A health check for your soil

A health check for your soil

30 April 2021

In the first of a three-part series on soil health, Ballance Agri-Nutrient Nutrient Dynamics Specialist Jim Risk looks at soil’s physical health.

What’s healthy soil?

A commonly used definition from the US Department of Agriculture is: “the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans”.

More precise definitions of soil health depend on how the land is used. Still, it goes without saying, healthy soil is important for producing pasture and has physical, chemical and biological aspects.

What’s physically healthy soil?

Healthy soil comprises 25 percent air, 25 percent water and about eight to 15 percent organic matter, both living and dead. The remaining 35 to 42 percent 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 how they’re arranged, including the pores (spaces) between them.

An aggregate’s shape depends on the soil’s composition and the conditions – such as wetting and drying, freezing and thawing cycles, foot and vehicle traffic and farming or cultivation techniques.

Well-structured soil has plenty of pore space for air, water movement and root growth. Ideally, soils have both large and smaller pores, allowing water that infiltrates the soil to move through the larger pores, draining surplus water, and be stored in the smaller pores for plant uptake. The pores also provide air for many soil organisms.

Soil structure affects pasture production. Poor structure can impact nutrient supply, seasonal growth patterns, pasture composition (including more weeds or undesirable species), tiller density, canopy cover and the infiltration, movement and storage of water in the soil, and potentially increase erosion susceptibility. Soil with poor structure is also more vulnerable to compaction by stock or vehicles.

The relative percentages of sand, silt, and clay determine soil texture, and it’s helpful to know which of the 12 textural classes of soil (ranging from clay to sand) your soil is. Soil texture also affects soil’s ability to hold and drain water.

Soil colour can indicate organic matter levels, with dark brown or black soils being high in organic matter. Brightly coloured soils are well-drained, whereas mottled grey, red and yellow soils are poorly drained.

What can farmers do to look after soil’s physical health?

  • Reduce or avoid cultivation, which can result in wind and water erosion of soil, or structure breakdown and the decline of organic matter.
  • Keep soil covered with vegetation, ideally a variety of species to support a great variety of soil microbes, also helps. Cover crops can prevent bare soil.
  • Minimise pugging and compaction, which can seriously affect soil structure. AgResearch studies showed that a single, moderate treading event reduces pasture production by 52 percent – 600 kilograms of dry matter per hectare (kg DM/ha) – over the two following months. Another study shows clover’s ability to fix nitrogen reduces by 60 percent after moderate treading and 80 percent after severe treading1.
  • Many physical soil properties can easily be assessed visually. Visual Soil Assessment (VSA) is a reliable, cheap way for anyone to score soil condition. Just some basic equipment and the VSA field guide are needed. The guide provides scorecards, information and images to help assess key bio-physical 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/.

Future articles in this series will focus on soil’s chemical and biological health.

1 Betteridge, K, Drewry, J, Mackay, A, Singleton, P. 2003. Managing Treading Damage on Dairy and Beef Farms in New Zealand. AgResearch.

Article supplied by Ballance Agri-Nutrients.