SOIL IMPROVEMENT 81 



some lime or wood ashes. The layers are repeated until 

 the manure is all in the compost. The moisture from rains 

 prevents the compost heap from rotting too fast and becom- 

 ing too hot. The sod layers save much of the plant-food 

 which would otherwise leach out. 



Rotting manure in a compost heap will kill the weed 

 seeds ; it will reduce the bulk of the manure ; it helps to unlock 

 the plant-food in the manure; it destroys the injurious effect 

 which fresh manure has on such crops as potatoes. Whether 

 to make such a compost of the manure will be determined 

 by the uses for which it is intended. 



Feeding Plants. — Those forms of plant-food which con- 

 tain nitrogen, potash, phosphoric acid, or Hme are considered 

 most precious on any farm. Soils are apt to become weak 

 in these foods. Many such compounds readily dissolve in 

 water and are lost from the soil at times of heavy rain by 

 leaching or washing away. They may be absent from other 

 causes. There are few if any other plant-foods which need 

 cause the farmer much concern. The others are usually 

 abundant enough. 



Commercial fertilizers containing nitrogen or phosphoric 

 acid or potash — or all three of these — are purchased in great 

 abundance in the Eastern and Southern States and for special 

 crops elsewhere. These are applied to the soils for the 

 direct feeding of crops. It is estimated that over eight mil- 

 lion tons of such fertilizers are used in the United States 

 each year. These cost the farmers over two hundred million 

 dollars. As a rule they have no beneficial effect in improving 

 the soil itself, as do the barnyard and the green manures. 

 A commercial fertilizer is valuable in proportion to the 

 amount and kind of these three plant-foods contained in it. 



Nitrogen is the most important element of such fertil- 

 izers, because soils are more often in need of it and it is the 

 most costly. When it is in a soluble form for plants to use, 

 it is apt to be lost from many kinds of soils, unless cover 

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