t'LANT FOOD IN SOIL 3? 



ComiDines with ammonia to form an amide. The 

 humus of soil always contains nitrogen; the amount 

 is very variable, depending on its past history. It 

 has apparently in part an amide nature, and jdelds 

 ammonia and soluble nitrogenous bodies when boiled 

 with dilute acids or alkalies. 



The humic matter of soils is of great agricultural 

 importance ; not only does it profoundly modify the 

 physical properties of soil, it is also the principal 

 source of the nitrogenous food of plants. A soil rich 

 in humus is rich in nitrogen ; a soil poor in humus 

 is poor in nitrogen. The old pasture at Eothamsted, 

 with roots removed, contains when dry, in the first 

 nine inches, 0"245 per cent, of nitrogen, and 3"36 per 

 cent, of carbon, corresponding to about 5*6 per cent, 

 of humus. In very rich English pastures the per- 

 centage of nitrogen will reach 0'5 or 0'6. The soil of 

 old kitchen gardens, 'and the black soil of Manitoba, 

 may contain a similar amount. The arable soil at 

 Eothamsted, a heavy loam, contains 010— 0*15 per 

 cent, of nitrogen ; and its clay subsoil, down to nine 

 feet, 0"04 — 0'05 per cent. A sandy subsoil will contain 

 less nitrogen. 



7. Plant Food in Soil. — The proportion of plant 

 food present in soils is very small, even when the soil 

 is extremely fertile, the bulk of the soil serving chiefly 

 as a support, and as a sponge to hold water. A good 

 arable loam may contain 0*15 per cent, of nitrogen, 

 0*15 per cent, of phosphoric acid, 0'2 per cent, of 

 potash (soluble in hydrochloric acid), and 0*5 per 

 cent, of lime : much larger quantities may, of course, 

 occasionally be present. Plant food is not equally 

 distributed throughout a soil. If a soil is separated 



