DK. SCHLEIDEN's THEORY OF AGRICULTUEE. 157 



leaves keeps off the direct sunbeams, and consequently impedes 

 evaporation. The moisture is therefore retained for the exi- 

 gencies of the plants, which are consequently more luxuriant, as 

 also from the increased absorption of carbonic acid ; while the 

 consumption of humus is either prevented or prolonged from the 

 stagnation of the air immediately in contact with it, since an 

 atmosphere of carbonic acid, the produce of decomposition, sur- 

 rounds the constituents of the humus, impeding the accession of 

 fresh oxygen, and consequently of further decay. Those parts 

 meanwhile which are slowly dissolved are stored up in the plants 

 which cover the ground, which constantly receives an addition 

 from the falling leaves, and the accession of vegetable matter is 

 eventually greatly increased by the ploughing in of the green 

 covering. The benefit of such fallows to poor and light ground 

 is at once evident. 



2. In the naked fallow, weeds are not allowed to come to 

 their full growth. The increase of humus is therefore slight 

 from this source, while it is sensibly diminished by the constant 

 ploughing, the dissolution of its mineral constituents proceeding 

 with equal rapidity. These unfortunately are subject to be 

 washed away by overwhelming rains. The naked fallow is only 

 useful then where the requisite looseness of soil camiot be at- 

 tained in any other May. 



The notion of rest so prevalent amongst cultivators is clearly 

 wrong, except it be rest from the destructi^■e influence of the 

 plough. 



E. The subject of manures has already been touched upon so 

 much that we must content ourselves with some general remarks, 

 without entering upon any special substances, which are treated 

 at considerable length by Dr. Schleiden. There is the less need 

 to follow all his observations, as much is accessible to English 

 readers in the work of Boussingault. 



The word manure, taken in its widest sense, comprehends 

 everything that is put to the land with a view to the improve- 

 ment of the crop, whether organic or inorganic. Ko agricultural 

 operation can satisfy one condition alone of vegetable life. It is by 

 manure almost exclusively that man is able to alter the character 

 of land, and the supply of nutriment from it is so far from being 

 the only object it answers, that in many cases it might be doubted 

 even whether this were the principal end, and therefore the indi- 

 vidual properties must be separated carefully from one another. 



If we consider the waste which takes place in the consumption 

 of what is carried out of the field, that in such crops as flax 

 scarcely anything is returned, that a large portion of the organic 

 matter is destroyed by the process of combustion, it will be 

 evident that, without having recourse to fallows or green crop- 

 ping, it would be impossible to cultivate land with mere farm- 



