The importance of soil testing

Soil testing supports good fertiliser practice and on-farm decision-making by providing robust fertility information rather than pre-empting a fertiliser strategy.

With the current prices of fertiliser, it makes good economic sense to soil test and only apply what the crop or pasture requires.

Hills Laboratories Market Sector Manager – Agriculture Fiona Calvert says, "soil testing is an increasingly important tool for optimising nutrient inputs, with a more precise approach driving soil test numbers at Hill Laboratories, up by 15 percent in the past five years. As well as more samples per farm, growers are interested in a broader set of tests per sample, particularly looking at organic matter and soil carbon to evaluate soil quality for their sustainability goals."

In support of Fiona’s comments, PGG Wrightson’s Technical Field Representatives (TFR) are seeing an increase in the number of soil tests being conducted. While basic soil tests are the most common, the inclusion of additional tests is increasing including total soil carbon percentage, which measures the carbon in the soil, and the organic matter test which measures decomposing plant and animal residues, soil biota and root exudates. These tests and more can be completed by your local TFR.

It is important to follow best-practice sampling methods when completing soil testing. Below are some tips for mapping your farm into blocks, ensuring a good cross-section is covered by the sample results. This will allow for more accurate fertiliser application decisions to be made.

• Take samples six to eight weeks prior to fertiliser applications. In some cropping situations, taking a sample as far out as 12 months is beneficial so capital fertiliser applications or corrections to pH can be addressed prior to planting crops.
• Mark and record the transect lines either by using a GPS unit, marking fence posts with the start and finish of the transect lines or recording on a map. By doing so, future tests can be carried out over the same transect lines to reduce spatial variability in the testing.
• Do not sample within three months of fertiliser or lime being applied.
• Avoid taking sampling from around and near gateways, fences, trees, hedges, stock camps, dung or urine patches as well as water troughs.
• Once five years of continuous yearly data has been collected within a usual soil sampling situation, tests can be conducted every two years on the same transect line.

Set up test sites according to the map you have created, showing the farm as blocks. Take into account soil type, slope, aspect as well as if the paddock is effluent and non-effluent. Sample crop paddocks separately. Refer to Figure 1 for an example of a map.

For further information on how to set up soil testing transect lines, and for a recommendation on which tests would benefit your farm, contact your local PGG Wrightson Technical Field Representative.

1 Ballance Agri-Nutrients. Store and Ute Fertiliser Guide. Electronic Version.