Australian Institute of Ecological Agriculture (AIEA)

Observing the nitrogen cycle in a green manure trial on an organic farm


As part of a 5 year Federal Government funded Landcare project in the Gippsland area of Victoria, a number of on-farm trials have taken place. One of these trials was monitoring a range of green manures on a certified organic vegetable farm.
Land to Market
The demonstration trialled and compared 6 different green manure crops each year for 3 years against a control. The control was immediately adjacent to the other strips and no green manure crops were sown into the control area. The intention was to demonstrate that soil quality characteristics would either be enhanced, or at least maintained, through the use of green manure crops and that soil nitrogen levels could be enhanced.

Organic horticulture practiced on this farm relies on the development of healthy fertile soils through the use of soil building crops of legumes, green manures, sound rotations and additions of compost and mineral fertilisers where appropriate. One of the major challenges facing organic growers is the limited research and extension available to support their enterprises and this approach to production horticulture, particularly in Australia.

In any intensive horticulture enterprise, over-cultivation and the compaction by agricultural machinery can cause a decline in soil quality parameters including:

  • soil structure
  • organic matter
  • soil carbon
  • soil biology and
  • nutrient availability.

Initial benchmarking consisted of visual soil assessment and a full soil analysis, which was repeated at the end of each year. As any good agricultural scientist will tell you, basing fertiliser decisions on nitrate nitrogen in a soil analysis is wrought with problems due to seasonal variation.
We decided to monitor nitrate nitrogen, phosphorus and potassium every month along with soil temperature and moisture to see what variations might be seen as a result of increased biological activity. Ammonium nitrogen was also included as part of the analysis. Over the three years both phosphorus and potassium remained reasonably constant while the nitrate and ammonium levels increased with soil temperatures along with sufficient moisture.

Although not new to science, we were able to demonstrate the strong relationship between soil temperature, moisture and nitrate nitrogen across the green manure plots. What is evident is that we were able to illustrate to producers (as part of an organic discussion group) biological information from a chemical soil analysis. The graphical representation of ammonium nitrogen and nitrate nitrogen illustrated how the nitrogen cycle was initiated at temperatures of about 14 degrees and then with further microbial action nitrate nitrogen was seen to rise.

The graphs and the data from the analysis enabled the producers to calculate the amount of nitrogen available for a full vegetable rotation as a result of green manuring. As expected as the season progressed and the vegetable crops took their nitrogen from the soil, the nitrate decreased in the analyses.

As part of this trial not only organic matter was monitored through the soil analyses but also labile carbon. Research indicates that over short-term trials organic matter movement is hard to see but labile carbon is far more sensitive to soil management.

The green manure crops in this trial with the addition of substantial amounts of organic matter managed to either increase or at least maintain soil carbon levels. Labile carbon remained reasonably constant.

In a previous trial on an organic farm the monitoring of labile carbon illustrated how sensitive it was to soil management as there were periods when due to seasonal condition cultivated land could not be sown with the green manure crops and were left with soil exposed. The labile carbon decreased as a result.

From the results that we have seen on two recent trials labile carbon should be an essential monitoring tool when looking at arable soil cultivation.

It should be remembered that this was not a replicated field research project but a farm trial attempting to illustrate some important soil management strategies and what data collection might be useful in assessing the sustainability of such a system. The strategy of monitoring nitrate nitrogen on a monthly basis was an enlightening exercise as was the demonstration of the sensitivity of labile carbon to farm soil management.

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