194 § 107. ANALYSIS OF SOILS AND EOCKS. 



fers no further change m weight, and then calculate the 

 amount taken up by 100 parts of the air-dried soil. 



As this box, with its wet soil, is used subsequently for 

 experiments in the course of which tlie soil is dried 

 again, this trial may then be repeated ; some soils shrink, 

 while drying, to a greater extent than others, and it will 

 be found that, in this second trial, the power of holding 

 water will not be the same as at first. The difference, 

 however, is but slight. 



The j)Ower of a soil to hold liquid water increases with 

 the proportion of humus, but diminishes as the quantity 

 of clay increases. A strong clay soil may retain 27.3° |^, 

 a moderately heavy soil 30-31° j^,, a sandy loam 33-36° |^, 

 a black loam, rich in humus, 41° 1^. When some soils, 

 that had been tested as above, were tested also in their 

 natural position in the field, after a rain of 14 days, when 

 they might be supposed to be saturated, they wxre found 

 to contain 10° \^ less than was indicated by the results of 

 experiments in the laboratory ; hence, the determinations 

 made with small quantities in zinc boxes, appear to have 

 value only in so far as they enable us to compare the 

 water-holding powers of different soils. 



2. To determine the readiness with which water evap- 

 orates from the soil, the wet or damp soil may be ex- 

 posed, in a shallow tray, to the air, at the common sum- 

 mer temperature, or at that of the laboratory; but so 

 long as a considerable proportion of water is present, the 

 rate of cvajDoration remains about the same for all soils, 

 provided only that the same amount of surface is exposed; 

 it is also very slow, months being required to bring 100- 

 150 grms. of soil, in a layer no moi-e than 4-6 cm. thick, 

 to the condition of air-dried soil. 



When, however, natural circumstances are more closely 

 imitated, and a sufficiently thick layer of soil is experi- 

 mented with and exposed to the usual alternation of direct 

 sunlight and shade, the characteristic differences of soils 



