SOILS 113 



the plant through the decomposition of the organic matter. 

 Barnyard manure serves in this way in addition to other more 

 direct benefits. In a peaty swamp soil or bog land, it has 

 been found that the potassium content is often very deficient, 

 and that its deficiency is a limiting element in the produc- 

 tion of crops. 



The commercial sources of potassium are kainit, potassium 

 sulphate, potassium chloride, wood ashes, and manure. 



It would seem, therefore, that in systems of general farm- 

 ing it would not be necessary to apply commercial forms of 

 potassium, but to utilize the supply already found in abun- 

 dance in the soil. Sometimes, however, in getting legumes 

 started, and in building up worn out land, it may be neces- 

 sary to make an application of some form of potassium such 

 as kainit, in order to furnish food which is quickly available 

 for the young plant. In truck and garden farming where 

 intensive methods are used, applications of potassium are 

 often profitable. 



Limestone. We must not omit mentioning limestone as a 

 factor in soil fertility. Its first chemical effect is to neu- 

 tralize the excessive acidity which prevents the growth of 

 legumes. Often the first step, therefore, in renewing a soil 

 is to correct the acidity, making it possible for the legumes 

 to grow so that organic matter may be incorporated into the 

 soil, thus furnishing a basis upon which to biuld the nitrogen 

 and mineral plant-food supplies. Certain plants require large 

 amounts of calcium as a plant-food, such as clover, alfalfa, 

 and blue grass. Lime has been used as a fertilizer for thou- 

 sands of years. It has been used in three forms : the ground 



