Chemically healthy soil?

Chemically healthy soil?

1 June 2021

Following on from last month’s article on soil physical health, Ballance Agri-Nutrient Nutrient Dynamics Specialist Jim Risk looks at soil chemical health.

What is ‘chemically healthy’ soil?

Chemical health refers to the amounts of plant available essential nutrients in soil.

Sixteen elements are essential for plant growth and development, including macronutrients (needed in greater quantities) such as nitrogen, phosphorus, potassium, magnesium and sulphur, and micronutrients (needed in smaller quantities) such as molybdenum and boron.

Plants can obtain three essential elements – carbon, hydrogen and oxygen – from soil water or the atmosphere. Many micronutrients, including iron, manganese and boron, are normally available in the soil, but some, such as molybdenum, may need to be added.

Production removes the major macronutrients from the soil, so these need regular replacement. Dairy production, for example, removes nutrients including phosphorus, potassium and sulphur.

In addition to removal of nutrients, other factors, such as soil’s capacity to hold nutrients, can affect their levels.

Most soil particles are negatively charged, and attract positively charged ions (cations), such as potassium, magnesium, calcium and sodium. Soil’s capacity to hold these cations is measured by its cation exchange capacity (CEC), which reflects the amount of negative charge present in the soil. This is largely determined by the ratio of sand, silt and clay, as well as by the amount of soil organic matter.

Conversely, soil’s capacity to bind negatively charged ions (anions) such as phosphate and sulphur is measured by its anion storage capacity (ASC).

While soil’s ability to hold macronutrients is a key factor in its chemical health, so too is its acidity. Is soil acidity optimal for the soil microorganisms responsible for converting non-plant available forms of nutrients into plant available forms? Soil acidity (pH) affects the range, survival and functioning of soil microorganisms, and a pH of around 6.0 is ideal.

Soil acidity can also impact on availability of essential micronutrients and aluminium, which at high levels impacts plant growth. When pH is managed within normal ranges aluminium is usually not an issue for plant growth, and in most situations sufficient micronutrients are available in the soil.

Why is soil’s chemical health important?

It’s known that pasture does better if the right nutrients are available. For example, the longest fertiliser trial under pasture in New Zealand, which started in 1952 at Winchmore Research Station in the Canterbury Plains, has shown the positive effect of phosphate fertiliser application on pasture production. In turn, stock performance also improves.

What can farmers do to optimise soil’s chemical health?

Test soil regularly – Regularly testing soil provides a reliable status of nutrient levels and pH, thus supporting the application of nutrients necessary for optimal production, avoiding both over- and underapplication.

For soil test results to be useful, they need to be directly related to the productive capacity of the land and matched against optimum pasture and crop production in New Zealand. For example, the Olsen P test has been extensively calibrated in New Zealand conditions and results provides valuable information to achieve an optimal phosphorus level for supporting pasture growth.

Maintain soil organic matter – Maintaining soil organic matter supports the soil’s ability to hold macronutrients, and also to supply nutrients such as nitrogen from mineralisation of organic matter.

In some environments, increasing soil organic matter over time can increase the number of cation exchange sites, and thus the soil’s CEC. ASC, on the other hand is fixed, and doesn’t change over time except in peat soils, where it increases as they progress from raw to developed.

Maintain soil fertility or correct low fertility – Ensure no single nutrient is limiting pasture production. Optimal fertility supports productive pastures and good levels of soil organic matter, thus providing a healthy environment to support the range of soil microorganisms within the soil.

Keep an eye out for an article on soil’s biological health in the next Enrich magazine.

Article supplied by Ballance Agri-Nutrients.