Does incubating bagged soil cores prior to oven drying effect extractable nutrient concentrations

The 2011 Australian Standards for Soil and Chemical methods state that soil samples should be kept cool or cold between field sampling and laboratory analysis to minimise biological transformations and other chemical reactions. This can be difficult in practice, as soil samples sent to commercial laboratories are often posted from their origin in sealed plastic bags and could spend several days in transit, before being dried for analysis. Depending on the air temperature during transit, it is possible that chemical and biological transformations may influence extractable nutrient concentrations. This replicated study used 2 different dairy pasture soils (red clay loam and sandy loam) to test the effect of storing field moist soil samples in sealed plastic bags prior to drying. Samples were stored at 4¢ªC, 20¢ªC and 35¢ªC for 48 h and the effect on Olsen and Colwell P, Colwell K and KCl S was measured. Results showed that the extractable nutrient concentration of soils were generally not affected by the incubation temperature, with extractable S concentrations marginally (P = 0.06) lower when incubated at 20¢ªC compared to cooled to 4¢ªC. It is possible that some S was immobilised by soil microbes under the warm, moist conditions, whereas temperatures of 35¢ªC may have been less conducive to microbial activity. These results suggest that some biological changes may have occurred as a result of temperature treatment, however the effect on extractable nutrient concentrations were minimal and the importance of cooling samples prior to extractable nutrient analysis, requires further investigation.