84 Pecan-Growing 



are usually present in the soil in much greater abundance 

 than the crops demand or can use, while most soils are 

 deficient in phosphoric acid, nitrogen and potash, in an 

 available form. A cover-crop contains only the amount 

 of phosphoric acid and potash that it has taken from the 

 soil itself, and at most it returns only that which was 

 borrowed. Nitrogen is obtained directly from the air in 

 the soil. 



As found naturally in the soil, both phosphoric acid and 

 potash are almost insoluble in water. However, in the re- 

 mains of plants the potash is in soluble form and the phos- 

 phoric acid is combined with lime and other bases which, 

 although somewhat insoluble in water, are in a form that can 

 be acted on much more readily by the roots of growing plants 

 than can the natural phosphates of iron and aluminum. 

 Therefore, while the cover-crop merely returns something 

 which it has taken from the soil, it gives it back in a more 

 available form. The effect, so far as the phosphoric acid in 

 the cover-crop is concerned, is much the same as that obtained 

 by treating rock phosphate with sulfuric acid to bring it into 

 a form to be readily utilized by growing plants. 



Cover-crops, especially the deep-rooted species, render an- 

 other important service in bringing a portion of the phos- 

 phoric acid and potash up from greater depths of the soil and 

 leaving it in the decayed plants in the surface layer, where it 

 will be better aerated and more available. As the pecan 

 is so deep-rooted it would seem that this would be of little 

 advantage ; however, the pecan obtains a very large percentage 

 of its plant-food from the surface layer of the soil, very little 

 deeper than that used for the support of the ordinary annual 

 farm crops. In special instances in which the tree draws much 



