The benefits of soil testing

December 11, 2020 by Shane Kable

There’s no doubt that properly preparing your paddocks for the next crop or pasture is a key element for a successful future harvest.

In the preparation phase, the focus of discussion is frequently on the relative performance between different seed options for sowing, including feed quality, yield potential, or persistence.

Unfortunately, soil testing — a vital step — is often overlooked.

The importance of soil testing

Without a soil test at an appropriate depth, you don’t know the fertility of your soil, which has a major influence on whether a species is suited to the paddock and how the pasture or crop will perform.

Soil testing also allows you to accurately predict the fertiliser needs of the paddock.

In some cases, the test will reveal that there is sufficient nitrogen available for the needs of your chosen crop, so the savings in fertiliser cost easily outweighs the cost of the test.

In other cases, a soil test will identify deficiencies in the soil, allowing you to apply the correct fertiliser and rate to ensure the best economic return from the crop.

Deep soil testing

Deep soil testing can also measure the changing pH down the profile, changes in subsoil texture, and sub soil chemistry that could potentially be hostile to plant roots.

This is particularly important for crops with deep root systems, such as lucerne, wheat and sorghum.

Requesting additional specific add-ons to a basic soil test, such as exchangeable cations extraction, will give you values for the elements important for the crop you will grow, without the expense of a full-range test.

The value of deep soil testing is diminished if it only gives you a partial picture of where the nutrients are in the profile.

My experience is that the deeper subsurface samples to 60 or 90cm should be segmented with both a surface test and then subsequent tests from segments lower in the profile. A single 0-60cm test does not consider variation down the profile, especially when considering nitrogen, sulphur, sodium and aluminium.

Soil testing range

My preference for sampling is a test of 0-10cm and 10-60cm. If possible, a four-way split of 0-10, 10-30, 30-60, and 60-90cm can give you a better picture of the soil profile in terms of soil texture and soil chemistry.

Soil testing should consider the effective rooting depth. Soil structure may mean you need to modify the depth segments you can test. A 10cm test may be the effective limit if there is rock or gravel in the surface layer.

Also, if the soil is very deep and roots are expected to grow deeper than 90cm, consider tests in the 90-120cm zone if possible. For shallow-rooted species like perennial ryegrass and white clover, deep soil testing is not necessary. 0-20cm is adequate.

It is important to determine nitrogen levels when considering spring/summer forage such as a brassica, millet, or forage sorghum. This is because nitrate levels in the plant are an important stock health consideration.

On more than one occasion, strategic deep soil testing has led to a complete reversal of the intended sowing plan, with changes in species mix due to high aluminium levels or sodic subsoil making the initial selections likely to under-perform or pasture species not to persist.

If you don’t have a comprehensive soil testing program in place, there are qualified and accredited agronomic advisers performing testing services.

For any queries, please don’t hesitate to get in touch, or talk to your S&W Territory Manager.

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