The effect of application of soy sauce cake on lettuce plants cultivated in a pot

Soy sauce is a traditional seasoning from ancient ages in Japan. Soy sauce is a fermentation product from soybean seeds and wheat with a high concentration of salt, and the liquid soy sauce is filtered by cloth, and the residual waste of the filtration is called soy sauce cake. Soy sauce cake contains a high concentration of protein, so it is good for animal feeds and fertilizers. However, soy sauce cake contains about 13% of salt (NaCl), so the application of a high amount of soy sauce cake may induce salt stress in crops and salt accumulation in the field. The chemical composition of soy sauce cake contained about 4% of N, 2% of P2O5, and 1% of K2O based on dry weight, respectively. By washing the cake, the P, K, and Mg concentrations were significantly decreased in the desalted cake, although the N was not washed out. The Na in the original cake was almost completely removed by washing. Pot experiments have been done for evaluating the effect of soy sauce cake on the growth of sunny lettuce (Lactuca sativa var. crispa) plants. In the first experiment, lettuce plant was grown in the sandy soil (about 3 L) with 0, 5, 25, 50, and 100 g of soy sauce cake in a 1/5000 a plastic pot, with 1-watering or 2-watering per day. The growth of the shoot and roots by 1-watering per day was increased by application of 5 g and 25 g of soy sauce cake but decreased with 50 g and 100 g of cake. The N concentrations and the nitrate concentrations of the shoot increased from 0 to 25 g of cake so the N may be provided by the decomposition of soy sauce cake. However, the concentration of Na was proportionally higher in the shoots to the application rates of soy sauce cake. It was 60 mgNa/g DW with 100 g cake was added. In the second experiment, a similar pot experiment was conducted to compare the application of 0, 5, 25, 50, and 100 g soy sauce cake or desalted soy sauce cake. We expected that the application of desalted soy sauce cake may give more effective than the original soy sauce cake due to the lack of salinity stress. However, the growth of lettuce was more severely depressed by the addition of desalted cake compared with the original cake. As the application rates of soy sauce cake increased, the water content in the surface soil increased. The poor growth of lettuce with desalted cake may be due to the excess water stress, which represses respiration and nutrient absorption of the roots.