The Composition and Use of Fertilizers. 277 



The richer; the soil- is in nitrogen, the less the crop takes from 

 the atmosphere. In order, therefore, to utilize leguminous crops 

 to best advantage as nitrogen gatherers, the soil should contain 

 only small amounts of available nitrogen, but generous amounts 

 of mineral plant food. Under favorable conditions a clover crop 

 will take from the air an amount of nitrogen equal to 50 pounds 

 or more per acre. It is possible by continuous growth of legu- 

 minous crops to accumulate an excess of nitrogen in the soil, 

 unless there is added an abundant supply of mineral plant food. 



With judicious management in growing clover or some other 

 leguminous crop for use as green manure, it is easily possible to 

 supply the nitrogen for one's crops without purchasing this, the 

 most costly plant-food element. 



" Nitrogen-Consuming " Crops as Green Manures. — Crops 

 other than leguminous plants are used as green manures, the 

 more common ones being rye and buckwheat. These crops do 

 not possess the power of using atmospheric nitrogen. All the 

 nitrogen they use comes directly from the soil. They do not, 

 therefore, add any nitrogen to the soil, but rather they take avail- 

 able nitrogen from the soil and change it to less available forms. 

 Such crops do not increase the fertility of the soil and cannot be 

 used as a substitute for leguminous crops or commercial forms 

 of plant food. The principle advantages coming from the use of 

 these crops as green manures are two: First, they furnish an 

 abundance of vegetable matter that becomes useful in supplying 

 humus-making material; second, they serve as cover crops, thus 

 preventing loss of plant food by leaching. In order to grow 

 these crops well, one must make sure that they have an abundance 

 of plant food of all kinds. 



11. Rotation of Chops. 



General Principles Underlying Rotation op Crops. — The 

 general object of systematically growing different crops succes- 

 sively on the same soil is to utilize the plant food in the soil most 

 economically, and maintain the soil in condition for raising maxi- 

 mum crops. The system of rotation to be followed must depend 

 upon the kind of soil and character of farming. A fruit grower 

 and a dairyman would follow different systems of rotation. Con- 

 tinuous grow T th of one crop on the same field is most exhaustive 

 of plant food, as a rule, and, in addition, the physical conditions 

 of the soil deteriorate. 



