BOOK THE ELEVENTH. 



ON MANURES IN GENERAL, AND THEIR APPLICATION TO CROPS. 



CHAPTER I. 



ON NATURAL MANURES. 



rTlHE practice of manuring land has existed from remote ages. The 

 J_ writings of Cato, Pliny, Columella, Varro, and Virgil, prove to us 

 that we have only been following in the steps, and endeavouring to 

 improve on the practice, of the ancient Romans, who evidently made 

 this important branch of agriculture a subject of careful attention. 

 The above-named old authors give repeated directions concerning the 

 choice, application, and preservation of various kinds of manures, both 

 liquid and solid. 



Crops, like stock, grow only in virtue of the food that is placed at 

 their disposal either by nature or by art ; and plants, like animals, 

 vary in their feeding requirements. The sources of the food of crops 

 are various, since the constituents of that food are diverse, plants being 

 built up of a number of different chemical elements. Rain or atmo- 

 spheric moisture, and the carbon that exists in the atmosphere in the 

 gaseous form of carbonic acid, furnish the greatest bulk of the mate- 

 rials of which plants are composed, and fortunately these sources of 

 food are practically unlimited in quantity. Unfortunately, they are, 

 on the other hand, insufficient in themselves, and the power of plants 

 to draw on the unlimited stores of moisture and carbon in the air is 

 regulated by the extent of the simultaneous supply of other equally 

 essential foods, in bestowing which nature is less lavish. 



Such foods are those obtained from the soil, and it is to their 

 economy and increase that most of the efforts of the farmer are, con- 

 sciously or unconsciously, directed. These foods comprise nitrogen 

 and mineral matters. It is true that certain plants obtain part at any 

 rate of their nitrogen indirectly from the air, but these are chiefly, if 

 not exclusively, leguminous plants. Their power of gaining nitrogen is 

 vested in micro-organisms or bacteria of a special type which are 

 present in most fertile soils. These bacteria " infect " the roots of the 

 plants, causing swellings or nodules, in which they form colonies. Living 

 thus in symbiosis with their host plants, the bacteria bring about 

 the assimilation of atmospheric free nitrogen, which thus becomes true 

 plant food. The micro-organisms can be grown independently of 

 green plants in artificial cultures, and have been thus raised and sup- 



