216 
ROTATION OF CROPS. 
give it consistency : alumina is the only substance in the soil which is not, under some circum¬ 
stances, incorporated into the tissues of plants. Magnesia, in some soils, is rare; it is an im¬ 
portant substance, being required in all the cereals, root crops and fruits ; all the edible roots, 
grains and fruits seem to require it, in combination with phosphoric acid. The main point I 
have aimed at in the foregoing remarks is to show that no system of rotation is of a general 
value. 
The crops are to follow each other in such a rotation as to use to the greatest profit the ex¬ 
pensive elements I have named. A fallow or rest; a clover crop comes in as a part of the 
rotation. I believe, however, the effect of cultivating clover has not been well understood. 
This I infer from the expression clover sick. Land I believe is clover sick only when some of 
the aliments which clover requires have been exhausted, and the manures employed do not 
contain them. Clover ash contains silica, phosphoric acid in combination with lime, lime in 
combination with the organic acids, magnesia, a large quantity of potash and soda, chlorine and 
sulphuric acid; each one of these is equally important to the plant, and the probability is that 
when land is clover sick, there is a want of potash ; or at least the probability is in favor of this 
view. In New-York I am not aware that any of our fields have, as yet, refused to bear clover. 
I think, however, that the change is advantageous on the ground of partial exhaustion of the 
elements, or a diminution to that extent which a plant begins to feel, it may be potash which 
has been removed, diminished to one half the usual quantity ; there is not enough to supply the 
wants of a potash plant. When, therefore, a plant which reonires less potash is substituted, it 
may yield a good crop, and in its growth there will be no evidence of a partial exhaustion of 
this alkali. Properly, the land can never be said to be sick, as though it were pining under 
some disease, it may always be regarded as an exhaustion ; and the true remedy is to supply 
those elements which analysis prove are either deficient or wanting. 
There are also other differences in the composition of vegetables which it may be proper to 
notice. We may divide cultivated plants, for example, into three classes. 1. Those in which 
the seed forms the main object of culture. 2. Those which are cultivated for their herbage. 
3. Those which are cultivated for the root. Each class differs essentially from the other in 
the amount of water which they respectively contain. To save the trouble of reference to the 
second volume, I collected the results, which I have obtained by carefully conducted experi¬ 
ments. I prefer to give my own work for two reasons. The results were obtained from sub¬ 
stances grown in the State, and second, my experiments were conducted uniformly, and hence 
should give uniform results. 
