IMPROVEMENT OF THE SOIL 87 



where the soil and climatic conditions are similar. 

 Soil surveys, by means of which the different soil 

 areas can be carefully mapped, are thus of great use 

 in determining the proper use of fertilizers. The 

 farmer can tell much, too, by closely observing his 

 crops, as to their need for the different food elements. 

 If the growth is slow and the leaves are small and 

 yellow, it is an indication of the lack of nitrogen unless 

 this condition has been caused by excessive drought. 

 This element if present abundantly tends to induce a 

 rank growth and gives a dark green color to the leaves. 

 If, on the other hand, the leaves are dark and the 

 growth rank while but little fruit or grain is pro- 

 duced, the lack of phosphoric acid is indicated. The 

 phosphates promote, fruitfulness and early maturity. 

 Potash, on the other hand, tends to promote vigor 

 and prolong the growing period. It is also supposed 

 to give firmness and higher flavor to fruits, and in 

 many cases it has a marked influence in promoting 

 the health of plants and warding off disease. In a 

 soil that is rich in nitrogen and phosphoric acid but 

 deficient in potash the crop will start well early in 

 the season, but it will be likely to show weakness 

 later. The foliage is liable to become spotted and 

 diseased, and the grain or fruit will be poor and 

 light. Cases sometimes occur where soils are only 

 deficient in some one or two of these elements, but 

 in the great majority of cases all three will be found 

 lacking, and a complete fertilizer will be required. 



Rotation of Crops. — The whole subject of soil 

 improvement, especially improvement by the use 

 of green manures, is intimately connected with a 



