474 NATURE AND PROPERTIES OF SOILS 



plant, and also to a thinning of the cell-wall, allowing a more 

 ready infection from without. 



While certain plants, as the grasses, lettuce, radishes, and 

 the like, depend for their usefulness on plenty of nitrogen, it 

 is generally better to limit the amount of nitrogen for the 

 average crop so that growth may be normal. This results in 

 a better utilization of the nitrogen and in a marked reduction 

 of the fertilizer cost for a unit of crop growth. This is a 

 vital factor in all fertilizer practice, and shows immediately 

 whether nitrogen fertilization is or is not an economic success. 



266. Influence of phosphorus on plant growth. — It is 

 difficult to determine exactly the functions of phosphoric acid 

 in the economy of even the simplest plants. Neither cell divi- 

 sion nor the formation of fat and albumen go on to a suffi- 

 cient extent without it. Starch may be produced when it is 

 lacking, but will not change to sugar. As grain does not form 

 without its presence, it very probably is concerned in the pro- 

 duction of nucleoproteid materials. Its close relationship to 

 cell division may account for its presence in seeds in compara- 

 tively large amounts. 



Phos phoric acid hasten sjthe maturity of the crop by its 

 ripemn^Jnfluences. This effect is especially valuable in wet 

 years and in cold climates where the season is short. The use 

 of acid phosphate is being advocated in the Middle West, espe- 

 cially for maize, as an insurance against frost-injury and a 

 means of avoiding soft corn. Phosphoric acid also encourages 

 root development, especially of lateral and fibrous rootlets. 

 This renders it valuable in such soils as do not encourage root 

 extension and to such crops as naturally have a restricted root 

 development. Phosphoric acid is especially valuable for fall- 

 sown crops, such as wheat. A sturdy root growth is developed 

 which tends to prevent winter injury and prepares the plant 

 for a rapid spring development. 



Phosphoric acid decreases the ratio of straw to grain in 

 cereals. It also strengthens the straw, thus decreasing the 



