20 



plants. It was also shown that the straw and haulm seldom contain more than one to tln-ee per 

 cent, of phosphoric acid, this substance being segregated in the seed. For the analytical evidence 

 of these positions I beg to refer to the Farmer's Dictionary, in which the admitted analyses of all 

 plants hitherto examined Avill be found. 



2. That phosphoric acid is the least developed of all the mineral bodies of the soil, being seldom 

 present to the extent of 0.5 per cent, and usually less than 0.1 per cent., in good soils. 



3. That many soils containing from five to twelve per cent, of humus are known to be 

 steril. 



4. That the amount of phosphoric acid removed by given seed crops far exceeds that removed 

 by the ordmary forage crops, being often five times as great. 



The evidence of these positions was set forth at length in that communication and is therefore 

 not worthy of repetition. The principle which I believe was fairly reached, and admitted, was that 

 seed crops exhaust the soil of phosphoric acid — the deprivation of which is easily perceived, even 

 in the best lands. It is not necessary for me to advance further evidence of this fact before 

 your Association. 



If it be admitted that phosphoric acid is segregated in the seeds, it is evident that the 

 exhaustion eifected by foliage plants, as tbbacco, cabbages, flax, hemp, etc., not intended for seed 

 and of the root crops, with perhaps the exception of turnips, is due to another cause. The experi- 

 ments of BoussAiNGAULT and our own observations on natural rotations will now throw light on 

 this other kind of exhaustion. Some plants draw all their azote from organized matters in the soil, 

 others from the air ; some families of plants appear only on rich soils and around dung-hills, whilst 

 others inhabit the mineral earth destitute of organic matters. It is evident that j^hosphoric acid has 

 nothing to do with this peculiarity, for none is removed from the soil, the dead plants restoring it ; 

 there is a diminution only in volatile matters or in the azotized products of the decaying organic 

 matter. Let us cultivate a few crojjs of cabbages or tobacco on a rich spot of land, how soon will 

 the organic matter disappear ! Practical men may tell us diat this is because the crops are hoed 

 and the soil exposed to the sun, but this is not the cause ; the hoeing improves the plant becajise by 

 introducing air it hastens the decomposition of the organic matters of the soil or assists the fixation 

 of atmospheric nitrogen. {See Mulder. Journ. fur. Pract. Chcm. XXXII. p. 344). AVhen putres- 

 cent manures are added to tobacco, potatoes and similar crops, the indication is to furnish azotized 

 matters, and is altogether different from the object in view when it is added to wheat and certain 

 grain crops. But if this point requires further evidence we may appeal to those plants which 

 exhaust the soil differently under different circumstances. A flax crop raised for its fibre exhausts 

 tlie soil of azote and may be followed by corn or beans, but if it be allowed to mature seeds it 



