Productiveness of Soils 97 



elements to suit the requirements of the crop. It is im- 

 portant in this particular to remember that forage crops 

 need large amounts of nitrogen, and grain crops much 

 phosphorus. The fruit crops require much potash. A 

 soil may even be deficient in any one or several of the 

 essential elements. The best and safest guide to learn 

 the special fertilizing needs of a soil is to try by test. 

 (See 1 133a.) 



139. Chemical Elements May Not Be in Balance. A 

 soil may contain so much nitrogen that the crop, say 



Fig. 50. Showing the effect of an excess of lime and magnesia on plant growth. 

 Excess of lime in pots on left; excess of magnesia in pots on right. Nearly 

 equal amounts of each in center pots. From Bull, United States Department 

 of Agriculture. 



grain or fruit, goes all to wood and leaf and does not 

 produce a harvest. In such cases, a potash or a phos- 

 phate fertilizer would be needed to balance the ration 

 of mineral food. Sometimes some element, even an 

 essential element, may be in excess. Plants require 

 magnesium and calcium (| 43), but an excess of either 

 may be the cause of a poor result. Fig. 50 shows the re- 

 sult of adding lime to balance an excess of magnesia in 

 the soil, and shows the effect of balanced and unbalanced 

 amounts of calcium and magnesium on plant growth. 

 The good effects that sometimes result from the appli- 



