This section contains six questions.
(Questions 2-6 were adapted from the book: Hunt N and Gilkes B (1992). Farm Monitoring Handbook. The University of Western Australia.)
Physical properties and processes of soil affect soil fertility by altering water movement through soil, root penetration of soil and waterlogging. Important physical properties that affect fertility include soil structure and texture. Structure is the amount of aggregation and pores in soil and texture is the proportion of clay and sand particles in soil. Both affect soil fertility by affecting water movement through soil, root penetration and water logging. Erosion is an important physical process that decreases soil fertility. When soil structure and texture are unfavourable for water movement through soil water erosion and waterlogging may be increased. Soil salinity is a chemical property but can affect soil physical fertility by decreasing the movement of water through the soil.
Physical soil characteristics important to soil physical fertility include:
Physical processes related to soil physical fertility include:
Soil texture is an approximation of the relative quantities of sand, silt and clay particles in a soil. Soil structure is a measure of the arrangement of these soil particles and the spaces between them. Soil structure is somewhat dependent on soil texture.
Good soil structure is one of the major factors for soil health and therefore, sustainable soil fertility. Good soil structure is present when the soil forms stable aggregates or cohesive groups of particles. This produces numerous pore spaces, which encourage root penetration and easy passage of water, nutrients and air and which also assist the growth of microorganisms.
There are two main types of structureless or non-structured soil:
There are four main types of soil structure:
Soil texture is not easily changed whereas soil structure can degrade or improve very quickly through various agricultural practices. The tendency of the soil structure to become unstable is related to soil type, texture (finer texture – higher tendency), water content and soil chemistry. Some soil chemistry factors that adversely affect soil structure include soil sodicity, acidity and salinity. The decline of soil structure will exacerbate the decline in soil health and fertility. Soil pores become smaller or less numerous which restricts water, air and nutrient movement. Therefore porosity, drainage and plant root growth are reduced. This can lead to an increase in either soil density or structural instability or both, especially in clayey soils. Sometimes a surface crust may form, inhibiting seedling growth, preventing water penetration and increasing erosion.
The good news is that all these changes are reversible. The actions required to improve soil structure depend on the individual soil conditions including stability of the soil structure. There are various options to improve soil structure, including some physical and chemical techniques such as: