Soil Physical Fertility

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.)

1. What is soil physical fertility?
2. What are soil texture and soil structure?
3. How do soil structure and soil texture affect soil health and fertility?
4. What are the main types of soil structure?
5. How are soil texture and soil structure affected by management?
6. How can management improve soil structure?

 

Question 1. What is soil physical fertility?

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:

• soil structure,
• soil texture,
• water repellence.

Physical processes related to soil physical fertility include:

• aggregation,
• water infiltration,
• waterlogging,
• soil erosion.

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Question 2. What are soil texture and soil structure?

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.

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Question 3. How do soil structure and soil texture affect soil health and fertility?

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.

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Question 4. What are the main types of soil structure?

There are two main types of structureless or non-structured soil:

• Single grain – like sands, and
• Massive – like compacted clays.

There are four main types of soil structure:

• Crumb structure – which has small rounded aggregates of soil particles loosely adjoining other aggregates. This soil is therefore porous and perme- able, yet retains moisture. It is the most ideal soil structure.
• Prismatic structure – which forms aggregates in columns with flat tops and separated by deep cracks. Aggre- gates usually form larger units called clods. Permeability is variable – better around the deep cracks and poorer inside columns.
• Blocky structure – aggregates are blocky and soil is moderately permeable.
• Platey structure – flat horizontal laminated aggregates like alluvial floodplain soil. Drainage and permeability are poor.

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Question 5. How are soil texture and soil structure affected by management?

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.

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Question 6. How can management improve soil structure?

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:

1. Maintaining continued plant cover on land by using appropriate stocking rates.
2. Using cultivating discs to elevate dispersive subsoils to the surface.
3. Deep ripping of compacted soils or layers.
4. Using minimum tillage practices especially when soil is wet.
5. Retaining stubble and green manuring to increase organic content and reduce compaction and erosion.
6. Applying gypsum on sodic soils. 

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