198 FUNDAMENTALS OF FRUIT PRODUCTION 



and eventually death of the roots are the primary symptoms. Dis- 

 orders proceeding from the causes just outlined should be distinguished 

 from the toxic effects produced by organic compounds or by excessive 

 soil concentrations, discussed previously. 



DEFICIENCIES 



The lack of a sufficient amount of any essential soil constituent may 

 lead to the development of distinct pathological conditions, or it may 

 result simply in checking vegetative development and fruit production 

 without producing obvious pathological symptoms. The use of fertil- 

 izers for correcting both of these conditions is discussed in the two 

 following chapters in which particular emphasis is accorded the correc- 

 tion of conditions interfering with fruit production on a commercial 

 scale. The discussion immediately following concerns the more impor- 

 tant pathological symptoms which are associated with the presence of 

 unduly small amounts or with the complete exhaustion of essential 

 mineral elements. 



Nitrogen. — A deficiency of nitrogen may become evident in several 

 different ways. The plant may be dwarfed, though it develops com- 

 pletely and produces flowers, fruits and seeds. As a rule, however, the 

 leaves are pale green because of the relatively small amounts of chloro- 

 phyll and the development of the mature fruit is affected in one way 

 or another. There may be an incomplete development of the sexual 

 organs, and consequent unfruitfulness;^^° in case fruits develop they may 

 be seedless, as in apples, pears and grapes, ^^^ or the fruit may develop 

 somewhat but drop prematurely. This is a common result of nitrogen 

 deficiency in apples and pears. The latter sometimes show excessive 

 thorn development in connection with a lack of nitrogen. ^'^^ 



Phosphorus and Potassium. — A deficiency of phosphorus appears 

 to produce no characteristic symptoms. Chlorophyll development is 

 not affected, but the plant does not increase in dry weight. 



A deficiency of potassium^''" is usuallj^ associated with a scarcity 

 of carbohydrate reserves. In trees, the terminal shoots show weak 

 development and eventually dry out, or shoot formation may be 

 suppressed wholly. Plants suffering from a lack of potassium often 

 maintain a health}'- appearance longer than those lacking nitrogen or 

 phosphorus. Whatever potassium is available apparently is used first for 

 vegetative growth and development and, if there is no residuum, the plant 

 does not blossom. Eventually the leaf blade becomes yellow on the 

 edges and between the veins, then brown and finally white, while the 

 veins and petiole remain green. This condition is known as a frenching 

 of the foliage. A potassium deficiency renders the roots susceptible to 

 rotting and the plant eventually dies. When nitrogen or phosphorus is 

 deficient, plants are likely to remain alive longer in a stunted condition. 



