244 SOILS. 



the latter compounds may be regarded as the ordinary physi- 

 cal absorption, that of the former as the so-called chemical 

 absorption. 



655. The matters absorbed b}* the soil may be released after 

 a time and pass into solution again, or they ma}' be displaced 

 from the soil-particles by the filtration of new solutions. When 

 it is remembered that rain-water exerts a powerful solvent action 

 upon some portions of the soil, and that, on the other hand, 

 the soil can remove from aqueous solutions some of the matters 

 therein dissolved, the complicated nature of the problem which 

 presents itself is at once apparent. Examination of the waters 

 which drain through soil, and which may fairly represent the 

 resultant of the solvent action of the water and the absorptive 

 power of the soil, shows that from thirteen to fifty parts of solid 

 matters may remain dissolved in 100,000 parts of water. (The 

 question of nitrogen compounds in drainage-water will be ex- 

 amined in a subsequent chapter.) 



656. Condensation of gases by soils. Soils have the power of 

 condensing in their pores certain amounts of different gases. 

 These condensed gases are released when the soils are subjected 

 to a high temperature, say 140 C., and their amounts can then 

 be measured. The figures below give the results of the meas- 

 urements in several instances, 100 grains of soil being taken 

 in each case. 



Soil. Cubic centimeters of gas yielded. 



Peat 162 



Clay 30 



Moist garden soil 14 



It is found that in the soil there is present a smaller amount 

 of oxygen and a larger amount of nitrogen than in the atmos- 

 phere. The percentage of carbonic acid in the soil is also some- 

 what larger than that in the atmosphere ; especially in soils 

 which contain much organic matter. 



657. Root- absorption of saline matters from soils. Having seen 

 that the soil, the principal medium in which roots extend, pos- 

 sesses the power of absorbing and retaining water, saline mat- 

 ters, and gases, attention must next be directed to the conditions 

 under which the root-hairs can abstract from it the matters 

 requisite for the plant. These conditions are (1) presence of free 

 oxygen, (2) a certain temperature, (3) the presence of saline 

 matters in an available form in the soil. 



658. Free oxygen is necessary to all protoplasmic activity, 



