166 PLANT GROWTH 



Percentage of the dry weight of 



Plant 



Ca K Mg N P 



Sunflower 2.2 5.0 0.64 3.6 0.56 



Beans 2.1 4.0 0.59 3.6 0.55 



Wheat 0.8 6.7 0.41 4.5 0.49 



Barley 1.9 6.9 0.54 4.7 0.52 



Peas 1.6 5.3 0.50 4.5 0.19 



Corn 0.5 3.9 0.40 2.9 0.39 



Note that the peas have three times as much calcium as 

 corn, more potassium and magnesium, nearly twice as much 

 nitrogen but only one-half as much phosphorus. Other 

 comparisons in the table are equally striking. (2) Carolus 

 showed that the absorption depends on the availability of 

 the elements. He found that when nitrogen and phos- 

 phorus were applied to the soil, the plants absorbed not only 

 more of these elements, but also more calcium and mag- 

 nesium. (3) Absorption of salts is increased by an optimum 

 soil moisture content. (4) A reaction of the soil with a pH 

 (see p. 171) of about 6 produces the best growth for most 

 garden vegetables with a minimum percentage of mineral 

 salts. These facts indicate that more consideration should 

 be given to the conditions of growth in the selection of our 

 vegetables as sources of mineral foods. 



Fertilizers are applied to soils to supplement the deficient 

 soluble elements of the soil. It must be remembered that 

 the soil contains an enormous store of insoluble elements 

 which are slowly becoming soluble. The fifty-year fertilizer 

 experiment at the Pennsylvania State College Experiment 

 Station shows clearly the stability of the soil. After forty 

 years of cropping without fertilizer the yields had fallen to 

 half the original, but if only phosphorus was added, in addi- 

 tion to lime to reduce the acidity, the yield was maintained 

 for fifty years, showing that all the other elements were sup- 

 plied by the soil. With the best treatment, namely, added 

 phosphorus, lime, potassium, and nitrogen, soil described 

 as in good state of fertility at the beginning of the experiment 



