Mitigating soil compaction and improving vegetable yield and quality using immature compost

The hardpan layer of soil, which inhibits the healthy growth of crops, is one of the most common and difficult to solve problems in many arable fields. Recently, a method of spreading pruned branches and immature compost together on the soil surface has been used by farmers as a technique that can solve this problem and further improve the growth and quality of crops. It is considered that by allowing indigenous fungi to grow fully on these organic matters before incorporating them into the soil, the soil’s aggregate structure will develop and the hardpan layer will disappear. In this study, we replicated the procedure of a farmer who successfully mitigated soil compaction and improved the growth and quality of vegetables, and compared the results when each organic matter was used alone, and when mature compost was used instead of immature compost. The field trials included five treatments: 1) control, 2) application of pruned branches only (B), 3) application of immature compost only (IMC), 4) application of pruned branches + immature compost (B+IMC), which is the farmer’s recommended method, and 5) application of pruned branches + matured compost (B+MC). Soil penetration resistance, soil inorganic nitrogen, microbial population and activity, aggregate structure, and yield and quality of komatsuna (Brassica rapa var. perviridis) grown after these treatments were examined. Maximum soil strength of hardpan layer was decreased in B+IMC compared to control. The weight and height of komatsuna were increased in B+IMC and IMC treatments. However, the concentration of nitrate in komatsuna increased and that of ascorbic acid decreased in IMC treatment. These results indicated that the combination of pruned branches and immature compost recommended by farmers was more effective in mitigating the hardpan layer and improving vegetable productivity and quality than using them alone or using mature compost.