Micro flora related to the nitrogen cycle in the tropical upland farm soils

Samples of upland-farm surface soils (0–10 em in depth) belonging to various great soil groups were collected in 28 upland sites in Thailand during the rainy season.

Among the microbes related to the transformation of nitrogen, namely ammonifiers, ammonia oxidizers, nitrite oxidizers and denitrifiers, the count of denitrifier showed the maximum value amounting to 10⁴ to 10⁵ per 1 g of dry soil, followed byammonifier. The population level of nitrogen-fixing blue green algae was unexpectedly high, being 10³ to 10¹

The microbial counts in Brown Forest Soils, Rendzinas and Grumusols with high content of organic matter, available phosphorus and exchangeable potassium tended to be high.

Non-calcic Brown Soils, Reddish Brown Lateritic Soils, Alluvial Soils, Red-Yellow Podzolic Soils and Gray Podzolic Soils which lack in some nutrients showed intermediate levels of microbial populations, while the counts of nitrogen-fixing blue green algae in Alluvial Soils and those of denitrifier in Red-Yellow Podzolic Soils were markedly high. In the case of Low Humic Gley Soils and Regosols with low content of organic matter, available phosphorus and available potassium, the population of microbes was generally small.

The relationship between the organic matter content and the microbial population of soils was positively significant at 0.1 % level only in the case of fungal population (r=0.551), while the relationship between the available phosphorus content and the microbial population was positively significant at 0.1% level only in the case of Azotobacter (r=0.682).

The relationships between the total nitrogen, the exchangeable potassium, the amount of NH₄+^-N, the amount NO₂ ^--N, or the amount of NH₄ ⁺-N+NO₂ ^--N and each microbial population were not significant in any microbial groups.

The count of denitrifiers in upland farm soils of Thailand was 9 times as high as that in non-volcanic upland-farm soils of Japan and was 23 times higher than that in volcanic soils though large variations were seen among the great soil groups of Thailand. Conversely, the population of non-spore-forming nitrite oxidizers in the upland farm soils of Thailand was 1/100 that in non-volcanic soils of Japan and 1/280 that in volcanic soils. In the case of Azotobacter, the count in upland farm soils of Thailand averaged 2,800 per 1 g of dry soil. while that in non-volcanic upland farm soils of Japan was 77 on the average.

The ratio of aerobic bacteria to actinomycetes in upland farm soils of Thailand was 2.31, while that of non-volcanic soils of Japan was 7.28.