Rotations. 
21 
while the surface \ soil may be drawn on by the one crop the 
lower layer is accumulating a supply against the next crop, 
and vice versa. 
4. Allow one crop to prepare the soil for another. 
An example will best explain this. Clover is a crop which 
finds its own nitrogen and takes little of it out of the soil — 
indeed increases the amount there through the action of the 
nitrogen -fixing “ bacteria ” in the tubercles on the roots. On 
the other hand, clover requires mineral manures. Wheat 
requires plenty of nitrogen but is much more independent of 
mineral manures than is clover. Hence clover is a good 
preparation for wheat. 
5. Check influence of insect and fungous pests. 
When the same crop is repeatedly grown on the same field 
there is liable to be a gathering strength from year to year of 
the insects and fungous pests which infect that crop. One of 
the most prominent illustrations of this is in the case of potato 
growing. The residue left in the land in the shape of haulm 
and tubers may be the means of continuing or even increasing 
the diseases from which this crop suffers. Thus potatoes after 
potatoes are more liable to be diseased than when they are not 
grown for several years afterwards, and when they are taken 
on absolutely fresh ground the percentage of disease is still 
smaller. Potatoes are often grown year after year in a garden, 
but then the early raising and use reduce the harm done by 
an attack of disease very much. Finger-and-toe in turnips is 
another illustration of the necessity of a change to reduce the 
chance of an attack from the disease. 
With insects a case in point is that of wireworm, where 
a bare fallow will help to eradicate the “ worms.” These 
usually live several years in the larva stage, so that the stoppage 
of the food supply compels them to go into the chrysalis stage 
and thus cease further depredations, whereas the repetition of 
a crop — especially a corn crop — would perpetuate the trouble. 
Clover “ sickness ” again is another illustration, as at least 
one form of it is due to fungoid agency, and leaving as many 
years as possible between successive crops of the same is 
beneficial. 
6. Influenced by soil texture. 
The texture of the soil has much to do, of course, with 
influencing the nature of the crops grown, but this takes 
effect often without altering the regularity of a rotation. For 
instance, on a heavy soil where root-growing would be 
expensive and precarious it is common to substitute a bare 
fallow ; it is a fallow in both cases, and therefore the circle of 
