Guideline Index

Chapter 4: Soil Properties

4.3 Chemical properties

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

4.3.1 Nutrient availability and cation exchange capacity

In the soil, a large portion of plant nutrients are 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 and biological processes. Thus, the chemical fertility of a soil depends in part on how easily the plants can access the nutrients in a form they require. This is referred to as the availability of a nutrient.

The availability of nutrients within the soil is also dependent on a range of factors such as soil pH, soil solution, soil type and the plant age, type and root system of the plant.

Plant nutrients are composed of single elements (for example, potassium (K)) or compounds of elements (for example, ammonium nitrate (NH4NO3)). In all cases, the nutrients are all composed of atoms.

Mineral nutrients are absorbed by plants from the soil solution as ions. An ion is an electrically charged particle formed by the removal or addition of electrons from an atom or molecule. 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–), phosphate (H₂PO₄-) and boric acid (BO₃—).

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.

Phosphorus availability is greatly influenced by adsorption reactions with calcium, aluminium, iron, manganese and reactive surfaces of certain clay minerals. These reactions can ‘fix’ the phosphorus and make it less available to plants. The degree of fixation depends on pH. In alkaline soils the phosphorus will combine with calcium, and in acid soils the phosphorus will combine with iron and aluminium, and in both cases less phosphorus is available to the plant.

Cations and anions are not equally held by the soil particle. More positive charges mean an increasing ability to bond with a negatively charged surface. More negative charges mean an increasing ability to bond with a positively charged surface. The order of strength of adsorption is; Al³⁺> H⁺> Ca²⁺> Mg²⁺> K⁺> NH₄> Na⁺. For example, plant root cells can secrete H⁺ ions that can displace weaker ions like K⁺ which then are available for plants to take up.

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 – See Figure 4.5). Cation exchange capacity is measured by a soil test which is discussed further in Chapter 9.2.9.


Figure 4.5  Illustration of cation exchange processes.  Source: Fertiliser Industry Federation of Australia 2006, pg. 6.
Figure 4.5 Illustration of cation exchange processes. Source: Fertiliser Industry Federation of Australia 2006, pg. 6.

T he 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 – See Table 4.2. Please note, on the edge of some clay minerals there is also a positive charge, which attracts and holds anions.

Table 4.2   Surface area of soil particles and organic matter.  Source: CSIRO (1979).
Table 4.2 Surface area of soil particles and organic matter. 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, unless the whole clay particle is carried away via erosion. However, they are not held too tightly and can be exchanged with other ions of a like charge that are in the soil solution. Within these exchanges some cations (such as Ca²⁺) are held more tightly than other cations such as Na⁺ and Mg²⁺. Once the nutrients are in the soil solution, they can be absorbed by the plant’s roots, used by soil biology or lost to leaching.

4.3.2 The soil solution

Soil water is the water 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 – See Section 4.2.4. 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 plant root zone that is sufficient to interfere seriously with plant growth 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. See Chapter 7.5 for information on managing salinity and Chapter 9.2.10 for information on salinity as measured by a soil test. 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 [i.e., more hydroxyl (OH⁻) 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. See Chapter 7.6 for information on managing acidity and alkalinity, Chapter 9.2.4 for information on pH as measured on a soil test and Chapter 5 for information on soil micro-organisms.

4.3.3 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 – See Chapter 7.2. 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, with consequent poor germination and survival.

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

Traffic on and grazing these soils while wet can make the situation worse. – See Chapter 7.2 for information on managing slaking and dispersion.