THE SOIL 59 



found to bo about 190 lb. per acre, while from plots of the 

 same soil bearing crops the loss was in some cases very small — 

 e.g.y with i^e grass only 2*3 lb.* 



The other substances carried off in drainage water, though 

 considerable in quantity, are of less importance from a prac- 

 tical standpoint. The largest is the calcium carbonate, and 

 this naturally varies very much in different soils. From soils 

 on igneous rocks its amount is estimated by Continental 

 observers at 500 lb. per acre per annum, while from the 

 chalk soils as much as 2700 lb. per acre may be removed 

 in a year. The amount is increased when ammonium com- 

 pounds are used as manure. English estimates are lower. 



The loss of phosphoric acid is probably very small, except in 

 the case of peaty soils, which, though often very deficient in 

 this constituent, generally lose much in the drainage, probably 

 because of the solvent action of the vegetable acids and the 

 carbon dioxide produced by the decay of organic matter. In 

 German experiments the annual loss per acre varied from about 

 8 lb. from clay to 19*6 lb. from peaty soils. 



The loss of potash is very variable, but seldom of much 

 importance in this country. Of course, under exceptional 

 circumstances drainage water may be very rich in dissolved 

 matter ; e.g.^ the drainage water from gardens, when excessively 

 large quantities of manure are used, may contain as much as 

 8'4: parts per million of potash and 33 parts of nitrogen 

 pentoxide. 



Analysis of Soils. — The presence of an adequate store of 



constituents of plant food in a soil is not sufficient to ensure 



fertility, and this is true even when the physical condition of 



the soil is suitable, for it is necessary that the constituents of 



plant food should be in such a state that they can readily be 



assimilated by the plant, A com'plete analysis of a soil, stating 



• These losses refer to nitrate, &c., carried off in the drainage water. 

 In some cases, a soil maj, in spite of such losses, actually become 

 richer in combined nitrogen owing to fixation of atmospheric nitrogen 

 by micro-organisms, e.g., A'^otohacter (seep. 66). 



