Soil fertility under various types of upland farming in northern Thailand. I. Case study of a village located in a transitional zone of hill evergreen and mixed deciduous forests

Soil fertility status under various types of upland farming was evaluated in a village in Mae Taeng District of Chiang Mai Province in northern Thailand. The farmlands were on sloping land in a transitional zone of hill evergreen and mixed deciduous forests (800-1000 m a.s.l.). Miang tea (Camellia sinensis var. assamica ), traditionally cultivated as the sole main cash crop, has been replaced by other cash crops. The studied soils originated from granite (n=30), limestone (n =16), and shale (n =6). Clay minerals were composed mainly of kaolin minerals in the granite soils and quartz, chlorite, gibbsite, mica, and kaolin minerals in the limestone and shale soils. Soil texture classes differed among soil types. However, granite and shale soils showed similar chemical properties to each other with an acidic nature and lower levels of exchangeable bases. Irrespective of the parent material, the CEC values were correlated with soil organic matter contents but not with clay contents. The levels of soil organic matter and the resulting CEC as well as ECEC values were higher in the limestone soils than in the granite and shale soils. These results suggest that higher negative charges in the limestone soils were derived from soil organic matter and that these charges developed under higher soil pH conditions. The C/ N ratios of the limestone soils were lower than those in the granite soils and shale soils, reflecting the well-decomposed status of soil organic matter. The granite soils were classified into three groups based on their physicochemical properties: foresttype soils (remnant forest, secondary forest, miang tea garden and lychee orchard), soils of annual crop fields, and soils of the home gardens. The forest-type soils were characterized by their strong acidic nature with lower contents of exchangeable bases and available P. Soils of the annual crop fields and those of home gardens showed a less acidic nature and nutrient accumulation occurring as a result of fertilizer application. However, a dif ference between these soils was that while the annual crop fields possessed preferable soil physical properties for crops caused by tillage management, soils of home gardens were subjected to compaction throughout soil profiles. In contrast to granite soils, the effects of different land uses on soil fertility were not evident in limestone soils. However, the annual crop fields and orange orchards had low total C and total N, engendering lower CEC values. Although the available P of these soils was high because of fertilizer application, fertilizer N in the annual crop fields seemed not to contribute to a buildup of total N and ammonium N. Both in lychee orchards with the granite soils and orange orchards with limestone soils, fertilizer P had accumulated around trees where fertilizer was usually applied. For shale soils, the subsurface soils under the oolong tea gardens (Camellia sinensis var. sinensis) were rich in total C and N and less compacted than soils of other land uses. Results show that dif ferent farming practices affected soil fertility status depending on soil types with different parent materials. 186 Sota TANAKA, Thanakorn LATTIRASUVAN, Kenji NAKAMOTO, Chalathorn SRITULANON and Katsutoshi SAKURAI