1864.] CHEMISTRY OF MANURES. 215 



fixed and exclusive relation, as the method in question supposes, 

 to its immediate influence on the crop. The reader who has 

 accompanied the reporter through the foregoing pages of this 

 section will be prepared to recognize that, under conditions of 

 frequent occurrence, a luxuriant crop, obtained by the use of an 

 artificial manure, so far from manifesting increased fertility, may 

 but be the sign and measure of accelerated exhaustion. He will 

 also understand that a manure may have added not a single sheaf 

 to the harvest, not so much as one blade to the yield of hay, and 

 yet may have solved the great problem of agriculture, by exactly 

 balancing the drain made on the soil by the crop. 



An unlimited supply of the former manure might be a positive 

 curse to a nation, by tempting them unduly to exhaust their 

 soil. The gratuitous gift of the latter, on the contrary, in due 

 adaptation to every field, would be the most precious boon a 

 nation could receive ; because it would place their agriculture on 

 a footing of perdurable prosperity. 



It may however be urged that the object of the Rugby experi- 

 ments is simply to determine the intrinsic value of the Rugby 

 sewage ; meaning its degree of richness in available plant-food of 

 all kinds, or its absolute crop-increasing power. And this infor- 

 mation, it may be contended, the direct test to which the sewage 

 is brought at Rugby (and which may be compendiously termed 

 tiie crop-test), seems, at all events, well adapted to elicit. 



But a very brief consideration of the matter, in the light of the 

 above-stated principles, will suflSce to show that these reasonings 

 also are illusory; and that the crop-test, of itself, cannot afford 

 any reliable or conclusive information as to the crop-increasing 

 power of sewage. 



For the benefit resulting to any given crop, from the use of 

 any given manure, will vary from absolutely 7iil up to the maxi- 

 mum attainable effect, according to the nature and composition 

 of the soil, which, in the Rugby experiments, does not appear to 

 have been determined. The richer the soil of the experimental 

 fields, the poorer must the Rugby sewage seem ; because, however 

 rich this sewage may be, the increase it can determine in the crop 

 depends, not merely on the wealth it brings, but also on the want 

 which it supplies. 



The blowing sands at Craigentinny, manured with the Edin- 

 burgh sewage, want every form of plant-food but silica, and con- 



