Abstract
The effects of large-quantity applications of N, P, K, Ca, or Mg or a compound of these five components on soil microbial biomass (biomass) and soil microbial respiratory activity (respiratory activity) were investigated. It was found that the microbial biomass and the respiratory activity were affected by both the quantity and the kind of element. The biomass in the P-, K-, Ca-, or Mg-treatment was not significantly different from that in non-fertilizer treatment. However, the biomass showed the maximum value in the compound treatment with 5 times the standard fertilizer level. The biomass tended to increase with the increase of the soil EC and did not decrease even with a high EC of 3 mS cm^<-1>. On the other hand, the respiratory activity decreased with an increase of the soil EC and remained at a low level under conditions of more than 1.0 mS cm^<-1>. Most fertilizer salts contained sulfate as an associated element. The relationship between the amount of sulfate application and the respiratory activity resembled that between EC and the respiratory activity. The respiratory activity remained at a low level under conditions of more than 2,000 mg kg^<-1> of sulfate. Thus, the respiratory activity decreased with an increase in the salt concentration, and EC or sulfate values that decreased the respiratory activity were almost the same as or somewhat higher than those recommended as a result of common soil diagnosis.
