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How Do the Properties of Soils Affect Plant Growth?

The characteristics of soil play a big part in the plant’s ability to extract water and nutrients. If plants are to grow to their potential, the soil must provide a satisfactory environment for plant growth.

If you think your pastures should be growing more dry matter per hectare than they appear to be, it is important to determine whether the reason for this lack of growth is due to another problem not related to nutrients before you apply fertilisers. The poor pasture production could be due to inherent problems in the soil, such as its texture or structure, salinity, acidity, waterlogging, or compaction. It could be caused by incorrect grazing management. Perhaps this season the soil temperature has been too high or too low for maximum plant growth.

What will you find out in this section?

You should be able to:

• Understand what makes up a soil and some of the terms used to describe soils.
• Understand the terms used to describe a soil profile.
• Understand the physical properties of soil (soil structure and texture) and their importance for plant nutrition and plant growth.
• Understand the chemical properties of soil (cation exchange capacity, pH, salinity and sodicity) and their importance for plant nutrition and growth.

3.1 Composition of soils

Plants obtain most of their oxygen and carbon from the air by photosynthesis ; and hydrogen is obtained, directly or indirectly, from the water in the soil. These three elements together make up over 90 percent of fresh plant tissue. However, plants cannot survive without the much smaller quantity of essential nutrients that they obtain from the soil, such as nitrogen, phosphorus, potassium, calcium, magnesium and sulphur.

Soil also provides the place for roots to anchor and grow. It holds the water in which the soil nutrients are changed into ions, which is the form that the plant can use. It holds the air that prevents the plants from becoming waterlogged. It holds the chemicals that determine your soils pH, salinity and dispersivity.

The things that your soil is made from—your soil’s composition—affect all of these aspects of plant growth. Knowing what soils are composed of will help you understand how soil affects plant growth.
Soils are composed of:

• Inorganic particles
• Organic matter
• Living organisms
• Pore spaces

The many soil types differ from one another because:

• They have different proportions of these components.
• The components are arranged in different ways.
• The inorganic particles have been affected in various ways by weathering, have different sizes and are made up of different minerals.

3.1.1 Inorganic particles

The inorganic portion of soil is made up of small rock fragments and minerals. These inorganic particles are classified by size as gravel or stone, sand, silt, or clay (Table 3.1). The size of the inorganic particles influences soil texture.
Table 3.1  Rock fragments and mineral particles classified by size

Common Name	Size Description	Diameter	Feel
Gravel, stone	Very coarse	Larger than 2 mm	Rocky
Sand	Coarse	0.02 mm to 2 mm	Gritty
Silt	Fine	0.002 mm to 0.02 mm	Smooth, silky
Clay	Very fine	Smaller than 0.002 mm	Sticky, plasticine

This portion of the soil is called ‘inorganic’ because it is made up of chemical compounds (combinations of elements) that form rocks and minerals rather than plant or animal (organic) matter.
The inorganic portion of the soil has formed over many years from unbroken, solid rock (bedrock) found in the earth’s crust. These rocks are classified as:

• Igneous rock (also called volcanic rock), such as granite and basalt, formed from volcanic lava.
• Sedimentary rock, such as limestone, sandstone, mudstone, shale, dolomite and conglomerates, formed from the deposit and cementation of the weathering products of other rocks.
• Metamorphic rock, such as gneiss, schist, quartzite, slate and marble, formed from igneous or sedimentary rocks subjected to high temperatures or pressures.

Weathering of the original bedrock produces parent material for mineral soils. Mechanical or chemical weathering of the bedrock causes fragments to break off. These rock fragments are subject to further weathering and become mineral particles. As the mineral particles continue to weather, they are further decreased in size and also release soluble materials, some of which become plant nutrients (Figure 3.1).

 

Source: Adapted from Buckman and Brady (1960).

Figure 3.1 Trends in weathering conditions that take place under acid conditions common in humid-temperate regions

Mechanical weathering is caused by:

• Temperature changes, such as freezing of the water in a rock or the different rates of expansion of the minerals composing the rock.
• Erosion and deposition from water, ice and wind.
• Plant and animal influences, such as tree roots and lichen.

Mechanical weathering essentially breaks up the bedrock into smaller and smaller pieces and may move it from its place of origin, but it doesn’t change its chemical composition.

