SOIL AND SUBSOIL. 133 



Amounts of Humus in Soils. While in peat, marsh and 

 muck lands the humus-content may rise above twenty per 

 cent, in ordinary cultivated lands it rarely exceeds about five 

 per cent, and very commonly falls below three per cent, even 

 in the humid regions. In properly arid soils we find a very 

 much lower average, rarely exceeding one per cent, and fre- 

 quently falling to .30 and even less. This scarcity of humus 

 manifests itself plainly in the prevalently light gray tint of the 

 arid soils. 



Meadows and woodlands generally show the highest humus- 

 content in their surface soils, gradually increasing while in that 

 condition; while when taken into cultivation the humus-content 

 gradually decreases, owing to the free aeration and consequent 

 " burning-out " caused by tillage. Hence the humus must be 

 from time to time replaced by the use of stable manure, or 

 green-manure crops, to prevent injurious changes in the tilling 

 qualities of the land. Not only humus as such, but according 

 to Schloesing also the insoluble colloid humates, produce in 

 the soil a loosening effect or tilth (Germ. Bodengare), which 

 apparently cannot be brought about by any other substance. 1 



Humates and Ulmatcs. That the insoluble humates of lime, 

 magnesia, iron, manganese and alumina are present in most 

 soils is conclusively shown by the composition of the solution 

 obtained by the extraction of soils with weak acid, as above 

 mentioned in connection with the quantitative determination 

 of humus according to Grandeau ; since these bases are almost 

 always extracted by the weak acid. When the brown solution 

 of alkali humate obtained in this process is carefully neutralized 

 with sulfuric or hydrochloric acid, fir is mixed with solutions 

 of the above bases, fiocculent, insoluble precipitates are formed, 

 while the solution is discolored. Similar precipitates may be 

 obtained with other metallic solutions, notably with that of 

 copper, which precipitates the humus-acids most completely. 

 Doubtless these compounds contribute greatly to the conserva- 

 tion of the humus-content of soils, protecting it to a certain 

 extent from oxidation, rind also preventing excessive acidity. 

 The brown tint of certain subsoils in the northern humid re- 



1 Tlie decrease of humus from wheat culture in the soils of Minnesota and North 

 Dakota lias been studied by H. Snyder and K. V. I. add, respectively. In the 

 prairie lands of the latter State the total organic matter in the first six inches of 

 soil ranges fram 15 to as much as 26 , and the bumus alone from 4 to 7.8 . 



