro-organisms that support plant growth.
- It often contains large amounts of topsoil
There are 16 nutrient elements required to grow crops (Table 1). Three essential nutrients—carbon (C), hydrogen (H), and oxygen (O )—are taken up from atmospheric carbon dioxide and water. The other 13 nutrients are taken up from the soil and are usually grouped as primary nutrients, secondary nutrients, and micronutrients.
The primary nutrients—nitrogen (N), phosphorus (P), and potassium (K)—are commonly found in blended fertilizers such as 10-10-10, or equivalent grades. Primary nutrients are utilized in the largest amounts by crops, and therefore, are applied at higher rates than secondary nutrients and micronutrients.
The secondary nutrients—calcium (Ca), magnesium (Mg), and sulfur (S)—are required in smaller amounts than the primary nutrients. The major source for supplementing the soil with calcium and magnesium is dolomitic lime (aglime), although these nutrients are also available from a variety of fertilizer sources. Sulfur is available in fertilizers such as potassium and magnesium sulfate, gypsum (calcium sulfate), and elemental sulfur.
Micronutrients—iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), and molybdenum (Mo)—are required in even smaller amounts than secondary nutrients. They are available in manganese, zinc and copper sulfates, oxides, oxy-sulfates and chelates, as well as in boric acid and ammonium molybdate.