Estimation of the amount of mineralized soil nitrogen from spring rainfall (air drying effect on ammonification)

We investigated the amount of soil nitrogen ammonificated in fields and we considered how to adapt it to the desing of fertilizer application. We obtained the following results. (1) The air drying effect on ammonification was evident when the water content of soil was between 18 to 20“, irrespective of soil types. (2) We noticed the inverse proportion in the relationship between the amount of soil nitrogen mineralized at 30degC for four weeks and the moisture of pre-flooded field soils. (3) By use of AMEDAS data, we obtained the estimated from the rain fall summed from March to April. The relationship between rainfall and soil moisture was proportional and the soil moisture was approximated as second dimensional equations for each soil types. The rainfall from November to February had week correlation to the extent of ammonification. Gley type soils were easy to dry, and muck type soils were difficult. Soils of other types showed intermermediate tendency toward dryness, and ther were strongly dryed with the less rainfall, on the contrary, they became wetter rapidly with increasing rainfall. (4) We noticed that the higher estimated level of ammonification makes the nitrogen uptake quantity of rice more than the normal value. So it is considered that the estimation method of the amount of ammonificated soil nitrogen is proper. (5) We estimated the amount of ammonificted soil nitrogen retrospective to the year of 1975, and we investigated the relationship between the index numbers of rice yield and the ammonification levels of soil nitrogen. We noticed that the high level of ammonification does not five good yieid. It seems to suggest that the growth promotion by the increased ammonificated soil nitrogen makes the growth regulation difficult. (6) We defined the rate of fertilizer application on the basis of air drying effect ammonification. When the sun of rainfall from March to April is below 100mm, 20“ of standard rate of fertilizer application should be diminished. The rainfall level of 100-120mm gives 90“ of the standard rate, the level of 130-180mm gives the standard rate, and the level of 190mm goes to 110“ of the standard rate.