Effect of drying temperature on amount of nutrient elements extracted with 0.01 M CaC12 soil extraction procedure

In the current soil drying protocol of the 0.01 M calcium chloride (CaCl2) procedure, soils are oven dried at 40°C for 24 h. At this drying temperature, as well as at lower drying temperatures, a change of the actual soil nutrient element status cannot be excluded because microbes will be active during part of the drying period. However, a higher drying temperature may affect soil characteristics and soil processes and also lead to a misinterpretation of the soil nutrient element status. An explanatory study was conducted to get more insight into the effect of i) oven drying temperature and ii) the use of forced-air ventilation at low drying temperatures on nutrient elements extracted with the 0.01 M CaCl2 procedure. The goal of the study was to investigate the perspectives of optimization of the soil drying protocol of the 0.01 M CaCl2 procedure. Three moist test soils with different soil characteristics were oven dried at 20 and 40°C with and without forced air ventilation and at 70 and 105°C 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-nitrogen (NH4-N), nitrate-nitrogen (NO3-N), ortho-phosphate (ortho-P), potassium (K), magnesium (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 from 40 to 105°C, ortho-P, NH4-N, org-N, and Mn extracted tended to increase two or threefold for each 30–35°C increase in drying temperature. Differences in ortho-P, NH4-N, org-N, and Mn extracted at 20 and 40°C were relatively small. The pH, K, Na, and NO3-N extracted were affected 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°C 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 different from the respective test values of the test soils dried at 20 and 40°C with and without forced-air ventilation. Based on the differences between moist and dried soils, it is questionable if soil drying should be recommended in the 0.01 M CaCl2 procedure. Therefore, further research should 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°C