Estimating Legume Nitrogen Mineralisation Rates

The Australian Society of Agronomy held its 17th Australian Agronomy Conference last year, and the result is a smorgasbord of papers freely available here. From pest management to climate change there is a paper to suit every interest.

One paper which is of use to grain growers in South East Australia is “Legume effects on available soil nitrogen and comparisons of estimates of the apparent mineralisation of legume nitrogen“: in short, if a farmer rotates a legume crop through their field, how can they estimate the nitrogen available to subsequent crops grown in the same field.


The introduction states three important factors in the decomposition of organic nitrogen and mineralisation, being:

  1. The amount of rainfall, which stimulates microbial activity;
  2. The amount of legume residues present; and
  3. The nitrogen content and quality of the residues.

Using these factors, the paper reports on a field experiment conducted in southern NSW that compared the soil nitrogen in fields treated as follows:

  1. Lupin grown for grain;
  2. Lupin grown for brown manure (the crop was killed before seed maturation to retain maximum nitrogen in the plant;
  3. Canola grown for grain;
  4. Wheat grown for grain.

By calculating the amount of residues of each treatment left after harvest and measuring the percentage of nitrogen content in those residues, the study used the data obtained along with the rainfall throughout the period to develop a method for grain farmers to estimate soil nitrogen content, in turn assisting growers to estimate fertiliser needs for crops grown following a legume rotation.

The experiment methodology and results

The soil was tested in April 2011 for nitrogen levels prior to growing each of the treatment crops. In April 2012, after each crop was harvested (and the brown manure crop was terminated), the soil was tested again and then all fields were planted with the same cultivar of wheat. In April 2013, the soil was once again tested.

The testing in April 2012 showed that the nitrogen content was 3 to 5 times higher in the lupin crop treatments than in the wheat treatments, and that the brown manure treatment had the highest quality of nitrogen content.

Further testing in April 2013 confirmed the increased nitrogen findings in the lupin treatments, but also showed that the mineralisation of nitrogen by microbial activity continued through the subsequent wheat treatment sown in 2012.  This resulted in a finding that an equivalent of 4 to 5 kg of nitrogen per tonne of the residue dry matter from Lupin treatments in 2011 remained in the soil and had become available for subsequent plant growth.

The conclusion

Using the data obtained and the rainfall measurements, the researchers delineate some useful rules to estimate soil nitrogen content.

The simplest estimate was to assume approximately 10kg of additional nitrogen have been added to the soil per hectare for each tonne of shoot residue. Using a rough estimate of the percentage of dry mass which is harvested as grain, and which farmers will know at the end of a growing season, the equation suggested for farmers to calculate the expected mineral nitrogen available for growing is given as:

Nitrogen (kg/ha) = 20 x tonne grain yield per hectare

The researchers do point out that rainfall between legume harvest and the following growing season will affect microbial activity and therefore mineralisation rate. Given the variation in rainfall, the equation is likely to be applicable only to areas with similar rainfall patterns to those in South East Australia.

Why is it useful?

Being able to adjust fertiliser rates has the obvious economic benefits for farmers. But it also has environmental effects. Over-application of nitrogen fertiliser increases nitrification of unused nitrogen, resulting the production of the greenhouse gas nitrogen dioxide, and when excess nitrogen runs off into adjoining streams, increased bacterial growth results in eutrophication and resultant negative effects on aquatic wildlife.

For the scientifically minded home gardener or market gardener, weighing your legume crops or green manure and calculating the total weight per hectare can give a rough idea of the nitrogen added to your soil and give greater confidence in the amount of fertiliser required in following growing seasons.





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