Pj. Van Erp et al., Effect of drying temperature on amount of nutrient elements extracted with0.01 M CaCl2 soil extraction procedure, COMM SOIL S, 32(1-2), 2001, pp. 33-48
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.