Guideline Index

Chapter 8: Assessing Soil Nutrients

8.2 Soil testing

Soil testing is very useful for assessing:

  • Fertiliser type and rate of nutrients required.
  • Lime requirements.
  • Gypsum requirements.
  • Changes in soil nutrient levels over time.

A recent survey has shown that about 82% of dairy farmers use soil tests to determine the next season’s fertiliser application, and 50 % of dairy farmers do this in consultation with independent consultants. A significant proportion (49%) are confident in the advice they receive which still leaves many dairy farmers that are only fairly confident to not confident in the advice, or are still basing their current fertiliser strategy on a soil test carried out 5 to 10 years ago or relying on past practices (Dairy Australia, 2012).

In order to know how much and where the nitrogen, phosphorus, potassium, etc. is in the soil and if it is plant available, you have to take a soil test. Essential plant nutrients in the soil are often in a dynamic state i.e. changing in terms of availability and form; nitrogen being the classic case. There are new soil tests being developed in different regions which allow more specific analysis. In some cases, soil tests are now able to give indications of soil health, in addition to a chemical analysis (See Chapter 5 ).

The accuracy of any soil test depends on:

  • A truly representative sample being supplied to the laboratory. Many incorrect recommendations associated with soil test results occur due to poor sampling.
  • The sample being packaged correctly and transported to an ASPAC or NATA accredited laboratory for a comprehensive and accurate analysis.
  • Correct calibration of the chosen test methods against local or regional field trials to allow reliable interpretation.
  • Basing the fertiliser, lime and gypsum recommendations on a broad range of other factors, such as pasture composition, homogeneous soil type, and stocking rate; in other words being site specific.