Chemical weathering is caused by:

• Hydrolysis.
• Hydration.
• Carbonation and other acidic processes.
• Oxidation.
• The solvent action of the soil solution (water and its soluble salts).

Chemical weathering continues the size reduction of the rock fragments and mineral particles and also changes their chemical composition.
Mechanical weathering also determines whether the parent material is considered to be sedentary or transported (Figure 3.2). Sedentary parent material is either still at its original site above the bedrock from which it was formed (residual soils) or has been moved by gravity down a slope (colluvial soils). Transported parent material has been moved by water (alluvial, marine, or lacustrine soils), ice (glacial soils) or wind (aeolian soils) from its place of origin.

Source: Adapted from Buckman and Brady (1960).

Figure 3.2  Diagrammatic representation of sedentary and transported soils

The parent material that formed your soil affects its soil properties. For example, quartz-based granite will weather into a sandy soil, which will have a lower water-holding and nutrient-holding capacity than a loam or clay soil. Soil formed from limestone may be alkaline (have a high pH) because limestone consists largely of the mineral calcite (CaCO3).

The weathering processes that formed your soil also affect its soil properties. For example, less chemical weathering occurs in arid (low rainfall) regions than in humid (higher rainfall) regions. This means fewer of the minerals in the parent material change to other minerals, fewer soluble materials are formed, and even fewer soluble materials are lost to leaching. This is part of the reason why arid regions often have alkaline soils, and humid regions often have acid soils. It also helps to explain why high-rainfall areas often have soils with poor fertility: many of the nutrients have been chemically weathered and then leached from the soil.

3.1.2 Organic matter

Strictly speaking, organic matter is either anything that is living or the remains of a living thing. However, in the context of soil composition, organic matter is a build-up in the soil of partly decayed plant and animal residues.

Organic soils, such as peats, contain from 20% to as much as 95% organic matter. Mineral soils contain anywhere from a trace to 15% or 20% organic matter.

Australian mineral soils contain up to 10% of organic matter, but most range from 1% to 7%. However, the influence of organic matter on soil properties, and consequently on plant growth, is much greater than this small portion might indicate.

The major roles of organic matter in soil are adding nutrients and improving the soil’s structure and water-holding capacity. Soils with low organic matter have ‘poor’ structure, hold little water, and erode or leach nutrients easily. Soils with high organic matter levels have ‘good’ structure, good water-holding capacity, and reduced erosion and nutrient leaching.

Nutrients are added as the soil micro-organisms break down the organic matter

When the organic matter is fully broken down, one of the things that is left is humus. Much of the humus is formed from the lignin in plants. Lignin is essentially indigestible by living organisms, and this is part of the reason why humus remains in the soil long after the other plant matter has decayed away (for several thousands of years). Humus ranges in colour from brown to black, and the intensity of its colour is influenced by climate (rainfall and temperature) rather than by the amount of organic matter in the soil.

Humus has some of the useful qualities of clay in that it adsorbs nutrients, adsorbs much higher quantities of water than clay can, and improves soil structure due to its low plasticity and good cohesion. Thus, organic matter also plays an essential role in maintaining a loose, friable soil structure.

3.1.3 Living organisms

Many living organisms are found in healthy soil, from large creatures, such as earthworms, to the smallest bacteria (Table 3.2).

Table 3.2 The major groups of organisms commonly present in the soil

Kingdom	Animals		Plants
Size, plant part, or plant type	Macro (large)		Micro (small)	Roots of higher plants, mosses	Algae	Fungi	Actino-mycetes	Bacteria
Source of sustenance or condition of living	Subsisting largely on plant materials	Largely predatory	Predatory or parasitic or subsisting on plant residues	Parasitic or subsisting on plant residues				Autotrophic or hetero-trophic Aerobic or anaerobic
Common names	Small burrowing mammals (rabbits, mice, etc.), insects (springtails, ants, beetles, grubs, etc.), millipedes, slaters, mites, slugs and snails, earthworms	Moles, insects (many beetles, ants, etc.), some mites, spiders, centipedes	Nematodes, protozoa, rotifers		Green, yellow-green and blue-green algae; diatoms	Mushroom fungi, yeasts, rusts, moulds, mildews	Many and varied, thread-like and profusely branched	Many and varied

Source: Adapted from Buckman and Brady (1960).

