Soil fertility is defined as the ability of soil to supply essential elements for plant growth without a toxic concentration of any element. Soil fertility refers to only one aspect of plant growth — the adequacy, toxicity, and balance of plant nutrients. An assessment of soil fertility can be made with a series of chemical tests.
On the other hand soil productivity is the soil’s capacity to produce a certain yield of crops or other plants with optimum management. For example, the productivity of soil for cotton production is commonly expressed as kilos, or bales of cotton per acre, or hectare, when using an optimum management system.
The optimum management system specifies such factors as planting date, fertilization, irrigation schedule, tillage, cropping sequence, and pest control.
Soil scientists determine soil productivity ratings of soils for various crops by measuring yields (including tree growth or timber production) over a while for those production uses that are currently relevant.
Included in the measurement of soil productivity is the influence of weather and the nature and aspect of slope, which greatly affects water runoff and erosion. Thus, soil productivity is an expression of all the factors, soil, and nonsoil, that influence crop yields.
For soil to produce high yields, it must be fertile for the crops grown. It does not follow, however, that fertile soil will produce high yields. High yields or high soil productivity depends on optimum management systems.
Many fertile soils exist in arid regions but, within management systems that do not include irrigation, these soils are unproductive for corn or rice.
The concept of soil as a medium for plant growth is an ancient concept and dates back to at least the beginning of agriculture. The concept emphasizes the soil’s role in the growth of plants.
Important aspects of the soil as a medium for plant growth are:
(1) the role of the soil in supplying plants with growth factors,
(2) The development and distribution of roots in soils, and the movement of nutrients, water, and air to root surfaces for absorption.
Soils are productive in terms of their ability to produce plants. The concept of soil as a medium for plant growth views the soil as a material of a fairly uniform composition. This is entirely satisfactory when plants are grown in containers that contain a soil mix. Plants found in fields and forests, however, are growing in soils that are not uniform.
Differences in the properties between topsoil and subsoil layers affect water and nutrient absorption. It is natural for soils in fields and forests to be composed of horizontal layers that have different properties, so it is also important that agriculturists and foresters consider soils as natural bodies.
This concept is also useful for persons involved in the building of engineering structures, solving environmental problems such as nitrate pollution of groundwater, and using the soil for waste disposal.