PLANT NUTRIENTS T 



son. If the crops develop to normal maturity, this excessive foli- 

 age growth has no harmful results, as the surplus material which 

 has been elaborated is properly translocated into the desired stor- 

 age organs; but, unfortunately, the retarding effect of the surplus 

 nitrogen supply upon the date of maturing of the crop is often asso- 

 ciated with premature ripening of the plants from other causes, with 

 the consequence that too large a proportion of the valuable food 

 material is left in the refuse foliage material of the crop. Crops 

 which are grown solely for their leaves, such as hay crops, lettuce, 

 cabbage, etc., profit greatly by abundant supplies of available 

 nitrogen; although when foliage growth is stimulated in this way 

 the tissue is likely to be thin-walled and soft rather than firm and 

 solid. 



Phosphorus is likewise an extremely important element in 

 plant nutrition. But phosphorus starvation produces no such 

 striking visible effects upon the growth of the plant as does lack 

 of nitrogen. Abundance of available phosphorus early in the 

 plant's life greatly stimulates root growth, and later on it undoubt- 

 edly hastens the ripening process; hence, this element seems to 

 act as the exact antithesis of nitrogen. 



The role of phosphorus, or of phosphates, in the physiological 

 processes of the cell seems to be difficult to discover. The element 

 itself is a constituent of some protein complexes and of the lecithin- 

 like bodies (see page 141) which are supposed by some inves- 

 tigators to play an important part in determining the rate of 

 chemical changes which take place in the cell and the movement 

 of materials into and out of it. It is an essential constituent of 

 the nucleus, and a meager supply of phosphorus retards, or inhibits, 

 mitotic cell-division. Photosynthesis of sugars and the condensing 

 of these into starch or cellulose takes place in plants in the absence 

 of available phosphorus; but the change of these insoluble carbo- 

 hydrates back again into soluble and available sugar foods does 

 not. 



Phosphorus is taken from the soil by plants in the form of 

 phosphates. Much study has been given to the problem of the 

 proper supply of available soil phosphates for economic crop pro- 

 duction. Any discussion of soil fertility and fertilization which 

 did not devote large attention to the conditions under which 

 phosphates become available as plant food would be wholly inad- 

 equate; but such a discussion would be out of place here. 



