PLANT NUTRIENTS AND THEIR ABSORPTION 109 



roots. Their solvent action is yet to be accounted for satisfactorily. 

 Crocker suggests that the strong, insoluble pectic acids found in the 

 walls of root hairs may be responsible for the absorption of bases and 

 the setting free of mineral acids, which would have a localized and 

 temporary solvent action on the soil. Various plants show great differ- 

 ences in the dissolving power of their roots, or at least in their ability to 

 obtain required nutrients. 



Hartwell*^ found that carrots secured all the phosphorus they required from 

 a soil in which rutabagas and cabbage were practically unable to grow, while 

 wheat, oats, white beans and soy beans ranged between these extremes. Simi- 

 larly, he found "an ability of the soy bean to obtain from the deficient [in avail- 

 able potassium] plots about two-thirds of their maximum requirements, whereas 

 carrots obtained about half their needs, mangels about one-fourth and summer 

 squash only about one-tenth." 



It is not clear to what extent this characteristic feeding power of 

 various plants may be due to varying ability to dissolve the materials 

 they encounter in a solid or colloidal form, what part may be due to 

 varying ability to use nutrients combined in different forms (e.g., potas- 

 sium in the form of a chloride instead of sulphate), or what part may be 

 due to varying ability to absorb from very dilute solutions. This 

 question needs careful investigation, particularly in its application to 

 orchard and vineyard fruits of different kinds and to the stocks upon 

 which they may be grown. It is conceivable that the high feeding power 

 of a certain stock in respect to some particular material may be of as 

 great significance in the success of a fruit plantation in a certain soil 

 as the question of "congeniality" of stock and cion. From the data 

 presented by Hartwell, the inference may .be drawn that the potassium 

 found in the soil and practically unavailable to mangels and summer 

 squash would be made available to them were soy beans first grown 

 upon the land and then plowed under, for, after the soy beans had 

 dissolved and used it, other plants wovild find it in a different form. 

 There may be little occasion for special efforts to make potash more 

 available to orchard trees by using intcrcultures, for evidence is presented 

 later that for fruit trees potash is seldom a limiting factor. Nevertheless, 

 the general principle involved may be important in relation to other 

 elements. 



Availability of Iron and Sulphur. — Certain types of bacteria oxidize sulphur or 

 hydrogen sulphide to sulphates and others, ferrous oxide to ferric oxide. These 

 organisms may play some part in rendering sulphur and iron available, though 

 the most important type of bacterial action in the soil is concerned with general 

 decomposition and particularly with the nitrogen supply. 



Availability of Nitrogen. — Just as sulphur is available only in the form 

 of sulphate and phosphorus in the form of phosphate, most nitrogen is 



