Introduction to the Physical Properties of Soils

Hello : ) In today's post we'll begin looking at the physical properties of soils. We'll discuss the components of soil texture, what a soil profile is, the importance of soil structure, as well as what factors lead to water repelling soils. 

Before we begin, you may be wondering why veterinary students have to learn about soils. Well, it turns out that many of the health problems in animals arise because of soil health issues. For example, a mineral deficiency in a soil may lead to mineral deficiencies in animals which eat the pasture grown from that soil.

It's also helpful to have a bit of background knowledge about the soils that can be found in Australia. Because Australia is quite an old continent our soils tend to be old, salty, clayey, nutritionally and organically impoverished as well as structurally challenging. In addition, not much new soil is produced so it is important that soil erosion is kept to a minimum. 

Soil Texture

Texture refers to a description of the proportions of sand, silt and clay in the soil. Sand includes particles which are between 0.02 - 2mm in diameter. Silt, also known as loam includes particles from 0.002-0.02mm in diameter. Clay includes particles which are less than 0.002mm in diameter. The texture of a soil influences several things including:

• the supply of air in the soil
• the availability and movement of water and nutrients
• the ease of root growth
• erosion potential
• organic matter level. 

Clay soils retain more moisture and nutrients than sands but a greater amount of water is unavailable to plants. These soils are also prone to water logging and compaction from livestock and machinery.

 Sandy soils have a poor ability to retain moisture and nutrients but they allow plants to extract these with little effort. However, these soils are prone to nutrient leaching but the retention of nutrients can be improved by increasing the amount of organic matter present and incorporating clay. They are also prone to the development of water repellency but the addition of clay can prevent this.

Soil Profile

Soil is made up of various horizontal layers known as horizons. There are three horizons:

1. A Horizon or Topsoil: most of the available plant nutrients and soil organisms are here. The upper part of this horizon is often darker because of a higher organic matter content. 
2. B Horizon or Sub-soil: this contains materials leached from the A horizon. The depth and water holding capacity of this layer greatly affects the value of the soil.
3. C Horizon or Parent Material: this is either rock or partly decomposed sand or clay deposited thousands of years ago. The ability of roots and water to travel through this layer has a large impact on plant growth.

 Soil Structure

 This refers to how soil components are arranged into aggregates. This is important for the permeability of the soil to water and air as well as root penetration and seedling emergence and the resistance to erosion. Aggregates are formed from a combination of sand, loam, clay organic mater and components of fungi. The more clay that is present, the greater the bond between the particles and the structure improves. Soil structure is important because good structure leads to an abundance of soil pores which allow the movement of water, air and microbes and provide a minimal resistance to root growth.

Water Holding Capacity

Soil can hold water in three ways:

• Chemical water: this is water which is tightly held by electrostatic forces to clay surfaces and is unavailable to plants. 
• Gravitational Water: this is held in large soil pores and rapidly drains out of the soil under gravity. It can only be used by plants while it is present. 
• Capillary water: this is water held in pores that are small enough to hold water against gravity. The smaller the pore, the harder it is to remove the water.

After the soil has been saturated and all the gravitational water has drained, the soil is at field capacity. When the plants have used up all the accessible water from the soil it is at wilting point. The structure and texture of soils affect the amount of water that is held in the soil and that can be used by plants. Smaller soil particles have a higher surface area that larger particles and the amount of water absorbed by the soil increases as the surface area does too. The leaves less water available to the plants. 

Water Repellence

Some soils can become water repellent and water is unable to infiltrate the soil. This is due to a hydrophobic material, derived from the decomposition of plants, which may coat the soil particles. Soils with lower clay amounts are more susceptible to water repellence. Water repellence leads to patchy pasture germination - some of the areas of the land have pasture growing on them while others don't. This leads to a loss in production for the farm.

That's all for this post, if you have any questions please feel free to ask in the comments section below : )