Words and image supplied by Ballance.
Healthy soil carries out many functions related to agricultural production and environmental quality, such as nutrient retention and cycling, carbon sequestration and storage, and water infiltration and storage. It is also the base of the food web, and provides habitat for culturally significant species and food sources.
So given its critical importance, what exactly is ‘healthy soil’?
The meaning of ‘healthy soil’ depends largely on land use, but it's commonly defined as ‘the continued capacity of the soil to function as a vital, living ecosystem that sustains plants, animals and humans’1.
Soil is formed by the interaction of rock, air, water and living things. Along with the interplay of these factors, human management also plays a part in determining its health.
Three broad, interrelated properties are measured to assess soil health: chemical, physical and biological.
The chemical aspect of soil refers to the levels of plant available macronutrients it holds.
Production removes the major macronutrients such as phosphorus and potassium from the soil, so these need to be regularly replaced. But aside from production’s impact, soil’s capacity to hold these nutrients can also vary.
Most soil particles are negatively charged, so attract positively charged ions (cations) such as potassium and magnesium. Cation exchange capacity (CEC) indicates the amount of negative charge in the soil, and its capacity to hold cations. CEC is largely a function of the ratio of sand, silt and clay, as well as 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).
Soil acidity (pH) is also important, affecting the range, survival and functioning of soil microorganisms, including those that convert non-plant available forms of nutrients into plant available forms. A pH of around 6.0 is ideal.
The physical aspects of soil are its texture and structure.
This is largely determined by the soil’s composition, but also by conditions such as wetting and drying, freezing and thawing, foot and vehicle traffic and farming or cultivation techniques.
Healthy soil is 25 per cent air, 25 per cent water and about 8 to 15 per cent organic matter, both living and dead. The remaining 35 to 42 per cent is varying proportions of soil particles (silt, clay and sand) depending on the soil’s parent material.
Over time, soil particles and organic matter arrange into small aggregates (clumps), which provide the soil structure – the size, shape and aggregation of soil particles and their arrangement, including the pores (spaces) between them.
Well structured soil has plenty of pore space for air, water movement and root growth. Ideal soils have both large and smaller pores, allowing water to move through the larger pores, and be stored in the smaller pores for plant uptake. The pores also provide air for many soil organisms.
Many physical soil properties can easily be assessed visually. Visual Soil Assessment (VSA) is a reliable, cheap way to score soil condition, using basic equipment and the Visual Soil Assessment field guide. The guide provides scorecards, information and images to help assess key biophysical indicators of soil quality and plant performance. VSA guides and soil management guidelines are available for free through some regional councils, or at www.landcareresearch.co.nz/ publications/vsa-field-guide/.
The biological aspect covers the organic matter, both living and non-living, soil contains.
Organic matter can improve soil structure, and increase its capacity to provide and retain nutrients and store water. Although organic matter is a relatively small part of soil (8 to 15 per cent) it holds a huge amount of life and differentiates soil from its parent material.
Living organic matter includes organisms and plants, such as protozoa, nematodes, arthropods and earthworms. Non-living organic matter includes decomposing plant and animal residues and substances exuded by plant roots.
Directly measuring soil’s biological health is difficult, but earthworm density and diversity can provide an estimate, as can measuring Hot Water Extractable Carbon (correlated with microbial biomass) or Anaerobically Mineralisable Nitrogen (indicates soil organism activity and health).
For more support, talk to your Ballance Nutrient Specialist or Ruralco Representative.