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plants. It was also shown that the straw and haulm seldom contain more than one to three 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 will 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 ordinary 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 effected by foliage plants, as tobacco, 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 phosphoric 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 crops of cabbages or tobacco on a rich spot of land, how soon will 
the organic matter disappear ! Practical men may tell us that 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 because 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. Chem. XXXII. p. 344). When 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 
the soil of azote and may be followed by corn or beans, but if it be allowed to mature seeds it 
