Why you should soil test
Have a clear purpose in mind to:
- support evidence-based fertiliser decisions – fertiliser costs are a major part of most farm budgets (prediction)
- have an evidence-based measure of changes in soil physical and chemical properties (monitoring)
- determine possible causes for differences in crop or pasture growth (diagnosis)
- minimise your impact on downstream waterways, by reducing excessive nutrients in soil (compliance)
What soil tests can tell you
Soil analysis measures physico-chemical properties of the soil including nutrient status, soil acidity and soil texture. These measures indirectly predict how plant growth and product quality will respond to nutrient supply and rainfall throughout a growing season.
Plant analysis directly measures nutrient concentrations in living plant tissue, with interpretation specific to the plant and its growth stage.
To build the best picture of overall soil fertility, both soil and plant samples are often used together. They complement each other because a soil test estimates what should be available to plants and the plant test measures if it is taken up by the plants. If there is some discrepancy between soil nutrient status and plant nutrient content, it may be necessary to investigate factors that can affect nutrient availability and uptake e.g. root diseases, herbicide damage, soil acidity, other soil constraints and water relations.
Soil test results
At a minimum, soil tests for high rainfall pastures should report on the primary soil chemical properties below:
- Primary properties
- the ability of the soil to hold and release P: estimated by the P buffering index (PBI)
- an estimate of the soil acidity (pH) measured in calcium chloride
- available phosphorus (P): as Colwell P in milligrams of P per kilogram of soil (mg/kg)
- available potassium (K): as Colwell P in milligrams of P per kilogram of soil (mg/kg)
- available sulfur (S): using KCl 40 technique.
- Secondary properties
- available nitrate nitrogen
- available ammonium nitrogen
- an estimate of salinity using electrical conductivity (EC)
- texture
- P retention index (PRI)
- trace elements and aluminium.
Plant tests are better for trace element analyses.
See Nutrient and fertiliser management on high rainfall pastures (Western Australia) for more information.
Using soil test results
Soil test results can be used to estimate relative pasture growth before the pasture has germinated. Soil test results can also be used to estimate the amount of each nutrient that needs to be applied (if any) to reach a desired production target. The test results will show how much of each nutrient is already in the soil, and can be used to estimate how much nutrient, fertiliser, or soil amendment such as lime if any, needs to be added to achieve the production target.
There is also an increasing opportunity and need to link sampling protocols and fertiliser recommendations with the risk of nutrient loss and environmental pollution.
How ofter should you get soil tests done?
We recommend testing every paddock each year for the best management results. At a minimum, soils should be tested once every three years to assess changes in soil properties, either by sampling a third of the paddocks on your farm every year, or each paddock on the farm every three years.
Sample more frequently on farms with lighter textured soils (sands), or where there is high nutrient removal in products (dairies), to ensure soil nutrient levels can be maintained.
For more information
- Moore, G, ed. 1998, A handbook for managing agricultural soils, Bulletin 4343, Agriculture Western Australia, Baron Hay Court, South Perth, WA 6152
- Peverill, KI, Sparrow, LA and Reuter, DJ, eds, 1999. Soil analysis: an interpretation manual. CSIRO publishing.
- Gourley CJP, Weaver DM, 2019, A guide for ‘fit for purpose’ soil sampling. Fertilizer Australia, Canberra, Australia.