The effect of application of soy sauce cake on potato plants cultivated in a sandy dune field

Soy sauce cake, a waste product by a soy sauce production, contains a high concentration of protein, so it is a good material for animal feeds and fertilizers. However, soy sauce cake contains a high concentration of salt (NaCl), so the application of a high amount of soy sauce cake as a fertilizer may induce salt stress of the crops and salt accumulation in the field. In this report, we carried out the field experiment of potato plants with the application of soy sauce cake and desalted cake by washing with water. One plot was 5.4 square m (0.9m×6m), and 20 potato tubers were planted in one plot. In addition to the basal dressing of P, K, and lime fertilizers, the soil was mixed with 0, 5, 10, 15, or 20 (gN/square m) of ammonium sulfate, or 10, 20, or 50 (gN/square m) of soy sauce cake, desalted cake or poultry manure. Ammonium sulfate (10 gN/square m) was added to the soy sauce cake, desalted cake and poultry manure treatments for supplementing fast-acting N fertilizer. The potato tubers (cv. Danshaku) were planted on 7 May 2001. Plants were harvested at the flowering stage (46 days after planting) and at harvest (96 days after planting). The harvested plants were washed, dried and ground into a powder, and the concentrations of N, P, K, Mg, Ca, Na were analyzed. The N concentrations of soy sauce cake and desalted cake were about 42 mgN/gDW and equivalent to that in the poultry manure. However, the P, K, Ca, and Mg concentrations in soy sauce cake and the desalted cake was much lower than poultry manure, suggesting that the fertilizer effect of cake and DSC are fundamentally restricted to the N supply. The different application rates of ammonium sulfate did not affect potato growth and the N, P, K, Mg concentrations, possibly due to the rapid leaching from sandy soil. On the other hand, increasing the application rates of soy sauce cake, desalted cake or poultry manure tended to increase the plant dry weight and tuber yield. However, the highest application rate (50 g/square m) of soy sauce cake did not increase the dry weight at the flowering stage, possibly due to salt stress. Application of soy sauce cake, desalted cake, and poultry manure increased the N concentration in each part and nitrate concentration in petiole juice, so these organic matters were rapidly decomposed in the soil, and absorbed by the potato roots. From the results obtained soy sauce cake or desalted cake are as efficient as poultry manure as N fertilizer, although a high dose of soy sauce cake needs caution not to give salt stress for the crops, or salt accumulation in soil.