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52 Agricultural Chemistry. 



Relation to water. We have learned that a good soil consists 

 of solid particles of fairly uniform size. The spaces between 

 these particles constitute about 40 per cent of the volume. If 

 the particles are a mixture of large and small, as for example 

 gravel and sand, the volume of these spaces is much reduced. On 

 the other hand if the particles are themselves porous, as in the 

 case of chalk, loam,- and especially humus, then the volume of 

 the spaces is increased. It is this volume of the inter-spaces 

 which determines the amount of water which a soil will contain 

 when perfectly saturated, or the amount of air which it will con- 

 tain when dry. 



Humus increases the capacity for a soil to absorb and retain 

 water and consequently a crop grown on a soil containing a fair 

 amount of humus is less likely to suffer from drought. The fol- 

 lowing table illustrates this point. It gives the amount of water 

 held by 1 cubic foot of different varieties of soil ; 



Lbs. of water in 

 Kind of Soil 1 cubic foot 



Sand 27.3 



Sandy Clay 38.8 



Loam 41 . 4 



Humus 50.1 



Farm crops will not grow in a soil permanently saturated with 

 water and from which air, consequently, is excluded; the best 

 growth is obtained from soils one-half or two-thirds saturated. 

 The surface of a soil is seldom saturated, except immediately 

 after a heavy rain; it is the quantity of water which a soil will 

 retain when fully drained which determines its capacity for sup- 

 plying a crop with water. The amount of water permanently 

 retained by a soil does not depend upon the volume of the inter- 

 spaces, but upon the extent of internal surface, the water being 

 held by adhesion as a film on the surface of the particles. The 

 smaller, therefore, the particles of a soil, or the more porous, the 

 greater is the amount of water retained. Two samples of pow- 



