Background of the study
Soil is an important medium for crop production. Emone (2003) stated that soil is an outer loosened layer of the earth’s crust which has the potential for supporting plant life. Olaitan and Omomia (2009) explained that soil is a thin layer of the earth’s surface which is loose, non-solid in nature and serves as a medium for plant and animal growth and development. Soil, according to the authors, is made up of inorganic particles (mineral matter), organic matter, living organism, water and air. In the context of this study, soil is the unconsolidated thin layer of the earth’s surface which serves as a home and medium for plant and animal growth and development. The level of nutrient support given by the soil to a particular crop growing on it represents its fertility. Fertility, in the view of Havlin, Beaton, Tisdale and Nelson (1999), is the state of richness of a soil in which it contains nutrients that support the growth and yield of crops. It is the ability of the soil to supply essential nutrients and soil water in adequate amounts and proportions for growth and reproduction of plants in the absence of toxic substances which could inhibit their growth and development. Brady and Weil (2010) explained that soil fertility is the quality of a soil that enables it to provide chemical elements in quantities and proportions for the growth of specified plants. The authors maintained that the quality of soil, to a great extent, determines the nature of plants, ecosystems and the capacity of an environment. Therefore, soil fertility is the capacity of a soil to supply the required nutrients and soil water in adequate amount and proportions for growth, development and yield of a specified crop in the absence of plant growth inhibitors.
Soil fertility is important in agriculture in many ways. International Fertilizer Industry Association (IFIA, 2011) reported that soil fertility is important to agriculture in that it supports high yield in order to feed a growing population; increases farmers’ return per unit of land cultivated; supports yields of renewable energy source (example, bio energy crops); supports plant growth to reduce nutrient runoff, surface water and water over-enrichments. It contains adequate organic matter which retains more water and favours efficient water use and cycle; favours the growth of cover crops which helps to control soil erosion; increases water retention capacity of the soil, thereby preventing desertification; houses microorganisms; serves as a hugereservoir of biological diversity and a powerful symbol of culture which represents the bounty of nature.
Soil fertility of an area, according to Esu (2008) may be low or high depending on the amount of chemical elements present in the soil, the rate and the proportion at which the elements are released to the crop and the nature of the crop(s) that grows on the soil. Brandy and Weil (2010) noted that there are 17 known essential elements for plant growth and yield namely: Nitrogen, Phosphorous, Potassium, Calcium, Magnesium, Sulphur, Manganese, Iron, Boron, Zinc, Copper, Molybdenum, Nickel, Chlorine, Carbon, Hydrogen and Oxygen. The first 14 of the 17 elements, according to Olaitan and Lombin (1985) are taken up by plants only in mineral form from the soil while the last 3 (Carbon, Hydrogen and Oxygen) are non-mineral elements and are supplied to the plants by air and water. The authors observed that plant nutrients are generally divided into macro and micronutrients depending on the relative amount needed by plants. Onwuegbuna (1994) clarified that macro or major nutrients are required in relatively large quantities by plants. These nutrients, according to the author, are Carbon, Hydrogen and Oxygen obtained from air and water; and Nitrogen, Phosphorus, Potassium, Calcium, Magnesium and Sulphur obtained from the soil by plants. The micro or minor nutrients, the author further stated, are required in relatively small quantities by plants. These nutrients are Iron, Manganese, Boron, Zinc, Copper, Molybdenum, Nickel, Cobalt and Chlorine, which are obtained from the soil. Hodges (2011) observed that even though micronutrients are required in relatively small amount by plants, they are as important as the macro nutrients. A deficiency of one or more of the micronutrients can lead to severe depression in growth, yield and crop quality. For instance, deficiency of Molybdenum leads to leaf chlorosis (that is, leaves turning pale yellow or yellowish-green), leaves drop, restricted root development and stunted growth of plants like that of Nitrogen deficiency. These deficiencies indicate low soil fertility and require management through application of fertilizer.