CONSERVATION OF N-MIN SOIL SAMPLES BY MIC ROWAVE DRYING

Rationalizing analytical lab work and preparing original soil sample r eserves for repeated analysis needs conservation of field moist N-min soil samples. Today's procedures e. g. common thermal drying or freeze drying modify the NO3 and NH4 content of samples. Regarding the resul ts of immediately analyzed field moist samples, effects of short term sample drying with a microwave oven research plant (max. 100 degrees C ) and a common household appliance, resp., were studied at 106 samples , dervied from different unfertilized arable soils. Related to field m oist analysis soil nitrate content was equivalent in microwave dried s amples. Although a 3fold extraction of field moist soil samples releas ed up to 30 % more nitrate than a single extraction, the nitrate conte nt of microwave dried samples did not increase. On the contrary, the s oil NH4 content was evidently raised when using both experimental and household microwave oven, but was still lower than in freeze dried soi ls or in samples stored in a drier cabinet at 105 degrees C. Generally , the increase of ammonia content by microwave drying was about the sa me in loamy and sandy soils. Interactions between drying conditions e. g. energy input, temperature, time etc. are suggested. Moreover, the NH4 analyzed is assumed to originate thermochemically from macromolecu lar organic matter. For methodological reasons the quantitative consid eration of soil samples' NH4 content is doubtful for fertilizer recomm endations, but the majority of the conventionally analyzed N-min sampl es contain less than 10 kg ha(-1), except organic soils or soils ferti lized with liquid manures or comparable substances. Therefore microwav e drying is a useful tool for the conservation of soil samples in the case of nitrate being the crucial N-min constituent. As a lab equipmen t microwave plants have to be able to control the soil sample temperat ure, but temperature limitations for drying process need further inves tigation.