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About Soil/Plant Testing

About Soil/Plant Testing

Many farmers are unable to confidently evaluate their soil test results, this makes them reliant on external recommendations that they are unable to critically consider. The aim of this article is to explain soil tests in "simple terms".

Making Sense of your Soil Test

Disturbingly many soil test results are being reduced to a point where information required for realistic farm assessment is omitted. Many of these "minimal tests" are supplied "free of charge". Perhaps the adage "You get what you pay for" applies.

Because seasonal soil cycles affect test results, soil testing should be carried out at about the same time each year (preferably similar seasonal conditions rather than the same date). Testing each year allows trends over years to be observed rather than looking at one year in isolation. Observation of the trends is important because individual tests can include anomalies.

It's your soil test. To be able to make informed fertiliser decisions, farmers should request the following tests as a minimum: pH, Olsen-P, P-retention, CEC, Calcium base saturation%, Magnesium base saturation%, Potassium base saturation%, Sodium base saturation% and Total base saturation%, (there may be an additional charge if you are taking advantage of a free test). Once every 3 to 4 years more extensive tests for sulphur and the trace elements should be included.

When looking at soil test results there are several columns. The first is usually the description of the test item, followed by the level found.This is an accurate scientific measurement of a generally rough soil sampling system.

The next column is usually "Recommended Range". The important thing to note is that this range is "someones" interpretation of what they believe the figure should be. This can vary considerably depending on which laboratory carried out testing. With "free tests" it is often the organisation that arranged for the test that influences what the laboratory places in this column.

Test results often include a graph to assist understanding the test results. The graph may be part of the laboratory report or it may be added by the organisation that arranged for the test to be done. The important thing to note is that the graphs are the "actual" results plotted against the "recommended range". The graph is intended to provide a "picture" of the test results to aid interpretation of the test report and highlight test results that are different to what has been defined as the 'Recommended Range'. In many instances, tests results that are outside this range (either high or low) are of no concern unless they represent a factor that is affecting farm production.

pH. A measurement related to the concentration of acidic Hydrogen ions in the soil. Different plants thrive in different soil pH levels. Azaleas like an acid soil of 4.5 while truffles require an alkaline soil pH of 8.0. Farm pasture is at its most productive in slightly acidic conditions of 6.0 to 6.5. At this pH range the major plant nutrients and trace elements are at their optimum plant availability, (see nutrient availability graph). Also between pH 6.0 and 7.0 the soils beneficial microbial life is at its most active, fixing free Nitrogen, cycling thatch and animal dung down into the soil, aerating the soil and generally making locked -up nutrients plant available.

Olsen-P, and Resin P tests measure plant available phosphate, the Resin P typically reports 2.5 times the P as the Olsen test in the same soil sample . The ideal level is a continuous issue. A cyclic would say the sellers of P recommend high levels while non-sellers of P recommend low levels. A major independent laboratory suggests an Olsen P level of 20, which is the same as a Resin P level of 50 is ample for maximum pasture growth.

P-Retention. A measurement of the soils potential to lock-up phosphate. On many NZ farms large quantities of Phosphate can be released and made plant available simply by raising the soil pH above 6.0. This is an opportunity to "mine" locked- up phosphate reserves. It is not unusual to see P-Retention levels in excess of 70% in ash soil types. There are no compelling reasons to leave it locked-up.

CEC. (Cation Exchange Capacity) This is an indicator of the nutrient retention characteristic of the soil. It is the capacity that the soil has to supply the base nutrients in a plant available form, (Calcium, Magnesium, Potassium and Sodium). The more composing organic matter present in the soil, the higher the CEC level and the more nutrient retentive the soil will be . In simple terms it's "how big the oils cation Fuel Tank is".

A CEC below 12 equals a low nutrient retention soil while a CEC above 30 equals a high fertility soil. This figure is used to determine the quantity of base nutrients to apply, (the higher the CEC level the bigger the tank that has to be filled). The cycling of animal dung and the breaking down of thatch layers by the soils microcbial life, or the addition or organic matter will lift CEC levels.

Calcium base saturation % (Ca) This is a measurement of how much plant available Calcium is present on the soil cation exchange sites. If we consider CEC (Cation Exchange Capacity) to be "how big the cation Fuel Tank is", then Calcium base saturation is what % of the fuel, in the tank is Calcium and this influences the soil pH.

To increase Calcium base saturation, add lime to the soil. The higher the lime purity and the finer the particle size the better. Calcium base saturation should be 60 - 68%.

Magnesium base saturation % (Mg) As for Calcium above. Magnesium base saturation should be between 5 and 12% in mineral soils to achieve adequate supply for the plant growth and animal nutrition.

Potassium base saturation % (K) As for Calcium. Potassium base saturation should be between 2 and 4 % in mineral soils for adequate supply of essential nutrients for plant growth. High levels of Potassium are associated with animal health problems.

Sodium base saturation % (Na) Sodium base saturation should be approximately 1/2 Potassium level or 1.5-2.0%

Total base saturation % Total base saturation is the total of the individual base saturation percentages above, (Ca + Mg + K + Na). Total base saturation is the measurement of the quantity of alkaline nutrients on the soils cation exchange sites and determines the long term pH level of the soil. It is "how full the soils cation Fuel Tank" is. A Total Base Saturation level of 75% will give a pH of approximately 6.3.

The most important consideration with the base saturation figures is to obtain a balanced relationship between them. i.e. Calcium 60-68%, Magnesium 5 - 12%, Potassium 2-4%, Sodium 1.5-2%. Most other figures on the soil test are the items above, expressed in different terms, or are self-explanatory.

Additional information on soil tests and assistance with interpretation of test results is a service provided by AgriFERT consultants. For more information freephone 0800 300 315 or contact us.