28 



On Agricultural Chemistry. 



in the well-defined limit to their action which is indicated, 

 further proof of the necessity of an artificial supply of nitrogen 

 in the soil for the increased production of corn, and the incapa- 

 bility of mineral supply to yield any increase, excepting when 

 nitrogen is thus provided. 



Turning now to Table X., we find that the 98 tons of farm- 

 yard manure which have been supplied during the last seven years 

 have only given an increase of 73 bushels of dressed corn and 

 3 tons 15 cwts. of straw over the unmanured plot. This is equiva- 

 lent to only f of a bushel of corn and f of a cwt. of straw for every 

 ton of farm-yard manure supplied! Farm-yard manure is, how- 

 ever, a very variable compound ; its composition being dependent 

 upon the amount and quality of the food consumed by the animals 

 which have produced it. According to the mean of several direct 

 experiments upon very rich box-manure, a ton of it, in round 

 numbers, is composed of 141 cwts. of water, and 5^ cwts. of dry 

 substance, the latter of which contains a large quantity of mineral 

 matter and nitrogen equal to about 20 lbs. of ammonia ; on the 

 other hand, a ton of manure composed merely of straw wetted to 

 the same degree, with of course the same amount of dry substance, 

 only gives nitrogen equal to about 5 lbs. of ammonia, and probably 

 much less than half of the more important minerals of the rich 

 box-manure. The farm-yard manure carried out of our yards, 

 as to nitrogen, will generally have a composition intermediate 

 between these two extremes ; and we may at any rate assume that 

 that which we employed would contain about 5 cwts. of dry sub- 

 stance, of which the dry organic matter, rich in carbon, would be 

 three or four times as great as, and the minerals also much in ex- 

 cess of, that required by the increase of corn and straw which we 

 have seen to be obtained by their use. It is evident therefore that 

 there must have been a great expenditure of non-nitrogenous 

 organic substance, and of mineral matter, without effect ; and we 

 conceive that the increase obtained was much more intimately 

 connected with the amount of nitrogen contained in the manure 

 supplied. This view, indeed, would appear to be beyond doubt 

 when we consider that the application of ammoniacal salts alone, 

 during the last six years^ as in plot 10a, has given an average 

 increase within one bushel of that which has been obtained by 

 the use of farm-yard manure. 



Neither mineral manures nor carbon then, are indicated by our 

 experiments as the special or direct manures for the growth of 

 wheat. Not so, however, with the turnip, for the successful 

 cultivation of which a liberal supply within the soil of carbona- 

 ceous substance and phosphates is found to be important. We 

 have here then a remarkable contrast — for if in practice the 

 Wheat plant be supplied with a sufficient amount of nitrogen. 



