106 FUNDAMENTALS OF FRUIT PRODUCTION 



and development of crops and that the magnitude of the concentration 

 is the same for practically all soils, one might easily be led to the belief 

 that fruit plants seldom suffer from lack of an adequate supply of mineral 

 nutrients. However, this inference is hardly warranted for, as is shown 

 later, mineral nutrients may be in solution and still be unavailable to the 

 plant. In other words solubility and availability are not synonymous. 



Furthermore, it may be noted that the 5 parts of water soluble nitrates 

 per million of dry soil found by Gom-ley and Shunk^^ in sod-mulched orch- 

 ards during the growing season were apparently insufficient for satisfactory 

 wood growth and fruit production, while a concentration of 15 to 40 parts 

 per million under certain other systems of culture proved entirely ade- 

 quate. In this case all the nitrogen measured was in an available form. 

 Whether in the sodded area nitrogen could be absorbed by the trees only 

 when the concentration in the soil reached a certain minimum, or whether 

 a very limited amount was absorbed even at the lowest concentrations, 

 cannot be stated from available data; they show clearly, however, that 

 the trees were unable to remove nitrates completely from the soil and 

 further, that a nutrient solution very dilute in respect to this element 

 provides only for very slow growth. 



Displacement. — The amounts of the various inorganic constituents 

 in the soil are subject to variation and exchanges of bases may occur when 

 they are present as silicates. Potassium, ammonium, magnesium, 

 sodium and calcium form a series in which each member is capable of 

 displacing any member following it in the series. One of two things 

 may happen: an essential element may be lost to the plant by becoming 

 soluble and being washed out of the soil, or it may be changed from an 

 unavailable to an available compound and so placed at the disposal of 

 the plant. 



Of most common occurrence is the displacement of calcium by 

 potassium or sodium, resulting in the calcium salts going into solution. 

 However, large amounts of calcium are capable of displacing small 

 amounts of potassium ^''^ or any other base standing ahead of it in the 

 series. Hence, calcareous soils are likely to be deficient in potash 

 and the application of calcium in great amounts tends to deplete the 

 potassium supply. Grape-fruit seedlings have been observed to show 

 injuries characterized by yellowing of the foliage due apparently to 

 the presence of ground limestone; more injury was evident in sandy 

 soils than in loams. ^® One type of this yellowing is "frenching," a 

 lack of green color in the areas between the largest veins, which is shown 

 later to be a characteristic symptom of potassium starvation. "French- 

 ing" was produced also by sulphate of ammonia or organic fertilizers 

 containing ammonia. This effect may be attributed to displacement 

 of potassium in the soil by relatively large amounts of ammonia. 



The effects on the plant of displacement of bases may be indirect 



