Guideline Index

Chapter 4: Soil Properties

4.2 Physical properties

Physical properties of a soil that affect a plant’s ability to grow include:

  • Soil texture, which affects the soil’s ability to hold onto nutrients (cation exchange capacity) and water. Texture refers to the relative distribution of the different sized particles in the soil. It is a stable property of soils and, hence, is used in soil classification and description.
  • Soil structure, which affects aeration, water-holding capacity, drainage, and penetration by roots and seedlings. Soil structure refers to the arrangement of soil particles into aggregates (or peds) and the distribution of pores in between. It is not a stable property and is greatly influenced by soil management practices.

4.2.1 Soil texture

Soil texture, or the ‘feel’ of a soil, is determined by the proportions of sand, silt, and clay in the soil. When they are wet, sandy soils feel gritty, silty soils feel smooth and silky, and clayey soils feel sticky and plastic, or capable of being moulded. Soils with a high proportion of sand are referred to as ‘light’, and those with a high proportion of clay are referred to as ‘heavy’.

Soil texture classes

The names of soil texture classes are intended to give you an idea of their textural make-up and physical properties. The three basic groups of texture classes are sands, clays and loams.

A soil in the sand group contains at least 70% by weight of sand. A soil in the clay group must contain at least 35% clay and, in most cases, not less than 40%. A loam soil is, ideally, a mixture of sand, silt and clay particles that exhibit light and heavy properties in about equal proportions, so a soil in the loam group will start from this point and then include greater or lesser amounts of sand, silt or clay.

Additional texture class names are based on these three basic groups. The basic group name always comes last in the class name. Thus, loamy sand is in the sand group, and sandy loam is in the loam group (see Figure 4.2).

Figure 4.2  Soil Texture Triangle. Source: Image adapted from Hunt and Gilkes (1992)  https://s3.amazonaws.com/soilquality-production/fact_sheets/28/original/Phys_-_Measuring_Soil_Texture_in_the_Lab_web.pdf
Figure 4.2 Soil Texture Triangle. Source: Image adapted from Hunt and Gilkes (1992)
https://s3.amazonaws.com/soilquality-production/fact_sheets/28/original/Phys_-_Measuring_Soil_Texture_in_the_Lab_web.pdf

P article size distribution can be determined by laboratory analysis, with the results shown in percentages. The texture is determined by drawing lines from the percentage point on the relevant axis parallel to the side of the triangle at the zero end of the same axis. Where the 3 lines intersect indicates the soil texture. A soil with 40% silt, 30% clay and 30% sand is a silty clay loam – See the red lines on Figure 4.2.

Soil texture influences many soil physical properties, such as water-holding capacity and drainage. Coarse-textured sandy soils generally have high infiltration rates but poor water holding capacity. Silt particles are much smaller than sand, have a greater surface area, and are generally quite fertile. Silts do not hold as much moisture as clay soils, however more of the moisture is plant available. Fine-textured clay soil generally has a lower infiltration rate but a good water holding capacity.

Soil texture also influences the soil’s inherent fertility. More nutrients can be adsorbed by a gram of clay particles than by a gram of sand or silt particles, because the clay particles provide a much greater surface area for adsorption. Clay is the active part of the soil. It is where soil nutrients are held and largely from where they are exchanged. The clay fraction also has a large effect on soil structural stability, and therefore erosion risk. See Section 4.3.1 Nutrient availability and cation exchange capacity for more information.

The texture of a soil can be easily estimated in the field by using the soil texture key – See Table 4.1. First, knead a small handful of soil into a ball about 4 cm in diameter, after removing any stones and plant material. Then slowly wet the soil and mould or press it into a ribbon between your thumb and forefinger. The length of the ribbon and the properties of the ball let you estimate the soil’s texture class.

Table 4.1   Soil characteristics indicative of soil texture.  Source: Euroconsult 1989, McDonald et al 1990 cited in Moody & Cong 2008.
Table 4.1 Soil characteristics indicative of soil texture. Source: Euroconsult 1989, McDonald et al 1990 cited in Moody & Cong 2008.

4.2.2 Soil structure

Soil structure refers to the arrangement of soil particles (sand, silt and clay) and pores in the soil and to the ability of the particles to form aggregates.

Aggregates are groups of soil particles held together by organic matter or chemical forces. Pores are the spaces in the soil.

