Physical Properties
The physical properties of soil describe the composition of particles within a soil. At a simple level we can describe the composition of sand, silt and clay based on the texture of a soil when rubbed between the fingers or when separated into different layers when mixed with water. These mineral particles have a dramatic impact on the properties of the soil.
Soil Texture
Soil texture is the particle size distribution of the solid inorganic or mineral part of the soil i.e. the proportion of sand, silt and clay in a soil. Texture normally refers to the particles less than 2 mm in diameter.
Mineral components (clay, silt and sand) are given the following size divisions in New Zealand (Milne et al. 1991):
Particles that are larger than 2 mm in diameter, and organic matter are usually omitted from textural names unless they are present in sufficient quantity to affect soil behaviour. In this case, they will be incorporated in the textural class name. For example, a soil with a large quantity of gravel might be named a gravelly sandy loam. Soils with large quantities of organic matter are called peats or humic soils.
As you can see from the above size categories, clay is much smaller than either silt or sand particles. In terms of relative size, a clay particle sitting on the surface of a silt particle on the top of a sand particle is like a bus ticket on a tennis court on a football field. This size difference is an important point – one that helps explain a great deal of the differences in soil behaviour, and one that will be constantly referred to throughout the course.
The texture of a soil has a profound influence on its characteristics and behaviour. There are three main factors: surface area, surface charge and particle shape. As the particle size decreases, the surface area of a set soil volume increases. This means an increase in the ‘reactive’ ability of the soil i.e. it is more likely to bond to other soil particles (for an increase in soil cohesion), and to retain water and nutrients.
Texture will influence the way a soil behaves: there will be numerous examples of this throughout the course. Below are some key examples of soil properties that are directly related to soil texture.
Soils of intermediate texture e.g. soils containing 15 – 30% clay, 15 – 30% silt and 40 – 55% sand (loams, sandy clay loams and sandy loams) will usually present relatively few problems for cultivation, drainage or ability to store availability of water and nutrients.
Mineral components (clay, silt and sand) are given the following size divisions in New Zealand (Milne et al. 1991):
- Clay – < 0.002 mm in diameter
- Silt – 0.002 to 0.06 mm in diameter
- Sand – 0.06 to 2.00 mm diameter.
Particles that are larger than 2 mm in diameter, and organic matter are usually omitted from textural names unless they are present in sufficient quantity to affect soil behaviour. In this case, they will be incorporated in the textural class name. For example, a soil with a large quantity of gravel might be named a gravelly sandy loam. Soils with large quantities of organic matter are called peats or humic soils.
As you can see from the above size categories, clay is much smaller than either silt or sand particles. In terms of relative size, a clay particle sitting on the surface of a silt particle on the top of a sand particle is like a bus ticket on a tennis court on a football field. This size difference is an important point – one that helps explain a great deal of the differences in soil behaviour, and one that will be constantly referred to throughout the course.
The texture of a soil has a profound influence on its characteristics and behaviour. There are three main factors: surface area, surface charge and particle shape. As the particle size decreases, the surface area of a set soil volume increases. This means an increase in the ‘reactive’ ability of the soil i.e. it is more likely to bond to other soil particles (for an increase in soil cohesion), and to retain water and nutrients.
Texture will influence the way a soil behaves: there will be numerous examples of this throughout the course. Below are some key examples of soil properties that are directly related to soil texture.
- Water movement: Water tends to move much more quickly through coarse textured soils than it does through fine textured soils.
- Aeration: Fine textured soils are often poorly drained and have poor aeration.
- Water retention: Plants growing in sandy soils are often at risk due to dry or drought conditions.
- Nutrient availability: Sandy soils often have relatively small reserves of some key plant nutrients.
- Ease of tillage: Clay soils are often difficult to cultivate (heavy) both in terms of the number of field operations required to prepare a seedbed, and the need to carefully time this field work.
- Susceptibility to compaction: Intermediate and fine textured soils are prone to soil compaction.
- Erosion: Soil particles that do not cohere well are at risk to erosion by wind and water.
Soils of intermediate texture e.g. soils containing 15 – 30% clay, 15 – 30% silt and 40 – 55% sand (loams, sandy clay loams and sandy loams) will usually present relatively few problems for cultivation, drainage or ability to store availability of water and nutrients.
How Composition Affects Soil Properties
A soil with a high proportion of sand allows water to drain through it and has good air circulation, but will also dry out quickly. While a soil high in clay has small mineral particles tightly packed together, resulting in poor water drainage and making it hard for air to circulate within it. A loam soil has a fairly even proportion of clay, silt and sand and allows good air circulation while also retaining water. Loam soil make the best agricultural soil type. The soil pyramid is used to classify a soil depending upon the proportion of sand, silt and clay it contains.
At a more advanced level we can also describe its structure, density, porosity, consistency, and colour of soil. Soil structure refers to the way in which soil particles are grouped or bound together to form lumps or aggregates. There are two main types of soil structure, single grained and compound structure. The structure of soil can be changed, modified and improved or damaged depending on the various soil management practices adopted like tillage, manuring, liming, rotation of crops, irrigation, drainage etc.
In between the particles there are empty spaces which are occupied by air and water and are termed as pore spaces. There is a higher percentage of pore spaces in sandy soils and these soils are free draining and never remain water logged.
Sandy soils have these properties to a much lesser degree than fine texture clay soils. Soils also have various shades of black, yellow, red and gray colours useful in soil classification. Soil colour is indirectly helpful in indicating many other properties of soils e.g. a dark brown or black coloured soil indicates its high organic matter content and fertility. A red or yellowish soil shows good aeration and proper drainage. A white or black colour due to accumulation of certain salts of alkali indicates deterioration of soil fertility and its unsuitability for normal growth of many crops.
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At a more advanced level we can also describe its structure, density, porosity, consistency, and colour of soil. Soil structure refers to the way in which soil particles are grouped or bound together to form lumps or aggregates. There are two main types of soil structure, single grained and compound structure. The structure of soil can be changed, modified and improved or damaged depending on the various soil management practices adopted like tillage, manuring, liming, rotation of crops, irrigation, drainage etc.
In between the particles there are empty spaces which are occupied by air and water and are termed as pore spaces. There is a higher percentage of pore spaces in sandy soils and these soils are free draining and never remain water logged.
Sandy soils have these properties to a much lesser degree than fine texture clay soils. Soils also have various shades of black, yellow, red and gray colours useful in soil classification. Soil colour is indirectly helpful in indicating many other properties of soils e.g. a dark brown or black coloured soil indicates its high organic matter content and fertility. A red or yellowish soil shows good aeration and proper drainage. A white or black colour due to accumulation of certain salts of alkali indicates deterioration of soil fertility and its unsuitability for normal growth of many crops.
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Determining Composition of Soil
A float test to determine mineral composition of soils. Soil consistency is a combination of properties that determine the resistance of the dry soil to crushing or pulverising action by implements and when wet its ability to be moulded or changed in shape. All soils have cohesive and adhesive properties.