Moisture of the Soil. 



491 



special cases, to secure sufficient herbage to afford an adequate 

 mulch. 



T7ie humus of the soil is the great store-house for nitrogen and 

 moisture. It is the accumulation of decaying vegetable or ani- 

 mal matter, and its presence in the soil, while not absolutely 

 necessary to the growth of plants, is the factor which makes the 

 land congenial for the very best development of the crop. The 

 constant use of commercial fertilizers, without being supple- 

 mented by barn manures or green manuring, will so reduce the 

 percentage of humus in the soil that its water-holding capacity 

 will be considerably diminished. This humus should be liberally 

 supplied by means of cover crops, rotations, and stable manures. 



148.— Showing the effect of 

 the roller in compacting 

 the surface layer. 



144.— Showing how the soil mulch 145.— The loose mulch on 

 should be restored by tillage forest soils, 



after the roller has been used. 



Underdrainage and how it acts as a conserver of moisture is 

 popularly misunderstood. It is usually supposed that under- 

 drains, instead of acting as conservers of moisture, produce ex- 

 actly the opposite effect. It has already been noticed that water 

 may exist in the soil as free or capillary and that the presence of 

 the free water within eighteen inches of the surface is positively 

 detrimental to the growth of most cultivated plants. Not only is 

 it necessary that moisture be supplied, but also that the soil shall 

 be in such condition that the air may have access to it, for a sup- 

 ply of oxygen is necessary to the breaking down and decomposi- 

 tion of organic matter and the making of plant food available. 

 The underdrain removes only the free water which may come too 

 near the surface and it leaves the soil above in a porous condition, 

 so that the water of rainfall may sink down instead of being 

 carried off by surface drainage. This rainfall water is not caught 



