35 



acid phosphate and as basic slag, "all of which contain 

 various amounts of phosphoric acid. Bone meal is also 

 a phosphate fertilizer and so are the various bat guanos. 



Nitrogen or ammonia is the third element contained 

 in fertilizers. It is present in all organic matter such as 

 the vegetable matter in the soil, and also in the air, from 

 which latter source leguminous plants can supply them- 

 selves. New land recently cleared from forest and con- 

 taining much vegetable matter is rich in nitrogen, and 

 frequently trees planted on such land suffer from gum- 

 ming, -die-back, etc., because they take up more of that 

 than of potash and phosphoric acid. For the same reason, 

 stable manure can not be used in large quantities for 

 citrus trees. For fertilizing purposes, nitrogen is sold 

 in the forms of nitrate of soda, sulphate of ammonia, 

 dried blood and the various slaughter house and fish 

 scrap offals as well as nitrate of lime which is the new 

 product -made where nitrogen is obtained from the air. 



Cover Crops is a name applied to plants grown in the 

 grove for the purpose of benefiting the soil. A cover 

 crop may or may not be a fertilizer according to what 

 kind of plant it is. Common weeds such as grasses usually 

 found in uncultivated groves do not add plant food to the 

 soil. They feed on whatever plant foods there are in the 

 soil and when they decay there is nothing more nor less 

 left than what there was before. There is a gain however 

 because the humus formed improves the physical con- 

 dition of the soil and the plant foods in dead vegetation 

 are in a form readily available for other plants. 



Leguminous plants, such as beans or peas do not alone 

 improve the physical condition of the soil but they also 

 add something to it. They feed on nitrogen, taking it 



