Nitrogen soil tests - Do they always reflect what is going on?

Do soil tests always reflect what’s going on? Emphatically, NO.

The usefulness of nitrogen (N) soil testing is frequently a topic of discussion in both academic and farming circles. Those for the negative generally cite variability involved in sampling, lack of analytical laboratory accuracy and/or precision and lack of response calibration as factors that influence their position. While they are correct in citing these as a source of variation, careful planning can alleviate many of these objections. The debate is then more about the effort versus the reward.

Those that find N soil testing valuable cite:

  • Optimising N strategy based on fitting crops to paddocks (first stage of precision N management – changing rate between paddocks)
  • Assessing residual from failed and underperforming crops
  • Helping develop fertiliser purchase plan (approximate quantity and when) as well as confirming nitrogen supplementation rates as where value lies.

Given that in the northern grains region most fertiliser N is applied at or prior to soil, soil testing is generally conducted some weeks to months prior to sowing and many months prior to harvest. This time-lag and its influence on soil processes introduces a degree of uncertainty and error about the final outcome which in-turn affects the relationship between predicted performance of N inputs and actual.

In my experience in this region a good outcome from N soil testing is being able to discriminate whether a paddock to be sown to a winter cereal will require a small amount (say 20-25 kg/ha), a modest input (40 - 60 kg/ha) or a high rate (90-120 kg/ha). Claims of accuracy and performance at a greater level from pre-sowing testing is probably over-optimistic and unrealistic considering the inherent variability even when employing the best sampling, lab and interpretation techniques.

To be able to get this level of discrimination there is a requirement that sampling, laboratory services and interpretation methodology are all of the best, affordable standard.

For N in the northern grains region, sampling needs to include assessment of at least the surface and one subsurface sample. Where other nutrients are to be assessed the surface sample is usually to 10 cm and the subsample to at least 60 cm in most soils. If N only is to be measured the primary sample it may be taken from 0 to 30 cm and deeper sample to at least 60 cm.

Selection of more reliable laboratories is now a little easier with publication of those who are QA Assured (, and Proficiency Accredited ( now searchable on the web

Interpretation is generally conducted through the N budget approach. Feature of N budget interpretation tools that provide most accurate predictions are:

• Crop demand is driven by a Realistic Yield Estimate, a target N content of the harvested material and a variable N transfer (uptake) efficiency that is linked to the N content of the harvested material.
• Calculation of mineral soil N makes allowances for both nitrate and ammonium-N forms and differences in bulk densities between soils and between depth increments
• Allowances made for potential mineralised N from sampling till harvest based on local climatic data and estimate of total N pool.
• Functionality to make allowance for the effect of previous legume crops.
• Functionality to make allowance for the effect of manure application.
When approach with commitment to good processes, and with the recognition that it is but one factor in decision making, can be soil N testing is a useful tool in managing crop production, profitability, soil health and environmental responsibility.

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