The pores between the aggregates are usually large (macropores). Their large size allows good aeration, rapid infiltration of water, easy plant root penetration, good water drainage, as well as providing good conditions for soil micro-organisms to thrive. The smaller pores within the aggregates or between soil particles (micropores) hold water against gravity (capillary action) but not necessarily so tightly that plants cannot extract the water.

A well-structured soil forms stable aggregates (aggregates that don’t fall apart easily) and has many pores of varying sizes – See Figure 4.3a. A well-structured soil is friable, easily worked and allows germinating seedlings to emerge and quickly establish a strong root system.

A poorly structured soil has either few or unstable (readily broken apart) aggregates and few pore spaces – See Figure 4.3b. A poorly structured soil can result in unproductive, compacted or waterlogged soils that have poor drainage and aeration. Poorly structured soil is also more likely to slake and to become eroded.

Figure 4.3   Soil structure
Figure 4.3 Soil structure

4.2.3 Pore spaces

The spaces between soil particles (clay, silt, and sand) and between and within aggregates (clusters of soil particles) are called pore spaces. They are the portion of the soil occupied by air and water. Water displaces air in the soil, and consequently the air content of a soil is inversely related to the water content. High water content in soils means there is less air within the soil. This results in higher levels of carbon dioxide and lower levels of oxygen within the soil which is not favourable for plant growth. These conditions also favour denitrification, the biological process that converts nitrate-nitrogen to the greenhouse gas, nitrous oxide.

Soil air differs to atmospheric air as the composition is more variable within the soil, can be more humid and has a higher carbon dioxide and lower oxygen content than the atmosphere.

The number and size of pore spaces are determined by the size of the soil particles (soil texture) and the arrangement of the soil particles into aggregates (soil structure). The larger pores (macropores) allow air and percolating water to move easily through the soil. The smaller pores (micropores) don’t allow air to move easily and also largely limit water movement.

Soil biology also plays a role in helping to bind soil. An example of this is the secretions of glomalin from arbuscular mycorrhizal fungi – See Chapter 5 for further information. A sandy soil may have insufficient organic matter to bind the sand grains into larger aggregates. In this case, the soil will have many large pore spaces and very few small pores. The plant roots will have plenty of air, but water will drain freely through the soil with very little storage. On the other hand, a compacted, heavy clay soil will have many small pores and few large pores. Plants suffer as water is so tightly bound in the small pores that plant roots are unable to extract it from the soil. The soil is poorly aerated, and drainage is poor. Consequently, the oxygen is exhausted.

4.2.4 Soil water

Water within the soil strongly influences plant growth and the biological functioning of the soil. It provides a medium for substances to dissolve into, including nutrient elements, allowing them to be accessible to plant roots. Water also enables nutrients to be transported off the farm, and contributes to erosion and weathering processes. The soil texture influences how water is held within the soil and also the rate that water will infiltrate the soil – See Section 4.2.1.

KM.4.003a

Too much water

When all the soil pores fill with water during rainfall or irrigation the soil can become saturated or waterlogged. Plants require both air and water within the soil. When a soil is waterlogged, especially for periods longer than a couple of days, plants can suffer. Plants require oxygen to respire and produce energy, without this they can’t grow. When soils are waterlogged fertiliser application should be avoided.

KM.4.003b

Too little water

As the soil dries out, the soil particles (particularly clay) tend to hold onto water more tightly than the plant is able to extract water. Therefore water is held in the soil with increasing strength as soil dries out. At this point, when the plant is unable to extract enough water it wilts and doesn’t recover. This is called the wilting point or the lower extractable limit.

Illustrations above adapted from Food and Agriculture Organisation of the United Nations 1985Illustrations above adapted from Food and Agriculture Organisation of the United Nations 1985

The right balance of air and water

Just after the soil has been saturated and starts to drain, the large pore spaces have air again and there is ample water available for plants. This is when the soil is at field capacity. Field capacity varies depending upon soil texture. Once plants have used up the water that’s readily available, the soil reaches refill point. The soil moisture level between the refill point and field capacity is called the readily available water (RAW). RAW is the water that plants can easily extract from the soil, and is also the level that irrigators aim to maintain, unless they are intentionally stressing plants. Figure 4.4 shows that sandy soils require less water before the water is available to plants compared to clay soils which require wetting up before water is available to plants.

 

Figure 4.4   Relationship between soil texture and water availability.  Source: Fertiliser Industry Federation of Australia 2006 pg.4
Figure 4.4 Relationship between soil texture and water availability.
Source: Fertiliser Industry Federation of Australia 2006 pg.4