(1) The effect of inoculating the paddy field with nitrogen-fixing blue-green alga: Tolypothrix tenuis upon the yield of rice plant was investigated at nine experimental farms located in various districts through Japan. (2) The experiments conducted during five successive years showed that the amount of ammonification was higher in inoculated soils than in non-inoculated ones.(3) On an average the increase of rice crop obtained by inoculating the alga in relatively nitrogen-poor paddy fields was: 2.0% in the first year, 8.0% in the second year, 15.1% in the third year, 19.5% in the fourth year and 10.6% in the fifth year.(4) The fact that the favorable effect of algal inoculation increased year by year may be attributed to the accumulation in the paddy field of the algal cells which would be partially decomposed and partially remain and proliferate in the following years to increase, at any rate, the nitrogenous fertility of the paddy soil.
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