Physical, Chemical and Biological Attributes of Agricultural Soils

Keywords: Soil mineral matter, soil organic matter, soil structure, plant nutrient uptake, root growth, assessing soil structure.

Crop yields are affected by many different factors above and below ground and the way they interact. Below ground, limitations to the freedom of the plant to develop a root system to its full potential are likely to result in an adverse effect on crop performance and will limit the efficient use of available resources. Water and nutrient uptake to achieve acceptable yields depends on the root system exploiting the available soil effectively. Soil texture, determined by the proportions of sand, silt and clay in the topsoil is a relatively stable feature of any soil but soil structure is, to a large extent, within the control of the farmer and grower and some of the factors that affect soil structure are discussed here. The mineral composition of soil is discussed in relation to the physical and chemical properties of soil; principle among these is the clay fraction. The mineral components of the clay largely control the inherent chemical properties, especially for the retention of nutrient anions, like phosphorus, and cations, like potassium and magnesium. Soil organic matter also has cation and anion exchange sites that retain nutrients in the soil.

Soil is a porous fabric made up of mineral and organic materials and a good soil structure depends on how these materials are organised into crumbs or peds such that the inter-particle spaces can hold both air and water. The oxygen in the soil air is essential for respiration of both roots and other living organisms within the soil mass. Water, the soil solution, contains dissolved organic and inorganic solutes, many of the latter are plant nutrients. This review discusses some of the biological, chemical and physical properties of soils that affect the root systems of plants and their functions related to crop yields.

A E Johnston, Soil Science Dept., Rothamsted Research, Harpenden AL5 2JQ, UK.

C J Dawson, Chris Dawson & Associates, Strensall, York YO32 5TD, UK.

40 pages, 18 figures, 11 tables, 34 references.