PERMEABILITY OF SOILS TO MANURES. 215 



able proportions are many. It is clear that the mass 

 and extent of the organs for the formation of grain (in 

 other words, the bulk of the straw) must bear a definite 

 relation to the product, that is, to the quantity of grain 

 produced: any excess or deficiency in the amount of 

 straw must always act injuriously upon the grain crop. 



When it is known that, on a given field, one part 

 by weight of corn to two parts by weight of straw is 

 the most favourable proportion for the production of 

 grain, then, according to theory, the manuring of the 

 field should not be such as to cause any marked altera- 

 tion of this relative proportion in the increased prod- 

 iice; that is to say, the several manuring substances 

 should be selected and laid upon the field in such quan- 

 tity and relative proportion, that the composition of 

 the soil may remain the same as it was before. 



It is well known that certain manuring substances are 

 especially favourable to the formation of the herbaceous 

 parts of plants, others to that of seed. Phosphates, as a 

 general rule, increase the grain crop : whilst of gypsum 

 it is well known that where that substance effects an 

 increase in the produce of clover-hay, this increase is 

 always attended with a marked diminution in the prod- 

 uce of seed. The cultivation of potatoes or Jerusalem 

 artichokes tends to reduce the excessive accumulation 

 in the arable surface soil, of substances which promote 

 the formation of the herbaceous parts of plants. 

 Theoretically, therefore, it is not impossible to main- 

 tain a certain uniformity of composition in the soil of a 

 field ; but this cannot be effected by carrying pn the 

 husbandry of an estate by the system of farm-yard 

 manuring. It will hereafter be shown that by the con- 

 tinuous and exclusive use of farm-yard manure, the 

 composition of the soil is found changed after each 

 rotation. 



The last point which claims our attention, in refer- 

 ence to the Saxon experiments, is the difference in the 

 permeability of the soil to the dung-constituents in the 

 different localities. The depth to which the alkalies, 

 the ammonia, and the soluble phosphates penetrate, 



