Effect of drying temperature on amount of nutrient elements extracted with0.01 M CaCl2 soil extraction procedure

In the current soil drying protocol of the 0.01 M calcium chloride (CaCl2) procedure, soils are oven dried at 40 degreesC for 24 h. At this drying tem perature, as well as at lower drying temperatures, a change of the actual s oil nutrient element status cannot be excluded because microbes will be act ive during part of; the drying period. However, a higher drying temperature may affect soil characteristics and soil processes and also lead to a misi nterpretation of the soil nutrient element status. An explanatory study was conducted to get more insight into the effect of i) oven drying temperatur e and ii) the use of forced-air ventilation at low drying temperatures on n utrient elements extracted with the 0.01 M CaCl2 procedure. The goal of the study was to investigate the perspectives of optimization of the soil dryi ng protocol of the 0.01 M CaCl2 procedure. Three moist test soils with diff icult soil characteristics were oven dried at 20 and 40 degreesC with and w ithout forced air ventilation and at 70 and 105 degreesC without forced-air ventilation. The moist test soils and the dried soils were extracted with a 0.01 M CaCl2 solution and pH and total N (N), ammonium-nitrogrn (NH4-N), nitrate-nitrogen (NO3-N), ortho-phosphate (ortho-P). potassium(K), magnesiu m (Mg), sodium (Na), and manganese (Mn) determined in the supernatant after centrifugation. Soluble organic N (org-N) was calculated as the difference between N and the summation of NH4-N and NO3-N. In the temperature range f rom 40 to 105 degreesC, ortho-P, NH4-N, org-N, and Mn extracted tended to i ncrease two or threefold for each 30-35 degreesC increase in drying tempera ture. Differences in ortho-P, NH4-N, org-N, and Mn extracted at 20 and 40 d egreesC were relatively small. The pH, K, Na, and NO3-N extracted were affe cted by drying temperature but the effect was variable. Magnesium extracted was not affected by drying temperature. The use of forced air ventilation at 20 and 40 degreesC had no significant effect on the amount of org-N, NH4 -N, ortho-P, K, and Mg extracted. There were significant effects of forced- air ventilation on pH and NO3-N, Na, and Mn extracted but the effects were variable. Test values (60-70%) of the moist test soils were significantly d ifferent from the respective test values of the test soils dried at 70;and 40 degreesC with and without forced-air ventilation. Based on the differenc es between moist and dried soils, it is questionable if soil drying should be recommended in the 0.01 M CaCl2 procedure. Therefore, further research s hould focus on the relationship between soil test values of moist and dried soils with crop response. If soil drying is preferable drying temperature should not exceed 40 degreesC.