Investigating the long-term impact of stockless organic conversion strategies

Abstract

Short-term leguminous green manures have traditionally been used to accumulate nitrogen during the organic conversion period. In a stocked system the in-conversion land may be used for grazing. However, in a stockless system land is effectively removed from production and substantial financial penalties may be incurred. Both the short and long-term effects of organic conversion are important. The legacy of the conversion strategies, i.e. the effects on the second (winter beans) and third (winter oats) organic crops were investigated on two soil textures. The strategies were:

1. two-years' red clover-ryegrass green manure
2. two-years' hairy vetch green manure
3. red clover for seed production, then a red clover-ryegrass green manure
4. spring wheat under-sown with red clover, then a red clover green manure
5. spring oats, then winter beans
6. spring wheat, then winter beans
7. spring wheat under-sown with red clover, then a barley-pea intercrop
   
   Post conversion the first organic crop was winter wheat.

Conversion strategy had a significant impact on organic bean yield, which ranged from 2.8 to 3.6 t ha^-1and organic oat yield, which ranged from 3.2 to 4.2 t ha^-1. In the organic bean crop, weed abundance prior to harvest, along with soil texture, accounted for 70% of yield variation. For the oats, soil mineral nitrogen in November together with weed abundance in April, accounted for 72% of the variation in yield. The impact of conversion strategies on soil mineral nitrogen levels were still detectable three years post conversion (after the winter oat harvest in 2004).

A land quality index (LQI) was developed which linked crop yield, through regression analysis, with crop price. Calculations of the LQI for the two organic crops showed that the spring wheat under-sown with red clover had the highest index value, but when the gross margins for the entire rotation were calculated, the red clover for seed production was ranked as the top strategy. However, this strategy relies on securing specialist markets and is unlikely to be appropriate for all growers.

Overall, the two years' red clover strategy would suit the risk-averse grower due to the high levels of soil mineral nitrogen and good weed control this strategy affords and the overall gross margin derived over the five year period from conversion to the end of the first three organic crops. These results highlight the importance of the fertility-building conversion period in terms of its effect on soil nutrient levels, weed abundance and economic viability.