- NUTRITION IN VEGETABLES. 69 
_ and to be frequently turned up by the plough that the clods 
may be broken down and pulverised. 
242. A loose sandy soil does not retain the moisture af- 
_ forded by the rain, which is so essential to the growth of the 
plant; and, owing to its want of cohesion, cannot fix the 
plant, so that when young it is liable to be rooted up by the 
winds. Besides these mechanical bad effects of a loose san- 
_ dy soil, it is also inadequate to the growth of crops from the 
_ absence of lime and alumina, which, there is every reason to 
_ suppose, exert such a beneficial agency on the plant. From 
_ these causes, this kind of soil is also deficient in natural ve- 
_ getable productions, the remains of which afford, in good 
soils, so large a proportion of the food of the plant. A soil 
such as this must be improved by the admixture of argil- 
_ laceous and calcareous earth, and must be well manured, or 
_ have a crop of some light vegetable ploughed in with it, be- 
fore it can be considered as fit for the Se of a good - 
crop. a : 
243. Besides these, there are many other Circatiistictoes 
which affect the productiveness of soils; such as, the facility 
with which they are heated by the rays of the sun; the 
_ length of time they retain their heat ; the power of absorb- 
ng moisture from the air; the degree of evaporation from 
heir surface; their power in acting upon, combining with, 
and retaining the organized matter in the soil, which is 
greatest in rich soils, those which contain much alumina 
and carbonate of lime, and least in sandy soils, in which the — 
organic matters, not being attracted by the soil, are decom- 
nature of the subsoil ; and many others, upon which it would - = 
posed by the air, or dissolved and removed by water; the _ 
