ORGANIC AMENDMENTS NECESSITATE A TRADE-OFF BETWEEN BUILDING SOIL ORGANIC CARBON AND SUPPLYING CROP NITROGEN

Experiments were established to investigate if the addition of various organic soil amendments could increase soil organic carbon levels, reduce requirements for the addition of inorganic fertilisers, and improve crop yield. The experiments compared repeated applications of organic soil amendments (lignite, chicken litter, silage and green organic compost) to ‘standard grower practice’ (SGP) on the yields of broccoli on a sandy Tenosol in south east Victoria. This experiment showed that organic amendments can be effective in significantly increasing the amount of organic carbon in the soil. The response in total organic carbon (TOC) is, however, dependent on the chemical composition of the organic amendment applied. Organic amendments with lower C:N ratios and more labile forms of organic carbon increased soil organic carbon levels in the short term, but these could not be maintained with smaller subsequent applications. Lignite, which had the highest C:N ratio and was the least labile of organic amendments applied, maintained significantly higher TOC levels, despite a smaller total application of carbon. Green organic compost and chicken litter were the only organic amendments able to significantly increase total organic nitrogen (TON). The application of lignite had little impact on TON but significantly increased the C:N ratio of the soil. The application of all the organic amendments, except lignite, resulted in broccoli yields that were statistically equal or greater compared to the SGP treatment, even when using only half the rate of nitrogen fertiliser. Lignite significantly decreased yields, but the mechanism for this yield decrease is unclear. Results from the experiments demonstrated that for the organic soil amendments tested, there was a trade-off between building TOC and the supply of plant available nitrogen. To gain the greatest benefit from the application of organic soil amendments, vegetable growers must be clear about the outcomes desired and apply the appropriate soil amendment to meet these outcomes.