The larger organisms help to decompose organic matter, and their burrowing habit incorporates the organic matter into the soil and also creates large pore spaces that aerate the soil and allow faster water infiltration. The smaller organisms, such as bacteria, fungi and algae, further decompose the organic matter, which releases nutrients in a form that ‘higher’ plants can use.

Living organisms are an important fraction of the soil, and their presence is encouraged by high organic matter levels, adequate soil moisture, and good drainage and aeration.

In a healthy soil, the domestic animal weight above the ground surface is substantially exceeded by the weight of the organisms living in the soil. For example, the earthworms alone can weigh from 16 kg/ha to more than 1200 kg/ha, depending on the suitability of the soil for earthworm survival. A normal population of soil moulds (a type of fungi) weighs between 1000 and 1300 kg/ha.

How to improve soil biology in your soil.

3.1.4 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.

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 to capillary movement.

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. Plant roots will suffer because water is tightly bound in the small pores, the soil is poorly aerated, and drainage is poor.

3.2 The soil profile

A soil profile is a vertical section through the soil that extends to or into less weathered parent material (see Figure 3.3). Soil profiles are divided into horizons. Each horizon is a layer within the soil profile that has distinct characteristics, such as colour, texture or structure, that differ from the layer above or below it.

 

Figure 3.3  The soil profile

Most soils have a well-defined uppermost layer, or topsoil, referred to as the A horizon. The colour and texture of the A horizon is usually different from the rest of the soil profile. The A horizon is also described as the zone of maximum leaching of elements.

A horizon usually contains the greatest proportion of organic matter, and the mineral portion may be more weathered than the minerals found in lower parts of the profile. The A horizon can range in thickness from 0 to 25 cm or more, depending on whether erosion or other forces, such as laser levelling, have removed the topsoil and on how permeable the soil is. Approximately 80% to 90% of the root mass of ryegrass/white clover is found in the top 8 to 10 cm of the soil profile. If the A horizon is very shallow, this rooting depth is substantially reduced, rendering that soil type vulnerable to drying out very quickly, over drying or possibly over wetting. Also nutrient access by the plant roots will be adversely affected.

The next layer, the subsoil, is referred to as the B horizon. This horizon contains less organic matter and is less weathered than the

A horizon, and it may contain clay particles that have been washed down the profile, making it heavier in texture. Only deep-rooted pasture species, such as phalaris and lucerne, extend their roots into the subsoil, unless the A horizon is very shallow.

The A and B horizons may be subdivided into further layers, which are called the A1, A2 and A3 horizons or the B1, B2, and B3 horizons.

The final layer, or C horizon, is made up of decomposing parent material.

Soils are called gradational, duplex or uniform based on how the soil texture changes from the A to the B horizon. In a gradational soil, the clay content gradually increases, so that the change from the A horizon to the B horizon is indistinct (Figure 3.4a). In a duplex soil, a sharp contrast in texture occurs between the A and B horizons, and the two horizons are easily distinguished (Figure 3.4b). In a uniform soil, the texture change throughout the profile is very small or nonexistent; in general, no textural boundaries can be found in the profile.

3.3 Soil properties

A soil’s properties are largely determined by the parent material and weathering during its formation. Topography and the age of the soil are also important. Agricultural practices can also affect a soil’s properties.

Three soil properties influence plant growth:

• Biological, or the organisms in the soil, such as bacteria, fungi, insects and earthworms (discussed in Section 3.1.3 above).
• Physical, or the texture and structure of the soil.
• Chemical, which affects both the fertility of the soil and its physical properties.

3.4 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, among other things. 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.

3.4.1 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’.

The names of soil textureclasses 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 (Figure 3.5).

Source: Adapted from Buckman and Brady (1960).

