116 FUNDAMENTALS OF FRUIT PRODUCTION 



When the acidity of the soil increases beyond the low value which is 

 most favorable to land plants, it becomes an important factor. 



Soil Reaction and the Availability of Phosphorus. — The effect of soil 

 acidity on the availability of phosphorus is shown by the following 

 quotation from Harris :^^ 



"In addition to the work that has been done, on determining the degree of 

 soil acidity, many investigations have been undertaken to determine the relation 

 of soil acidity to the quantity of available phosphorus in the soil. As a result 

 of the work of Wheeler, Thorne, Whitson and Stoddart, it has been show that the 

 content of this element is generally low in acid soils and largely unavailable for 

 use by plants. Stoddart explains this by saying that acid soils convert any 

 calcium phosphate that may be present into soluble compounds which are either 

 washed out or are fixed in an insoluble form by the formation of iron and alumi- 

 num phosphates." 



Soil Reaction and the Availability of Iron.- — An excess of calcium salts 

 affects the availability of iron in such a way that many plants grown on 

 calcareous soils suffer from lack of iron, even though iron is present in 

 considerable amounts. It is from this cause that grape vines and fruit 

 trees become chlorotic on some of the calcareous soils of France and 

 England, pineapples and sugar cane on Porto Rican soils containing 

 large amounts of lime and citrus fruits in Florida when ground limestone 

 is added to the soil. 



"In Porto Rico the extension of the pineapple industry has been retarded 

 by a disease known as chlorosis, the principal external mark of which is the 

 yellowing of the foliage and the consequent poor nutrition of the plant. From 

 investigations by Gile and by Loew it appears that the yellow color of the leaves 

 and the accompanying weakness of the plant are due to the lack of iron, and that 

 where the soil contains an excess of lime the organic acids which are needed to 

 dissolve the iron of the soil are themselves neutralized and the iron, although 

 present, is not available for absorption by the pineapple roots. "^* 



According to Gile and Carrero," sugar cane grown on the calcareous soils 

 of Porto Rico suffers from chlorosis. Analysis shows that the ash of these chlor- 

 otic leaves has less iron then normal leaves. 



Floyd^^ describes two types of injury to grape-fruit seedlings from the presence 

 of ground limestone in the soil. In addition to frenching which has been dis- 

 cussed, chlorosis occurs. This type of injury may be attributed to iron deficiency 

 and is probably quite distinct from frenching, since no case of the latter was 

 observed to develop into complete chlorosis. The larger the amount of limestone 

 in the soil the greater was the injury observed. 



The unavailability of iron in calcareous soils is probably attributable 

 to the alkaline reaction produced by an excess of calcium salts in solution. 

 Colloidal iron hydroxide is formed in alkaline solutions and is for the 

 most part unavailable to plants. Similar conditions prevailing in man- 

 ganiferous soils confirm the idea that the basic reaction of the soil solu- 



