|| its seeds, a larger quantity of the nutritive mat- 
ter of the soil is sucked up by it than when it is 
brought only to its less advanced stages. When 
crops of plants, therefore, are suffered to arrive 
at maturity, they are greatly more exhausters of 
the soil on which they grow than when they are 
cut down while they are green; and if those 
seeds are in whole or in part carried off the farm, 
the crops are exhausters of the farm, as well as 
of the ground which had produced them. Were 
the ripened seeds to be wholly returned to the 
soil, it may be believed that they might give 
back to it all the nutritive matter which had 
been derived from it. But, in practice, seeds 
are employed for many purposes, and are gene- 
rally carried off the farm which produces them. 
When this is done in whole or in part, the plants 
produced are in an eminent degree exhausters of 
the farm, as well as of the soil on which they 
have grown.” 
Plants differ widely from one another in the 
encouragement they give to the growth of weeds 
which either smother useful vegetation or rob 
the soil of nourishment. “The cereal plants 
take precedence of all others in this respect ; 
since their tall, slender stalks, and long, narrow 
leaves but very partially shade the ground, and 
thus not only permit such weeds as are natural 
to the soil to spring up in their intervals, but 
also encourage their growth by the protection 
which they afford to them against the heat and 
winds. Herbaceous plants, on the other hand, 
|| their stalks rising only to a moderate height, 
and their leaves covering the entire surface of 
the soil, effectually prevent the growth of every- 
thing about their roots, and consequently the 
soil is kept clean, and free from weeds. It must, 
however, be remarked, that this latter effect will 
only be produced, where the soil is suited to the 
growth of the plants, and at the same time 
abundantly supplied with manure, so that there 
shall be a luxuriant and vigorous vegetation ; 
since, in cases where these favourable circum- 
stances do not exist, we shall observe the same 
plants to be in a feeble and languishing condi- 
tion, so that they become gradually overshadow- 
ed by others of a more sturdy and less useful 
character, until at length they are entirely dis- 
placed by them. When plants are cultivated in 
furrows, as is the case with roots and most of the 
legumes, large intervals are left which become 
occupied by weeds; the soil however may be 
kept clean in this kind of culture, by frequently 
passing over it with the hoe, or with some other 
suitable implement ; and it may in this way be 
maintained in a state sufficiently fertile to pro- 
duce another crop, especially if the plants grow- 
ing upon it be not suffered to mature their seed. 
The seeds of weeds are often mixed with those 
of the useful plants, and sown at the same time ; 
nor can the farmer be too careful in avoiding 
this. But much more frequently these seeds 
are conveyed’ by the winds, or deposited by 
ROTATION OF CROPS. 
water, or committed to the soil along with the 
manure.” . 
All cultivated plants, particularly such as are 
grown in large quantities in the fields, may be 
classified into comparative exhausters or compa- 
rative amelioraters according to the predominant 
principles which they abstract from the soil. 
“Different genera of plants,” remarks Liebig, 
“require for their growth and perfect maturity 
either the same inorganic means of nourishment, 
although in unequal quantities and at different 
times, or they require different mineral ingredi- 
ents. It is owing to the difference of the food 
necessary for the growth of plants, and which 
must be furnished by the soil, that different 
kinds of plants exert mutual injury when grow- 
ing together, and that others, on the contrary, 
grow together with luxuriance. Very little dif- 
ference is observed in the composition of the 
ashes of the same plants, even although they 
have grown on different soils. Silica and potash 
form invariable constituents in the straw of the 
Graminee ; and, in their seeds, there are always 
present phosphate of potash and phosphate of 
magnesia. A large quantity of lime occurs in 
the straw of pease and in clover. We know, 
further, that in certain kinds of plants, the pot- 
ash is replaced by soda, and the lime by mag- 
nesia. Jt has been shown by the experiments of 
Boussingault, that the five following crops grow 
in succession on an equal surface of the same 
field once manured, removed from the soil :— 
Ingredients of 
the soil. 
Lbs. 
1 Year crop of potatoes (tubers without herb) 246°8 
Dine SePey nes Wheat (straw and corn) 371:0 
BES BESS Clover : . 620:0 
Ales j Wheat 5 4880 
Fallow Turnips 108°8 
Ca ie Salar de Oats (corn and straw) 215:0 
By a crop of Beetroot (roots without leaves) 399-6 
“ss Pease (pease and straw) 6180 
Sos. ke MeL LUV .e viaeaie : : 5 284°6 
<<“ Helianthus tuberosus . C 660:0 
These numbers express the quantities of inor- 
ganic substances removed from the same soil by | 
different plants, and carried away with the crops. 
They, therefore, prove that different plants take up 
into their organism unequal weights of these in- 
gredients of the soil. It is shown by a further 
consideration of the composition of these ashes, 
that they differ essentially from each other with 
respect to their quality. One thousand parts of 
beet, turnips, or potatoes, yield by incineration 
90 parts of ashes; the latter are easily fusible, 
and contain a large quantity of carbonate of pot- 
ash, and of salts with alkaline bases. Of these 
90 parts, 75 parts are soluble in cold water. Two 
thousand parts of dry fern yield also 90 parts ot 
ashes; but of these 90 parts none, or only a 
trace, is soluble in water. The ashes of wheat, | 
barley, pea and bean straw, differ in like manner 
in their composition. Equal quantities of their 
ashes contain very unequal amounts of ingredi- 
