312 IMPROVEMENT OF THE SOIL 



prefer it. But the green manure shows its superior effects in 

 the harvest. The best rule is to apply to hoed crops a small 

 quantity of rotted manure in the hill, to give the young plant 

 a vigorous " start," but to spread the greater portion in a 

 green state, to act upon the crop in mid-summer. The gen- 

 eral practice of manuring in the hill is, by the best farmers, 

 almost wholly discontinued. 



Sect. 5. Saline Manures, or those consisting of inorganic 

 Salts. 



Mineral substances act as manures, when they enter in- 

 to the composition of plants. They act as amendments 

 or correctors, when they improve the texture or neutral- 

 ize acids. They act as solvents or converters, when they 

 induce changes in animal and vegetable bodies, or con- 

 vert them into vegetable food. They act as stimulants, when 

 they excite the living powers of plants by producing electri- 

 cal changes, and other effects not well understood. 



The substances, classed as mineral manures, are salts ; that 

 is, they consist of acids combined with alkalies, alkaline 

 earths and metallic oxides. As fertility depends upon salts 

 and geine, and, as the base of the salt, or alkaline portion, 

 acts wholly upon geine, and in one uniform manner, p. 220, 

 salts may be classed, with reference to the peculiariti/ of their 

 influence, by their acids. 



In this respect, salts may be divided into two classes. 

 1. Those salts whose acid nourishes plants ; such are nitrates, 

 carbonates and phosphates. 2. Those salts, whose acid 

 poisons plants, or yields but a small quantity or no nutri- 

 ment ; such as sulphates, hydrochlorates or muriates. 



I. Salts ivhose acid contains the elements ivhich nourish 

 plants. This class includes three families, which may be de- 

 scribed as nitrates, phosphates and carbonates. 



I. Nitrates. In this family of salts, nitric acid is com- 

 bined with several bases. The three principal salts which 



