Improvement of soil physical and chemical properties by the cultivation of green manure and compost application at early stages of land reclamation in the Isahaya Bay [Nagasaki, Japan]

To prepare farmland of 645 ha and reinforce protection from damage due to flood and high tide, the Isahaya-bay Sea Reclamation project aims to complete the reclamation in 2007 fiscal. It is a full-scale project to use the reclaimed land as upland fields. To do so, physical and chemical properties of the soil needed to be improved at its initial phase of land draining besides correcting the problems of high salt concentration and poor drainage. Hence, to establish the initial farm management in the upland fields, the effect of the cultivation and plowing-in of green manure crops and compost application had been studied since 2000 and the following conclusions were obtained. (1) A cropping system comprising feed corn, sorghum, and sesbania for the summer and Italian ryegrass and plants of genus Hortem for the winter was combined with farm management and drainage measures, such as mole drain. The combination speeded the drying process in the upper soil layer, lowered the gley layer to as low as 35-40 cm in two years (four croppings), and hardened the soil. The concentration of harmful, water-soluble chlorine ion declined to 100 mg/kg dry soil in the plowed soil layer. (2) The repeated cultivation and plowing-in of the green manure crops improved the soil in physicochemical properties, such as bulk density, liquid phase, total carbon, and total nitrogen. (3) Gypsum increased the amount of exchangeable calcium and bulk porosity in the plowed soil immediately after application but did not show clear effects in desalinization and the improvement of the physical and chemical properties. The cultivation and plowing-in of the green manure crops, and improved field drainage due to cracks were estimated to exert great influence. (4) A simple regression equation involving EC measured by non-dried soil and the concentration of water-soluble chlorine ion enabled the quick and simple judgment of the concentration of water-soluble chlorine ion and the risk of salt injury occurrence. (5) Cattle compost was smoothly decomposed into carbon and nitrogen compounds during the growth period of winter cabbage. Although the compost can be expected to release inorganic nitrogen compounds soon after application, the decomposition was slower than the decomposition of bark compost and halophile compost, requiring a longer time.