Studies in soil fertility with special reference to organic manures. III. Residual effects of the organic matter.

The results of two field experiments and a pot-culture experiment to assess the residual effects of organic manures of the plant-residue type are reported. In the first field experiment, to which superphosphate was added at seeding, no residual effects of the organic manures were observed. In the second field experiment, with cabbage as the test crop, rice-hull pretreatment gave a highly significant yield increase of seven tons per acre. This is attributed to increased phosphorus supply, and the interpretation is supported by analytical data for the cabbage heads. A parallel pot-culture experiment with tomatoes confirmed the findings of the field experiment and indicated that the nitrogen status of the soil had also been slightly improved by rice-hull pretreatment. It is shown that at the beginning of the cabbage and pot-culture experiments, rice-hull pretreatment had increased the inorganic but not the organic soil-phosphorus fractions. The development of the interaction of current phosphorus treatment with rice-hull pretreatment is illustrated by seedling-harvest data for the cabbage experiment, and weekly leaf-area measurements are used in a detailed study of treatment interactions in the pot-culture experiment.