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07Jan

Time for a physical

Words and images supplied by Ballance Agri-Nutrients.

A fertile soil with physical health issues can still perform poorly, so it could be time for a physical health check.

Physically healthy soil?

Aerated structure is a very important aspect of soil’s physical health. Structure affects the pore space in the soil, and allows water to infiltrate the soil rather than running off, and for roots to penetrate. A healthy soil is 25 per cent air and 25 per cent water.

Soil particles and organic matter (8 to 15 percent of a soil) arrange into small clumps (peds), which give the soil structure. The shape of the clumps depends on the soil’s composition but also on conditions, such as wet and dry, freezing and thawing, foot traffic and farming or cultivation techniques.

The remaining 35 to 42 per cent of a soil is varying proportions of silt, clay and sand. This determines the soil’s texture – one of 12 classes, ranging from clay to sand.

Soil colour is influenced by its mineralogy, but can also reveal other aspects. Dark brown or black soils are high in organic matter. Well-drained soil is brightly coloured whereas soil that is often wet and soggy has a mottled pattern of greys, reds and yellows.

The bulk density of soil is measured by the mass of dry soil that fits in a specific volume. Low bulk density soils are fluffy and easy for roots and water to pass through, and high bulk density soils are heavy and often hard to dig or plough.

‘Unhealthy’ management practices

Leaving soil undisturbed is good for its physical health. Cultivation results in losses of soil from wind and water erosion, and losses of organic matter due to mineralisation (release of nutrients from organic matter).

Soil also does better if it is covered with vegetation, ideally a variety of species, as different species exude different carbohydrates from roots to feed greater variety of soil microbes. Growing more vegetation also produces more food for soil microbes, so maintaining a moderate to high soil fertility is beneficial.

Pugging and compaction can seriously affect soil structure. AgResearch studies showed that a single, moderate treading event reduced pasture production by 52 per cent (600 kg DM/ha) over the two following months. Another study showed clover’s ability to fix nitrogen was reduced by 60 per cent after moderate treading and 80 per cent after severe treading1.

Assess soil visually

Many physical soil properties can easily be assessed visually. Visual Soil Assessment is a reliable, cheap way for anyone to score soil condition.

All that is needed is some basic equipment and the Visual Soil Assessment field guide (available free online) developed by noted soil scientist Graham Shepherd. The guide provides scorecards, information and images to help assess key bio-physical indicators of soil quality and plant performance.

The indicators are underpinned by extensive research and linked to economic performance. Once you have determined your overall rating, you can refer to the relevant suggestions in the companion volume of Soil Management Guidelines to maintain or improve your score.

For soil quality the indicators are: soil texture, soil structure, soil porosity, number and colour of soil mottles, soil colour, number and size of earthworms, soil smell, potential rooting depth; surface ponding and surface relief. Soil indicators are generic and generally independent of soil type.

Soil can be visually assessed without knowing the soil type, it is a hugely beneficial complement to soil testing, and results can inform management practices.

VSA guides and soil management guidelines can be purchased by emailing gshepherd@bioagrinomics.com and through some regional councils. Free PDFs can be downloaded from the Landcare Research website (www.landcareresearch.co.nz/publications/books/visual-soil-assessment-field-guide/download-field-guide).

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

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