THE BIOLOGY OF 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 soil- 
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not withdrawn from the soil more quickly than they can 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 : 
First Year. | Second Year.| Third Year. | Fourth Year. 
First field - He B. L. W. 
Second field B. Ti W. R. 
Third field - Ibe W. R. B. 
W. Ri B i 
Fourth field 
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