20 CHEMICAL FERTILIZERS 



salts suitable for manure are soluble in water. Any ordinary 

 soil contains an ample supply of insoluble potash com- 

 pounds, and, whatever salt may be added to the soil, the 

 potassium contained in it is readily fixed near the surface. 

 Potash is very intimately connected with the process of 

 carbon assimilation by the green parts of a plant. Appli- 

 cations of potash manures to plants stimulate the syn- 

 thesis of carbo-hydrates, such as sugar and starch. Potash 

 tends to improve the quality of grain and roots ; it is also 

 particularly powerful in promoting the development of 

 leguminous crops, although these latter have a very strong 

 power of extracting potash from very insoluble potash 

 compounds, provided they are supplied with a sufficiency 

 of phosphatic manure. 



When the supply of potash to the plant is quite insufficient, 



the leaf loses its natural green tint, and presents an abnormal 



appearance. Potash-starved plants exhibit a very poor 



colour, and their leaves have a tendency to wither at the 



tips and edges. The production of sugar by swedes and the 



production of starch by potatoes is very intimately connected 



with the supply of potassium compounds. In experiments at 



Rothamsted 7255 pounds of leaves accompany 14,684 pounds 



of mangolds where potash food was deficient, but where potash 



was supplied the production of leaf amounted to 8508 pounds, 



and of root to 40,328 pounds. Thus with less than 20 % 



increase of leaf there was nearly three times as much root. 



Unlike phosphates and nitrates, potassium compounds have 



a very marked effect on the weight of the individual grains 



of barley ; poor, stunted grains may very easily be produced 



by withholding potash. Potash-starved plants are the first 



to succumb in a bad season, whilst those plants that are 



over-manured with nitrogen exhibit the same lack of resisting 



power under unfavourable weather conditions. The legumi- 



nosae also seem to stand in considerable need of potassium 



salts. Potash is well known as a good stimulant for clover, 



especially on light soils where potash is naturally deficient. 



The effect on barley of potash salts is given in Tables 2 and 



3 (p. 18), which show that potash is less important than 



