Abstract

In order to monitor spatial inorganic N concentrations in soils over time, collected soil samples are composited and thoroughly mixed. The sticky characteristic of Vertisol when wet makes it difficult to thoroughly mix wet samples. There is the need for appropriate drying of Vertisol samples to facilitate thorough mixing and storage to retain mineral N content reflective of field status. We determined the effects of drying method, storage period and C:N ratio on inorganic N contents of tropical Vertisols. The treatments comprised of three soils with different C:N ratios, five drying methods inclusive of a check (method 1), and five storage periods including a check (period 1). Storage temperature was -10°C. In method 1, field-fresh soil samples were kept at -10°C and later used for mineral N extraction using 0.5 M K₂SO₄. Period 1 involved storing soil samples for two days from sampling date before mineral N extraction. Ammonium-N and NO₃ -N were determined using colorimetric method. Method 1 x period 1, for each soil, resulted in NH₄⁺-N and NO₃^--N contents significantly different from other treatments indicating that extraction for mineral N should be done on field moist soil not later than two days after sampling.

Key words: Drying method, mineral nitrogen, storage period, soil preparation, Vertisols