Introduction to Soil Chemistry

measuring soil water 87
soil is reported as the kilopascals (kPa) of pressure holding the water in the
soil. The importance of reporting soil water in this fashion can be seen in
Figure 4.2.
4.13.1. Laboratory Methods
The most important laboratory measure of soil water is the percentage water
on a dry-weight basis. In most cases, soil will be extracted or analyzed while
still moist to minimize changes that occur during drying. To obtain comparable
data from multiple analyses, the soil sample weight is corrected using the
percent water on a dry-weight basis. Because the water content of soil is highly
variable, the dry weight is used, as it is more constant. Soil is typically dried at
105–110°C for 24h in a drying cup as shown in Figure 4.9, and the amount of
water lost is divided by the dry weight of the sample and multiplied by 100.
The basic, simplified formula is given in equation (4.3a); formula representing
the actual calculation usually made (for soil dried in a drying can; see also
Figure 4.9) is given in equation (4.3b):
soil wet weight - soil dry weight
%water =
¥ 100 (4.3a)
soil dry weight
[ ( soil wet weight - can weight)-( soil dry weight - can weight)
]
=
¥ 100
( can weight - soil dry weight)
(4.3b)
Thus the can weight must be subtracted to carry out the calculation.
The percentage of water on a dry-weight basis is used to calculate the dry
weight of soil taken for an analysis. First, a 50-g sample of soil is taken and
dried. A second 25-g sample is taken and analyzed. The dried sample is found
to weigh 48g and thus lost 2.0g of water; thus:
2
%water = ¥ 100 = 4.% 2
48
(4.4a)
The water in the moist sample, taken for analysis, is found using this
percentage:
42 .
= 0. 042
100
weight of sample taken
Dry-weight sample analyzed =
1 + 0. 042
25
= = 24 g
1. 042
(4.4b)
(4.4c)