Test results on the void ratio and the compressive strength of the hardened porous concrete are shown in Table 2. In general, when the aggregate grain diameter increases, the voids increasingly represent a structural defect and the compressive strength of the concrete decreases. The strength of the concrete is inversely proportional to the void ratio when the void ratio of the aggregate is different, even if the diameter range is identical. Using a hard aggregate, like iron ore, it is possible to increase the compressive strength without changing the void ratio. The addition of fertilizer lowers the void ratio by only a minute amount. However, the mixture of fertilizer hardly influences the compressive strength at an age of 28 days.

　

　

　

Results of the dissolution tests on nitrogen and phosphorus are shown in Table 3. They confirm that ammonia and phosphoric acid dissolve in the EPOC type mixture containing the granulated fertilizer after 90 days in the sea. The cumulative leaching of ammonia was 2.3%, and the cumulative leaching of phosphoric acid was 0.025 percent. It is confirmed that the elution function of the fertilizer component works effectively even if the enriched porous concrete is immersed in seawater.

　

　

　

Some test plates were sunk in the experimental sea area, and the adherent condition of the seaweed was observed. For this experiment, seaweed was naturally inserted into the test blocks. Therefore, a moderate seaweed community existed around the test plates, and an experimental site was now required which would provide hydrographic conditions suitable for the growth of seaweed. An experimental site satisfying the above conditions was selected, as shown in Figure 2. It is located in the SETO Inland Sea of Japan, and a community of Ecklonia cava exists within the circumference (Fig.3).

　

　

　

Three kinds of test plates were made with the mixtures shown in Table 1. The shapes of the test plates used at experimental site were 2 m in height x 1 m in width x 20 cm in thickness. They were immersed in the water at the low water depth line -0.5 m - -2.5 m on a caisson wall. The condition of the test plates is shown in Figure 4.

　

　

A biological study was carried out two years after the installation of the test plates. On the surface of the test plates, the type and the degree of cover of all the organisms which could be confirmed macroscopically were observed.