Figure 3.5  The proportions of sand, silt and clay in some representative soil texture classes

Soil texture influences many soil physical properties, such as water-holding capacity and water infiltration rates. Coarse-textured sandy soils generally have high infiltration rates but poor water-holding capacity, whereas a fine-textured clay soil generally has a low 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 (see Section 3.5.1).
The texture of a soil can be easily estimated in the field by using the soil texture key (Table 3.3). 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 3.3  Soil texture key

Ball	Feel of Ball when Manipulated	Ribbon Length	Soil Texture Class
Will not form a ball	Single grains of sand stick to fingers	none	Sand
Ball will only just hold together	Gritty feel; organic stain discolours fingers	0 - 0.5 cm	Loamy sand
Ball just holds together	Sticky when wet; sand grains stick to fingers; clay stain discolours fingers	0.5 - 1.3 cm	Clayey sand
Ball just holds together	Very sandy to touch; visible sand grains	1.3 - 2.5 cm	Sandy loam
Ball just holds together	Fine sand can be felt and heard	1.3 - 2.5 cm	Fine sandy loam
Ball holds together strongly	Sandy to touch; sand grains visible	2.0 - 2.5 cm	Light sandy clay loam
Ball holds together	Spongy, smooth feel but not gritty or silky	2.5 cm	Loam
Ball holds together	Slightly spongy; fine sand can be felt	2.5 cm	Loam, fine sandy
Ball holds together	Very smooth to silky feel	2.5 cm	Silt loam
Ball holds together strongly	Sandy to touch; medium-sized sand grains visible	2.5 - 3.8 cm	Sandy clay loam
Ball holds together	Plastic; smooth to touch	3.8 - 5.0 cm	Clay loam
Ball holds together	Plastic; silky to touch	3.8 - 5.0 cm	Silty clay loam
Ball holds together	Fine sand can be felt and heard when manipulated	3.8 - 5.0 cm	Fine sandy clay loam
Ball holds together strongly	Fine to medium sands can be seen and heard	5.0 - 7.5 cm	Sandy clay
Ball holds together strongly	Smooth and silky feel	5.0 - 7.5 cm	Silty clay
Ball holds together strongly	Smooth to touch; slight resistance to forming a ribbon	5.0 - 7.5 cm	Light clay
Ball holds together strongly	Smooth to touch; slightly greater resistance to ribboning than light clay	5.0 - 7.5 cm	Light to medium clay
Ball holds together strongly	Plastic; smooth to touch (can be moulded into rods without fracture); sticky if wet	5.0 - 7.5 cm	Medium clay
Ball holds together	Plastic; handles like plasticine; can be moulded into rods without fracture; smooth; very sticky when wet; firm resistance to ribboning	7.5 - 10 cm	Heavy clay

Source: Northcote (1979).

3.4.2 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), and their large size allows good aeration, rapid infiltration of water, easy plant root penetration, and 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 (Figure 3.6a). A well-structured soil is friable, easily worked and allows germinating seedlings to emerge and to quickly establish a strong root system.

A poorly structured soil has either few or unstable (readily broken apart) aggregates and few pore spaces (Figure 3.6b). A poorly structured soil can result in unproductive compacted (see Section 4.2) or waterlogged (see Section 4.3) soils that have poor drainage and aeration. Poorly structured soil is also more likely to slake (see Section 4.1) and to become eroded.

Figure 3.6 Soil structure

3.5 Chemical properties

The chemical properties of soils that are important to plant growth are:

• Nutrient availability and cation exchange capacity, which affect the soil’s inherent fertility and its ability to hold nutrients.
• The chemical characteristics of the soil solution, which affect pH and salinity.
• The sodicity of the soil, which affects soil stability.

3.5.1 Nutrient availability and cation exchange capacity

In the soil, the larger portion of plant nutrients is bound up in complex compounds that are unavailable to plants. The smaller portion is in simpler, more soluble forms, which are useable by plants. The complex compounds are gradually changed into the simpler compounds by chemical weathering. Thus, the fertility of a soil depends in part on how easily the complex compounds can be changed to the simpler forms. This is referred to as the availability of a nutrient.

Plant nutrients are composed of single elements (for example, phosphorus (P)) or compounds of elements (for example, ammonium nitrate (NH4NO3)). In either case, the nutrients are all composed of atoms.

Most of the soil nutrients that a plant takes up must be in a soluble form (in other words, mixed with water). When an atom is in water, it usually becomes electrically charged and is called an ion. An ion with a positive electrical charge is called a cation. An ion with a negative electrical charge is called an anion. Cations include sodium (Na+), potassium (K+), calcium (Ca++), magnesium (Mg++) and aluminium (Al+++). Anions include chloride (Cl-), nitrate (NO3-), sulphate (SO4--), carbonate (CO3--) and borate (BO2--).

One plus sign or one minus sign means an ion has one positive or negative electrical charge.

