( 53) 
3d. Vegetables, whether of the same family or not, 
having a similar structure of roots, should not succeed 
each other. It has been observed that trees suffer 
considerably by the neighborhood of sainfoin and 
lucern, on account of the great depth to which the — 
roots of these plants penetrate—whereas culmife- 
rous grasses do them no harm. 
4th. Annual or biennial trefoils prevent the escape 
of moisture from sandy and arid soils, and should 
constantly cover them in the absence of other 
plants;(1) while drying and dividing crops, as beans, 
cabbages, chickory, &e. &. are best ithe lo correct the 
faults of stiff and wet clays. 
5th. When plants are cultivated in rows or - hills, 
and the ground between them is thoroughly worked, the 
earth is kept open, divided and permeable to air, heat 
and water, and accordingly receives from the atmosphere 
nearly as much alimentary provision as zt gives to the 
plant. This principle is the basis of the drill hus- 
bandry. 
6th. All plants permitted to go through the phases 
of vegelation (and of course to give their seeds) ex- 
haust the ground, ina greater or less degree ; but if 
cut green, and before sceding, they take litlle from the 
principle of fertihty. 
. 7th. Plants are exhausters in pronorléon lo the 
length of time they occupy the soil. Those of the 
culmiferous kinds (wheat, rye, &c.) do not ripenun- 
— 
Romans. We have proof of this in the following passage of Festus: “ Restibilis ager 
fit qui continuo biennio seritur farreo spico, id est aristatp, quod, ne fiat solent, quz 
pradia locant, excipere.” 
(1) The “ Sterilis tellus medio versatur in stu,” of Virgil, shows the opinion he 
entertained of a husbandry that left the fields without vegetation, 
