63. The fertility of soils depends mainly on the presence 

 in sufficient quantities of four essential constituents of plant 

 food, viz. Nitrogen, Phosphoric acid, potash, and lime. In 

 fact, lime and potash being almost invariably present in suffi- 

 cient quantities, the excess or deficiency of Nitrogen and 

 Phosphoric acid are mainly looked to in judging of the che- 

 mical character of soils. If a soil contains *i to '5% of Nitro- 

 gen and *o8 to '5 of Phosphoric acid, it may be classed as a 

 fertile soil. Soils containing i/ of potash or lime (the latter 

 not as insoluble silicate but as carbonate) are to be considered 

 quite rich in these substances. A sample of dry soil showing 

 i of Nitrogen, phosphoric acid and potash would yield out 

 of a depth of 9 inches, two to three thousand pounds of each of 

 these constituents, per acre ; but no crop ordinarily takes up 

 more than 50 to 60 Ibs. per acre of these substances. So that 

 properly tilled a soil even left without manure would raise 

 hundreds of crops. A soil containing *2% of N. (calculated as 

 NHs), *2/o of P 2 O 5 and '5/ of potash, and weighing when per- 

 fectly dry i ,600,000 Ibs. per acre to a depth of 5 inches, is cap 

 able of affording 3,200 Ibs. of N. (calculated as NH 3 ), 3,200 Ibs. 

 of P 2 O. and 8,000 Ibs. of Potash. A good crop of 20 maunds 

 of wheat and 30 maunds of wheat straw per acre would not re- 

 quire more than 4olbs. of N., 2olbs. of P 2 O 5 and 261bs. of K 2 O. 

 The object of manuring is to give a larger quantity of readily 

 available plant food to growing crops and to help in dissolving 

 the plant food of the soil and thus augmenting its quantity. A 

 judiciously manured soil, also forest and pasture land, may go 

 on getting more and more fertile. So few pounds of P 2 O 5 , 

 K 2 O,N, and CaO are taken up ordinarily by crops, that it is 

 easy to more than recoup these by the use of proper manures. 

 To ascertain however whether a particular soil needs the addi- 

 tion of K 2 O,P 2 O 5) N, or CaO in the form of manure, or if 

 it is already sufficiently rich in this or that constituent and 

 it will be superfluous to use one or another of the manures, it 

 is not absolutely necessary to have recourse to chemical an- 



