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have no knowledge imparted of what substances. ' It is not enough to say of manure, for this is a 
compost of all the bodies necessary for plants. If we still further advance the speculations of prac¬ 
tical men and assert that the exhaustion is of organic matter or humus, the position is denied by the 
second precept, for seed crops such as beans are exhausting whilst they require little humus 
— whilst on the other hand, many forage plants as cabbages, turnips, beets, are not seeds crops, but 
exhausting. We do not deny that excellent rotations devised by practical men do exist, but we do 
deny that every rotation based upon the foregoing indefinite precepts is necessarily good, and if they 
be no guide without the assistance of experience gained at great cost and by separate observations in 
the field, they are worse than useless. The defect of the precepts rests in this, that we are not 
informed in what respect the food of different plants varies, nor in what particular seed crops exhaust 
the soil. The apologists of the system may assert that these are remote facts not within the reach 
of the pmpounders, but this being the case the time has now arrived when a closer approximation to 
truth may be made and the former precepts abandoned or improved by modern investigation. 
V. Of the Exhausting Qualities of Crops. 
The soil may be exhausted to such a degree that it will cease to produce certain forage plants 
* 
without the introduction of a single seed crop. If we enrich any field so that it produces tobacco 
. and follow this crop by cabbages, turnips, flax, taking no seed from either, we speedily reach a 
period when none of these plants will yield a remunerating crop. This is one kind of exhaustion, 
but it is not complete exhaustion, for corn, wheat, oats, beans and clover seed may be obtained in 
good quantity from the same field. On the other hand, a few crops of hemp seed, linseed, corn, oil 
grains, wheat, will run down the land to barrenness; but this exhaustion is altogether different from 
the preceding; it is, in truth, the specific exhaustion produced by seed crops, and it matters not 
which are the seeds. Hence there are two distinct kinds of exhaustion well known to practical men 
and it behoves us, who desire the advancement of agriculture, to make the line of demarkation 
between them bold and distinct. There are other kinds of exhaustion to which we shall refer 
presently. 
In a paper I had the honor of reading before the Association last year, I made a thorough 
examination into the nature of the exhaustion of lands by seed crops. The object of the commu¬ 
nication was to prove the following points: 
1. That all seeds contain an excess of phosphoric acid, amounting usually to thirty-five or 
forty per cent, of the entire ash, nearly the whole of the ash being in many cases phosphates ; 
this was demonstrated in the case of corn, wheat, beans, hemp seed, flax, peas, cotton and other 
