600 AGRICULTURE. 



phate of lime. They must get all this from the plants they eat. 

 We see, then, how important this compound is. Our cultivated 

 soils are usually more in need of this than any other plant food, 

 as it is less generally common ; and its importance in any manure 

 can readily be seen. Another important element is sulphur. 

 This, in the form of sulphuric acid (sulphur and oxygen), readily 

 unites with lime, and forms in nature large beds of sulphate of 

 lime, or plaster, as it is commonly called. Plaster is largely used, 

 and found profitable on such soils as are supplied naturally with 

 potash combined with silica. The plaster "pushes out," as some 

 one has said, the potash, so that plants can get it. 



Silica (silicon and oxygen) is common sharp sand in its pure 

 state, and in this shape only serves to loosen and lighten the 

 soil, but in combination with potash forms silicate of potash, 

 which dissolves slowly in rain water. Silica is needed in plants 

 to stiffen the straw of wheat and other grains. It is always in 

 plenty, but sometimes needs to be made soluble by lime, etc. 



Nitrogen is one of the most essential elements needed by 

 plants. Nitrogen is found free in the air, and exists in soils in 

 the form of nitrates of potash, lime, soda, and ammonia. 



Nitrogen in some form is absolutely essential to well devel- 

 oped growth in plants. Nitrates continually form in cultivated 

 soil, but, being very soluble in water, are more rapidly washed 

 away and lost than any other plant food. We find that an 

 ordinary mellow soil will take and hold on to all the various ele- 

 ments of plant food, except the nitrates, which are rapidly washed 

 away. Some plants, such as peas and clover, have in their roots 

 a "ferment"; that is, a microscopic organism, which rapidly 

 promotes the formation of nitrates in the soil. Hence the great 

 value of this class of plants in improving land for other crops. 



But the plants get nitrogen, also, in the shape of various com- 

 pounds of ammonia, which is hydrogen and nitrogen. We are 

 all familiar with the sharp odor which rises from a pile of horse 

 manure heating. This is from the carbonate of ammonia, which 

 is rapidly flying off into the air, and being lost by the farmer. 

 But if we mix with this manure a supply of plaster, which is 

 sulphate of lime, the carbonic acid unites with the lime, and the 

 sulphuric acid parts with it and unites with the ammonia, and 



