82 Practical Farming 



used, and we do not use sulphur as sulphur. (In fact, as 

 we shall try to explain, we do not use any of the elements 

 in the soil as pure elements.) Sodium is essential mainly 

 to marine plants, and is of Uttle use in the plants usually 

 grown as farm crops. There was some years ago an effort 

 made by the salt manufacturers to make the farmers be- 

 lieve that sodium could take the place of potassium. But 

 you have been told that plants will not thrive where there 

 is a deficiency of potash, though there may be a large per- 

 centage of sodium. None of our cultivated crops use 

 sodium to any great extent, though the chloride of sodium, 

 common salt, may have some effect in the solution of 

 other matters in the soil, and to some extent may be useful 

 to such crops as cabbage and beets, which in their wild 

 state are natives of the seashore. Lime exists in the soil 

 as the carbonate of calcium, and is largely used by plants 

 to render harmless certain acids, such as oxaHc acid, which 

 are formed in the plant. The oxaHc acid is located in 

 crystals of the oxalate of lime in certain cells, and this 

 form being insoluble in the cell sap, the acid is rendered 

 harmless to the plant. Lime is also found in all the cell 

 walls. But the supply of lime needed as plant food is 

 usually abundant in all cultivated soils, and it is mainly 

 valuable as a reagent, as will be explained in full. 



Nitrogen is a gas which we have shown to be the larger 

 part of the mixture of nitrogen and oxygen which we call 

 air. Hence it must be got into combination with other 

 matters before it can be located in the soil and become 

 available to plants. Considerable quantities of nitrogen 

 are brought to the soil in the rain water in the form of 

 ammonia, which is a hydride of nitrogen. The decay of 



