FERTILIZERS 1 5 



the nitrogen supply less, the growth is hard and 

 firm. As illustration : note the strength of chap- 

 arral wood, compared with marsh land growth. 

 Nitrogen is deficient (comparatively) where the chap- 

 arral grows and the mineral or soil derived elements, 

 including potassium, relatively abundant. In a 

 word, potassium gives starch or stiffness to the 

 plant. Young peppers, acacias and eucalyptus trees 

 bend frequently to the ground because their roots are 

 in the surface soil, feeding on the excess of nitrogen 

 and water. Older trees with deeper roots have not 

 this tendency. (71). 



Sulphur, iron, calcium and silicon also perform 

 special parts in the building of plant tissue, but a 

 discussion of these would extend our subject unneces- 

 sarily; They are not "essential" plant foods. (23). 



PHYSICAL CONDITIONS. 



20. In California the mechanical or physical con- 

 dition of the soil is of greater importance than the 

 chemical composition. Especially is this true of cit- 

 rus culture, where irrigation and cultivation are so 

 frequent and thorough. The first sign of trouble is 

 usually a slight 3 r ellow color of the foliage and is 

 usually traceable to loss of humus and its conse- 

 quent nitrogen. Yet the cause may be due to the 

 roots entering a "hard pan" or coarse gravelly 

 strata, less favorable to growth. (99). In any 

 event the soil's mechanical condition should be 

 known to a depth of at least five feet, and deeper if 

 possible. The loss of humus so changes the soil's 

 condition that the trees cannot derive the benefit of 

 the water, the cultivation or the labor gives to it, 

 and the first sign is the loss of the healthy green color. 



