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Microwave Radiation and Incubation Effects on Resin-Extractable Nutrients: I. Nitrate, Ammonium, and Sulfur

Dep. of Plant and Soil Science, Montana State Univ., Bozeman, MT 59717

*Corresponding author.

ABSTRACT

The phytoavailability soil test (PST) is a resin-extraction methodology being developed for use on fresh soil samples. Mineralization of N and S during shipment of samples to a soil-testing laboratory is of concern with this soil-test approach. The objective of this research was to investigate microwave-radiation (MWR) treatment of soil samples to arrest microbial conversions of nutrients during incubation of fresh samples. Fresh samples from three Ap horizons of Montana soils (Haploborolls) with different fertility histories were subjected to MWR for 0 to 4 min, and then incubated at 20 °C for 0 to 12 d. Subsamples were also dried before being subjected to the same treatments. Nutrients were extracted with mixed-bed ion-exchange resin extractors placed in saturated pastes of each sample and by standard soil tests on dried soil samples. Results for NO₃-N indicate that MWR prevented nitrification, but extractable NH₄-N and S varied in a manner suggesting release of interlayer NH₄ and thermal decomposition of organic components. Compared with standard soil tests, PST was similar in sensitivity to changes in NO₃-N and S and more sensitive to changes in NH₄-N. These soils had up to three times more NH₄-N than NO₃-N in either the fresh or dried condition. Based on these results, MWR is not recommended for arresting microbial activity prior to extraction for soil-test purposes, either by standard or PST methods. Changes in the two forms of plant-available mineral N occurring during shipment or increases in total mineral N or S may need to be dealt with through other means of microbial arrest, or through calibration adjustments.

Contribution no. J.2438 from Montana Agric. Exp. Stn.

Received for publication January 12, 1990.