220 General Botany 



in the nuclei of cells. It is essential for cell division and many- 

 enzyme activities. 



Manganese, like iron, is a catalyzer and is said to be associated 

 with iron in the formation of chlorophyll. 



Fertilizers. Fertilizers are added to agricultural soils, (i) to 

 increase the supply of the essential mineral elements, (2) to 

 improve the texture of the soil and its water-supplying quahties, 

 (3) to liberate other mineral elements by breaking up insoluble 

 compounds in the soil, and (4) to correct acidity. 



Of all the elements needed for plant growth, phosphorus, 

 potassium, and nitrogen are most frequently found in quan- 

 tities insufficient for the best yields of agricultural products. 

 Phosphorus may be added in the form of crushed phosphate rock, 

 or as '' acid phosphate " (rock phosphate treated with sul- 

 furic acid), which is more soluble and contains sulfur in addition. 

 Potassium may be suppHed by the use of potassium chloride. 



Nitrogen may be added to soils in the form of sodium nitrate or 

 ammonium sulfate, but these salts are very expensive and can 

 only be profitably used on truck gardens where the value of the 

 crops amounts to hundreds of dollars per acre. In agricultural 

 soils generally, the nitrates are best secured and maintained by 

 making the soil favorable for the growth of the nitrogen-fixing 

 bacteria. Calcium is usually added to soils in the form of 

 crushed, or burned, limestone, not only to furnish this element to 

 the plants, but also to improve the physical condition of the soil. 

 Lime also helps to liberate potassium from insoluble compounds. 

 Acidity of soils. Most plants grow best in soils that are nearly 

 neutral. The use of Hme in neutrahzing acid soils has been men- 

 tioned previously. There are some plants, however, that are 

 favored by acid soils. Cranberries, blueberries, azaleas, laurels, 

 and rhododendrons flourish only under these conditions. In 

 growing these plants in cultivation the acidity of the soil is some- 

 times maintained by adding ammonium sulfate, by watering 

 occasionally with tanbark extract, and by adding alum. 



