Soil Testing: What is it and why is it important?

Soil Testing: What is it and why is it important?
March 18, 2016 Comments Off on Soil Testing: What is it and why is it important? Chicken Litter,Farming,Rural Services,Soil Care Charmaine Acworth
Why Soil Test? – Report by Agronomist Tony De Vere…

The ultimate goal for soil testing is profitability. You want to know what investment in fertiliser will give you the best dollar return. Achieving and maintaining soil fertility is important for sustaining an acceptable level of crop production & quality.

Guesswork and trial & error come with risks – cost of wrong fertilisers & lost productivity. Wrong fertilisers may be more than wasted up-front costs; they could lead to hidden ongoing costs due to detrimental effects that reduce production.

Profitable fertilising starts with a good soil test & an interpretation that identifies the soil fertility limitations. The purpose of soil testing is to identify nutrient deficiency, excess or imbalance & other factors that affect soil fertility. To get reliable information from a soil test, you need accurate sampling, accurate analysis & a meaningful interpretation.

Soil Sampling 

An accurate soil sample is just as important as an accurate analysis. Here are some tips:-

  • Take a composite sample of >10 sites, at the appropriate depth (usually 0-15cm). Samples must be a consistent slice or core from top to bottom. If not, soil test results may be inaccurate.
  • Sample within one soil type, paddock history & slope (upper & lower slopes may be different). Do not mix different soils into one sample.
  • Avoid non-typical areas, e.g. recently fertilised areas, urine & manure patches, old fence lines & building sites, animal yards & near tracks & roads.
  • Use clean equipment – sampling tube, auger, spade, sample container, sample bag.
  • Do not use galvanized pipe (stainless steel or plastic push tubes are suitable).
  • Avoid sample contamination from sweaty hands, dust, etc.
  • If soil is stored in plastic bags, keep out of the sun. Excess heat & humidity change pH, humus & nutrient levels.
  • Sample at the same time of year if comparing with previous tests. Available nutrient levels, pH & Humus often fluctuate with the seasons.
Soil Analysis

Laboratory analyses for soil fertility assessment will extract a portion of soil nutrients that is classed as plant-available. These tests are designed to mimic what plant roots can extract. Different methods extract different amounts of nutrient and have different guidelines for interpretation, based on crop response research.  This can cause confusion when inexperienced people try to compare results from different labs.

Soil testing laboratories can vary in accuracy. Use a reliable lab & use the same lab when doing repeat testing to monitor changes.  DIY test kits are an option for in-field nutrient monitoring, but are usually less accurate than laboratory tests.  Accurate analysis may be of limited use if the soil has not been sampled & handled correctly.

Meaningful Interpretation

Soil productivity is limited by the least available nutrient. This is the concept of “the weakest link”.  Rebuilding soil fertility is comparable to repairing a chain with damaged links.  If the weakest link is not repaired, the strength of the chain will not increase.  Similarly, a fertiliser program that does not supply the most limiting nutrient will give poor results, or maybe no result.

One approach is to fertilise with all possible nutrients. This method wastes fertiliser and dollars, can pollute ground and surface water with excess nutrients, and risks toxic reactions to your plants from too much of an otherwise good thing.

A meaningful interpretation of test data involves recommendations to optimise crop yields without adverse environmental impact. A good interpretation involves:

  • supplying sufficient nutrients to feed the crop and;
  • improving soil nutrient balance to enhance nutrient availability, soil structure & root development & to encourage a good balance of soil microbes.

Knowledge of recent fertiliser history is important when designing a fertiliser program. Some fertilisers break down slowly & may be still releasing nutrients that are not accounted for in the soil test.  Ignoring fertiliser history can lead to over-application & possible detrimental effects.

Soil fertility improvement should also include soil structure assessment & treatment. Soils with surface crusting & compaction layers are prone to poor root development & poor nutrient uptake.  Poor soil structure can seriously affect response to a fertiliser program.


Fertilising without soil testing can be bad for plants & animals and bad for profits. Knowledge is power, and some knowledge of soil fertility & soil testing will enhance ability to make a profit, make soils healthy, produce healthy animals & produce quality crops with good flavour & nutritional value. 

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