Two or more plus or minus signs means an ion has two or more positive or negative charges.

More positive charges means an increasing ability to bond with a negatively charged surface.

More negative charges means an increasing ability to bond with a positively charged surface.

The cations and anions can be:

• Absorbed (taken up) by plant roots.
• Leached from the soil via the soil water.
• Adsorbed (attached) to the surfaces of negatively and positively charged soil particles.

The soil’s capacity to adsorb nutrients in the form of cations is called its cation exchange capacity. Cation exchange capacity is measured by a soil test .

The cations are held on the surface of soil minerals and organic matter and within the crystalline framework of some clay minerals. The greater the surface area available to adsorb cations, the higher the soil’s inherent fertility. Thus, soil texture has an effect on soil fertility because of the sizes of the particles that make up the various soil texture classes, and so does the amount of organic matter (Table 3.4).

Table 3.4 Surface area of soil particles and organic matter

Particle	Square cm per gram
Coarse sand	23
Fine sand	90
Very fine sand	230
Silt	450
Clay	around 8,000,000
Organic matter (humus)	around 8,000,000

Source: CSIRO (1979).

As you can see, soils with a high clay or organic matter content provide a much greater surface area for cations to adsorb onto.

As long as the nutrient cations and anions are adsorbed onto the soil particles, they cannot be absorbed by plants or leached from the soil. However, they are not held too tightly and can be exchanged with other ions of a like charge that are in the soil solution. Once the nutrients are in the soil solution, they can be absorbed by the plant’s roots or lost to leaching.

3.5.2 The soil solution

Soil water is the water (H2O) held within the soil pores. Soil solution is the soil water together with its dissolved salts (cations and anions). The soil solution is the medium by which most soil nutrients are supplied to growing plants. It also has a role in soil salinity and pH.

Soil salinity

Soil salinity is an increased concentration of salts in the soil solution. In general, as soil moisture is reduced, especially by evaporation, the concentration of soluble salts of sodium, calcium, magnesium, and potassium in the soil solution increases. These salts may already be present in the soil solution or they can be carried upward from the ground water by capillary action if the watertable rises.
The concentration of soluble salts can become so high as to interfere with the growth of plants. Soils that have a salt concentration in the surface soil that is sufficient to interfere seriously with the growth of plants are called saline soils.
Salinity can occur on dryland farms and on irrigated farms. The salinity that occurs is the same in either case, only the initiating causes and management methods may be different and is measured in soil tests.

Soil pH

The soil solution can be neutral, acid, or alkaline. This is called the soil pH. The pH measures the concentration of positively charged hydrogen ions (H+) in the soil solution on a logarithmic scale ranging from 0 to 14. When a soil solution contains more H+ ions, it is acidic. When there are fewer H+ ions, the soil solution is alkaline.

The level of acidity or alkalinity in a soil affects the availability of soil nutrients and the activity of soil micro-organisms and can affect the level of exchangeable aluminium.

Information on managing acidity and alkalinity

Information on pH as measured on a soil test

3.5.3 Sodicity

The sodicity of the soil refers to the amount of exchangeable sodium cations compared to other cations adsorbed onto the soil. A soil with 6% or more of its exchangeable cations as sodium is called a sodic soil.

Excessive exchangeable sodium can cause clay particles to disperse when in contact with water. A typical sign of dispersion is the blue-grey puddles found in winter in the older basalt areas around lake margins and where drainage is poor.

Sodic soils have poor structure and disperse readily when wet. Seedlings have difficulty penetrating a drying dispersed surface, and their roots have difficulty penetrating into the soil, with consequent poor germination and survival.

The dispersion is caused by weak positive charges, such as sodium, and responds to gypsum application, which replaces the sodium ions with calcium ions.

Moving traffic on and grazing these soils while wet can make the situation worse. Read more information on managing slaking and dispersion

There are examples of sodic soils scattered throughout the dairy areas of south west Victoria, the northern irrigation areas and very occasionally in Gippsland.

3.6 Summary

• Soils are composed of weathered minerals, organic matter, living organisms and pore spaces.
• A soil’s texture describes the amount of sand, silt and clay particles in the soil.
• A soil’s structure is determined by the size and arrangement of aggregates and pores.
• A healthy soil is stable and friable and contains a reasonable level of organic matter and a large and varied population of soil organisms.