This research determines to assess the benefits (economic and environmental) of a green manure phase under current cropping systems in areas with a poor nutritional status. 

To evaluate crop types and incorporation techniques to rejuvenate poor performing soils using a green manure phase, as well as to identify and evaluate the potential longevity of benefits on a heavy loam soil in Mullewa, Western Australia over five years.

Study Conclusions

Potential nitrogen contributions are assumed to be 40% of the total nitrogen contribution, assessed from the dry weight and nitrogen content (N% tissue) of the green manure crops measured at anthesis. Legume crops provided the most potential nitrogen of all treatments, due to high tissue nitrogen and moderate to high biomass production. Oats had the highest biomass but were low in tissue nitrogen and potential nitrogen benefits were therefore negligible. Any benefits from this type of crop, would be largely associated with the contribution of organic carbon to the soil. Emergence of both canola and mustard was poor, whilst the lathyrus and vetch competed poorly with weeds due to slow initial growth.

The percentage of undecomposed material remaining on the soil surface at seeding in 1999, varied from approximately 35% for oats down to 10% for lathyrus. The general pattern was that the crops with higher tissue nitrogen content broke down quicker. This indicates the microbial degradation of legume crops is likely to be faster than for brassica or cereal crops.

Differences in water infiltration observed may indicate the potential for treatments to increase the initial sorptivity of soil (ability for more rapid entry of rainfall) and decrease the flow rate through the soil (effectively increasing moisture retention). This may allow earlier sowing after summer rains, increased moisture availability during the growing season and potentially longer availability under dry finishing conditions. These results are variable and subject to trial/operator error. Further evaluation of these attributes is required prior to evaluation of renovation techniques.

The high cost of seeding and maintenance of the green manure crop, can significantly influence the gains or losses experienced over this three year period. It may be most economic, to green manure legume crops in seasons with low potential returns (poor yield potential or price).  The cost of harvesting a lupin crop for example may prove less beneficial than using it as a green manure to improve wheat yield and quality, where prices are depressed or where disease/season has reduced yield potential. Strategic use of green manure crops as a tool in integrated weed management or potential productivity gains through addressing specific problems may result in little or no short term economic gain where employed in better performing paddocks and must be viewed as a long term strategy to overcome yield constraints. However, strategic use of these techniques for poor performing paddocks, or paddock that would have otherwise been ‘rested’, are likely to result in economic responses, particularly where using low cost crops or pasture has been employed as there is no requirement to recover the cost of foregone yield – only those associated with growing a crop and the implementation of techniques.

Data supports the hypothesis that a green manuring:wheat:wheat rotation, may be as profitable as a legume:wheat:wheat rotation using either a field pea or Lathyrus crop. Given this assessment does not include any additional benefits that may be gained from green or brown manuring in terms of additional weed control and potential improvement of soil characteristics such as the addition of organic material, improved soil structure and longer term productivity gains, these techniques are likely to prove both economical and sustainable by providing additional benefits that may be observed in lower herbicide costs through better weed management, improved soil health and a more flexible farming system.