Rice husks of Koshihikari (Oryza sativa L.) were used for the following experiments. Rice husk ash was obtained by burning the husks in a field-scale boiler system (incineration capacity: 100 kg rice husks h−1) (Fig. 2).
The details of the boiler system are described by Tateda et al. (2016b). The silica in the ash was confirmed as amorphous using an X-ray diffraction analysis (XRD). The solubility of the silica was more than 50%, thereby, indicating its amorphous state. The solubility percentage of silica is an official indicator, which is highly correlated with the amorphous state of silica. Higher the solubility percentage, the more amorphous the state will be. Details of the solubility measurements are described by Tateda et al. (2016a). Komatsuna (Brassica rapa var. perviridis), a Japanese mustard spinach, was used for all the experiments. Possible contamination by radioactive cesium in the rice husk ash was also analyzed, because all the experiments were conducted after a severe incident involving an explosion at a nuclear power plant that had been damaged by the strong earthquake, which hit the northern area of Japan on March 11th, 2011. Levels of radioactive cesium (Cs-134 and Cs-137) were measured using the standard method for measuring radioactive cesium in fertilizers (MAFF 2011).
Pot experiment for comparison with different individual fertilizers
The purpose of this experiment was to investigate the negative effects of rice husk ash as a fertilizer for komatsuna compared to other common fertilizers and compost, using cultivation pots. The experimental procedure followed the standard method for solubility tests of silica (MAFF 2007). Neubauer pots (10−6 ha; inner diameter 11.3 cm × height 6.5 cm) were used for the experiment. Diluvium was used as the base soil, and its characteristics are shown in Table 1. For comparison, a dried cell fertilizer and farm compost were used, and their characteristics are shown in Table 2. Because nitrogen content was less than 2% for SF and CF2, an application of 5 g (dry base) was set as the standard. For CF1, 100 mg of nitrogen was set as the standard because the nitrogen content was more than 2% (dry base). The experimental design of the fertilizer and compost applications are shown in Table 3. The cultivation was conducted in a glass room and the room temperature was kept above 15 °C, with germination and growth observations conducted on days 5 and 9, and on days 14 and 21, respectively.
Pot experiment for comparison with different mixtures of fertilizers
The purpose of this experiment was to investigate the negative effects of rice husk ash mixed with compost on komatsuna using cultivation pots. The experimental procedure followed the standard method for solubility tests of silica (MAFF 2007). Neubauer pots (10−6 ha; inner diameter 11.3 cm × height 6.5 cm) were used for the experiment. Komatsuna was cultivated in a glass room at an average room temperature of 25 °C. Compost produced by mixing bark, poultry manure, tea dregs, pruned branches and/or leaves, and urea was used for making the bed materials for komatsuna growth. Tea dregs consisted of leaves left over from brewing tea, which is one of the most popular drinks in Japan. The experimental design is summarized in Table 4. Bed materials A, B, and C are compost products that are commercially available. Volcanic ash subsoil was used as the base soil. 150 ml of the bed materials was added to the base soil (350 ml), which was 71 g, 70 g, 45 g, 61 g, 50 g, 49 g, 57 g, 51 g, and 45 g for pots #1, 2, 3, 5, 6, 7, 8, and 9, respectively. 360 g of base soil was used for the control.
Field experiment with different quantities of silica
The purpose of this experiment was to investigate the negative effects of rice husk ash on komatsuna in field cultivation. Two greenhouses were used for the experiments. The configuration of the greenhouses and the experimental design are shown in Fig. 3.
Elemental components of the base soil in greenhouses A and B, and the pH of each section are shown in Tables 5 and 6. The pH of the soil in each section (Table 6) was measured by dissolving the soil in tap water (pH 7.31) at ratio of 2:1 (soil:water) by weight. Resistance values to lodging were also obtained to evaluate the negative effects of the rice husk ash on komatsuna (Fig. 4). Seeding was conducted at the end of November 2013 and harvesting was conducted at the beginning of February 2014.