Comparison of sodium hydroxide-tetraborate and sodium pyrophosphate as extractants of Al/Fe—Humus “complexes” in dystrandepts, Japan

Abstract

Extractions were made with tetraborate and pyrophosphate of the samples of 26 A 1 horizons of soils derived from volcanic ash differing in origins and ages. The samples were divided into three groups. Group 1 comprised eight samples from soils in ash less than, and Group 2 five samples from soils in ash more than 1,000 years old. Group 3 comprised thirteen samples from soils in ash more than 2,500 years old that had also been buried. Effectiveness of the two reagents was gauged by the amounts of C, Al and Fe removed by the treatments. Amounts extracted by tetraborate ranged from 0.26 to 7.02% of C, 0.06 to 1.05% of Al and 0.01 to 0.07% of Fe. Those extracted by pyrophosphate ranged from 0.32 to 13.02% of C, 0.09 to 3.92% of Al and 0.05 to 3.16% of Fe. Tetraborate extracted smaller quantities of all three elements from every sample than did pyrophosphate. Similar results were obtained by multiple extractions. The comparisons thus indicate that pyrophosphate is the more effective extractant of Al/Fe—humus complexes in A 1 horizons of soils formed in volcanic ash. Amounts of the elements, especially Al, extracted by the two reagents differed among the three groups of soil samples. The amounts of Al were smallest for samples from the youngest ash deposits, larger for the samples from older ash deposits still at the land surface, and smaller again in the samples of buried soils. Ratios of the amounts of Al and C extracted by tetraborate and those extracted by pyrophosphate were progressively lower from soils in the youngest ash to soils in the older ash to the buried soils. These ratios and the differences in amounts extracted from samples of the different groups suggest that the nature of the Al-humus complexes changes as soils become older and also if they are buried.