THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS. 



323 



is that of the reaction of the soil on litmus or other test paper, 

 to ascertain its acid, neutral or alkaline reaction. Should the 

 latter occur quickly (by the prompt blueing of red litmus 

 paper), " black alkali " would be indicated; but a blueing after 

 20 to 30 minutes means merely that a sufficiency of lime car- 

 bonate is present. An acid reaction (the reddening of blue 

 litmus paper) of course indicates a " sour " soil (see chap. 8, 

 page 122). 



Clicinical Analysis of Soils. When the observations men- 

 tioned above give no very decisive results or inferences as to 

 the soil's chemical character, the more elaborate processes of 

 qualitative and quantitative chemical analysis may be called in. 

 It would seem at first sight that these ought to yield very de- 

 finite results to guide the cultivator ; yet such is by no means 

 always the case. Both the previous history of the land, and 

 the method of anaylsis, influence materially the practical utility 

 of the results of chemical soil analysis. 



The cause of this uncertainty becomes obvious when we 

 consider the three groups of ingredients outlined above, viz., 

 the insoluble or unavailable, wholly undccomposed rock mine- 

 rals ; the " reserve," consisting of compounds not soluble in 

 water but soluble in or decomposable by weak acids; and the 

 water-soluble portion, either actually dissolved in the water 

 held by the soil, or held by the soil itself in (physical) absorp- 

 tion. While the latter portion is directly and immediately 

 available to plants, the amounts thus held are usually quite 

 small, and (outside of alkali lands) would rarely suffice for 

 the needs of a crop during a growing season. 1 This demand 

 must be materially supplemented by what can be made avail- 

 able from the soil minerals and the " reserve " by weathering, 

 conjoined with the direct action of the acids secreted from the 

 plant's root-hairs upon the soil particles to which they are 

 attached. It is obvious that the greater or less abundance of 

 the plant-food in the soil-material upon which these processes 



1 The investigations of King (On the Influence of Soil Management upon the 

 ^Vater Soluble Salts in Soils and the Yield of Crops, Madison, 1903) show that from 

 some soils at least, a sufficiency of plant-food ingredients for a season's crop may 

 be dissolved by distilled water alone, if the soil be repeatedly leached and dried 

 at iio\ Whether such a supply can be expected under field conditions, remains 

 to be tested. 



