Visual Soil Assessment

Visual Soil Assessment is a practical, science-based system for scoring key bio-physical indicators of soil and plant condition. It is a cheap and effective tool for farmers to assess and monitor soil quality and pasture and crop performance. The assessment process takes about 40 minutes per sample and requires little more than a spade, a piece of board, a score card and pen, and at least one good eye. The results link soil, plant and animal health to management practices and so guide farmers as to what needs to change in order to improve performance and profitability.

The ability of humankind to feed itself lies in its husbandry of the thin layer of soil on farmable land. For that layer to be productive it must contain millions of organisms per cc, from bacteria, fungi and other microorganisms to worms and insects, all thriving in a complex biological ecosystem. A critical part of that system is the two-way trade in nutrients between plant roots and soil microbes, the roots swapping carbon compounds (resulting from photosynthesis in the leaves) for minerals, nitrogen and moisture from the soil biology.

This picture of active interaction between living things above and below the soil surface seems to have been deleted from the collective memory discs of conventional agriculture. The soil is treated as a chemical soup into which grasses dip their roots and suck up what they need. The soup needs regular topping up with chemicals that will give production the quickest boost – mainly soluble forms of phosphate, potassium and nitrogen. The amount to put on is determined by chemical analysis of soil and herbage samples. If pasture production is inadequate after chemical fertiliser is applied, the answer is to put more on. If stocking rate rises the answer still is to put more on. This is the “more-on” approach.

Chemical analyses are useful but they provide only part of the picture of soil health. Relying on them alone tends to give a view coloured by convention and the interests of the chemical fertiliser industry. A more balanced approach, and one that puts farmers back in the role of observers and decision makers, is to include visual assessment of soil and herbage. This involves taking a spade width and depth cube of soil and going through a series of steps to gauge its condition using a set of visual and physical standards.

A visual method for assessing soil condition and plant performance and relating it to management practices (VSA) was first developed by soil scientist Graham Shepherd at Landcare Research in the late 1990’s. It has been developed further and refined by Shepherd, and has now been adopted and promoted for use in many countries by organisations including FAO. VSA is based on extensive research and experience using laboratory and field measurements.

The 2nd Edition of the Field Guide For Assessing Pastoral Grazing And Cropping On Flat To Rolling Country was published last year. A Hill Country guide will be published in 2012.

In the preface he writes: “To the untrained eye, visual messages provide an immediate and effective assessment of the condition of the soil and the performance of the pasture and crop. To the trained eye, visual images provide extremely insightful semi-quantitative information regarding the condition of the soil and pasture/crop performance. The trick is to train the untrained eye. This is the ultimate purpose of the VSA, which will in turn improve farm management practices.”

The VSA process for pasture involves taking a 200mm x 200mm square x 200mm deep cube of soil, dropping it onto a hard board to shatter it, and then going through a series of observations of key physical attributes and scoring them against standards shown in the Guide. These include assessments of:

• Texture

• Structure

• Porosity

• Number and colour of soil mottles

• Colour

• Earthworms

• Smell

• Potential rooting depth

• Surface ponding

• Surface relief

Pasture plants are also assessed for:

• Quality

• Clover nodules

• Weeds

• Growth

• Colour and growth relative to urine patches

• Utilisation

• Root length and density

• Area of bare ground

• Drought stress

• Production costs to maintain stock carrying capacity

The VSA procedures are not difficult and can be learned quite quickly. Shepherd and others he has trained run training sessions for those interested. With experience, individuals can complete an assessment at one location in about 40 minutes. This will give a considerable amount of information on the condition of soils and plants, how management practices have brought about those conditions, and what changes should be made to improve productivity, performance and profit.

The results will also make for better decisions on fertiliser type, quantity and application.

The Guide also details visual indicators that can be used to assess the potential for:

• Nutrient loss into groundwater and waterways

• Carbon sequestration into soil

• Greenhouse gas emissions.

More information is available at www.BioAgriNomics.com