Poicer of Soils to absorb Manure. 



141 



Fresenius found that 1,000,000 parts of air contained on the 

 averag:e of 40 days and nights 0.379 parts of carbonate of 

 ammonia, or 0.133 parts of ammonia itself; and although this 

 proportion is very small, yet inasmuch as every time the air is 

 renewed in the porous soil, the whole of the ammonia is removed; 

 and as in a highly-worked soil one-third or one-half of the 

 1000 tons, of which an acre 1 foot deep consists, may be sup- 

 posed to be ever on the watch for this prize, it is quite conceivable 

 that the balance between the supply and that required by the 

 crop may be in favour of the former ; but this result can only 

 be hoped for when the soil is brought into and kept in a con- 

 tinual state of subdivision and porosity so as to offer the freest 

 welcome to the enriching air, and this it is the object of the 

 Tullian system, as practised by Mr. Smith, to effect. Then again, 

 with ammonia comes carbonic acid, and as this gas constitutes 

 1 part in 1000 of air, it will be supplied in quantity nearly 

 10,000 fold more than the ammonia ; I do not therefore see whj^, 

 between the absorption of carbonic acid by the leaves and its 

 supply to the roots in a porous soil, sufficient carbon should not 

 be derived from the atmosphere for all the wants of a growing- 

 crop of wheat. But here again we can only conjecture — it is 

 impossible to say positively, on theoretical grounds, that such 

 icould be the case, but on the other hand it is equally impossible 

 to deny that it jniglit be. 



With regard to the mineral matters required by a crop of wheat, 

 the inquiry is far more simple. We know very accurately how 

 much the different parts of plants remove from the soil of potash, 

 magnesia, phosphoric acid, iScc. ; and we know that if they are 

 not added in manure they must be derived from the internal 

 resources of the soil itself — for the air will do nothing here. 

 We further know, from the analyses of soils that have been made, 

 what amount of these substances are usually to be met with in 

 the land. 



Now, although it has been a constant axiom in the instruc- 

 tions of chemists to farmers to " return to the soil what the crops 

 remove " — and every candid agricultural chemist will own, that 

 at the outset of his career he has somewhat overrated the im- 

 portance of literally fulfilling this obligation, — it is certain that 

 most soils of fair quality contain an amount of the different 

 mineral substances far greater than is necessary for many suc- 

 cessive crops of the most impoverishing character. 



In the table which follows I have given the quantities in pounds 

 of the different mineral substances required by a crop of wheat 

 of 35 bushels of grain and 2 tons of straw and chaff; a second 

 column of the table shows the amount removed by twenty such 



