THE BIOLOGY OP THE SOIL 



97 



pose it is placed in a compact mass below the plants, that 

 it may ferment, and the heat evolved during the putrefac- 

 tion warms the soil above and stimulates the growth of 

 the plants rooted in it. 



2. Artificial Manures. — These^ are manures applied to 

 the soil in a form which requires no further decomposition 

 by soil-bacteria — e.g., the nitrate, potash, soda, and phos- 

 phatic manures of commerce. Guano, being very rich 

 in nitrates, belongs practically to the same category. 

 They should all be applied in very dilute solutions (p. 92). 



3. Living Manures. — There have been several attempts 

 in recent years to place on the market extracts of soU- 

 bacteria. They are intended to make poor soils fertile, 

 or to increase the weight of the crops taken from them. 

 Extracts of root-nodules are regarded in some quarters 

 with special favour. The seeds are infected with the 

 extract before sowing, or the ground is watered with it 

 when the plants are coming up. 



The Rotation of Crops. 



Different crops withdraw from the soil its nutrient 

 salts in different proportions, one crop demanding 

 more of one salt than another. For this reason, the 

 same crop raised year after year in the same soil would 

 soon impoverish it. To obviate this, crops are grown in 

 rotation, each crop differing from the rest in its demands 

 upon the soil. By this means, nutrient substances are 

 not withdrawn from the soil more quickly than they can 

 be replaced by the ordinary methods of manuring. 



As an example of the rotation of crops, we will set forth 

 the so-called Norfolk, or Four-Year Rotation, popular on 

 light sandy soils in this country. It will be seen that the 

 same crop is never raised from the same field two years 

 in succession, but only at intervals of four years : 



