Physical and Chemical Properties of Tropical Soil for Rice Cultivation in Upper Northern Thailand during Wet Season, 2020/2021

Abstract

The upper northern region of Thailand, comprising eight provinces, has been classified into land suitability classes for rice cultivation as follows: highly suitable (S1; 1.9 million rai), moderately suitable (S2; 1.3 million rai), marginally suitable (S3; 320,000 rai), and unsuitable (N; 660,000 rai). This study aimed to analyze the physical and chemical properties of paddy soils in this region during the 2020/2021 wet season to provide a scientific basis for appropriate soil and fertilizer management for sustainable glutinous rice production. Soil samples were collected using purposive sampling and analyzed for their physical and chemical characteristics at the Phrae Rice Research Center laboratory. The results revealed that sandy loam was the most common soil texture, accounting for 43% of all samples. While this texture offers good drainage, it has limitations in retaining water and nutrients. Conversely, clay soils, which have high potential for moisture and nutrient retention, were found in very small proportions. Significant differences in chemical properties were observed among the land suitability classes. S1 soils had median values of organic matter at 2.34% (IQR: 1.76-2.92), available phosphorus at 12 mg/kg (7-23), exchangeable potassium at 81 mg/kg (47-120), and soil pH at 5.61 (5.16-6.17). S2 soils showed lower nutrient levels and higher acidity, with organic matter at 2.05% (1.59-2.58), phosphorus at 11 mg/kg (6-26), exchangeable potassium at 80 mg/kg (52-113), and pH at 5.31 (4.97-5.95), particularly in Chiang Rai and Lampang. S3 soils had the highest available phosphorus content at 30 mg/kg (15-66), with 2.18% organic matter (1.68-2.68), 88 mg/kg exchangeable potassium (52-143) and pH 5.61 (5.05-6.24). N-classified soils had organic matter at 2.12% (1.57-2.58), available phosphorus at 16 mg/kg (7-40), exchangeable potassium at 80 mg/kg (48-115), and pH 5.64 (5.08-6.17). The findings support that fertilizer application based on soil test results can improve nutrient use efficiency, reduce production costs and prevent excessive nutrient accumulation. In particular, phosphate fertilizer application should be reduced in S3 areas where phosphorus accumulation is high. This fertilizer management approach has the potential to enhance glutinous rice production in the upper northern region of Thailand.