Effects of compost application on fruits yields, sugar and mineral contents and delta15N values of tomato fruits

We examined the effects of chemical fertilizer and compost application on the yields and sugar and mineral content of tomato (Lycopersicon esculentum Mill. Saturn in an isolated bed. Five treatments were conducted within a two-year period of 4 continuous croppings. CDU and LSR (Low-sulfate slow-release fertilizer) were used for the chemical fertilizer plots. A mixture of cattle manure and CDU (CM+CDU) , a mixture of poultry manure and CDU (PM+CDU), and a mixture of cattle and poultry manure (PM+CM) plots were arranged as compost-using plots. We also measured the d sup(15)N values of tomatoes and the soils of each treatment, and estimated the correlation of the d sup(15)N values between fruits and soil to certify compost applied products. We did not find any reproductive differences in the yield or sugar content among the treatments. As to inorganic content of tomatoes, there were no significant differences except for Mg content among the plots. These results showed that it is difficult to assay regular benefit of organic fertilizer application to tomato yields and quality. On the other hand, d sup(15)N values of tomato fruits showed significant differences among fertilizer applications. d sup(15)N values of the chemical fertilizer were +1.6 permilage and - 1.1 permilage for CDU and LSR, respectivery. Those of mixture of chemical and compost were+ 12.2 permilage and + 11.2 permilage] for CM+CDU and PM +CDU, respectively. The mixture of PM and CM showed the highest d sub(15)N values (+17.9 permilage) among the treatments. d sub(15)N values of the soils and fruits reflected those of the fertilizers and were positively correlated (R**2=0.89). It may be possible to use d sub(15)N values as an indicator of organic products by setting the threshold point, e.g. +5.0 %o, to distinguish them from the products cultivated with chemical fertilizer.