66 THE SOIL OF THE TAR^,U 



acted as a powerful and economical supplement to farm- 

 yard manure wlien the latter was used in moderate 

 quantity. But there are many cases in which artificial 

 manures must be solely relied on. AVhere land is very 

 steep and hilly, and outlying, it i;? often cheaper to pur- 

 chase light and portable manures ; and to sup2)lement 

 their use in such cases, not with the manure-cart, but 

 by feeding sheep on the field on the roots or other green- 

 crop grown on the land. 



Tlie imj^ortimcc of artificial fertilizers cannot be oyer- 

 estimated in regard to the maintenance of fertility on 

 poor lands. Many of them have acquired also an ad- 

 ditional value on account of their special character, and 

 their special action on the quality of various crops grown 

 for industrial puri30ses, as sugar-beet, flax, etc. 



The action of manures is not, however, fully explained 

 by their affording a direct supply of plant food ; for many 

 of them operate indirectly to feed crops by their chemical 

 effects upon the soil. Thus, farm-yard manure, in under- 

 going decomposition yields a supply of carbonic acid, 

 which mav act on the mineral constituents and liberate 

 their elements. Many mineral manures also, common 

 salt, gypsum, and other saline matters, may react on the 

 soil, converting potash and magnesia, for instance, into 

 soluble forms, and thus giving the same result as would 

 follow an immediate use of the last-named substances. 



Again, certain manures which are used in large doses 

 influence the fertility of the soil by amending its texture, 

 or otherwise modifying its ph^^sical characters— as we have 

 already seen in the case of marling and mixing of soils. 

 Farm-yard manure is of great benefit in this respect, both 

 on heavy clays and on light sandy soils ; for in the one 

 case it diminishes tenacity, and in the other it lessens 

 porosity and helps to retain moisture. 



FarDi-yard manure is a mixture of the dung and urine 



