NUTRITIVE FUNCTIONS OF SALTS 115 



theory and, according to its proponents, the failure of a crop 

 following itself is due to the production in the soil of autotoxins 

 or poisonous metabolic products excreted by roots into the soil. 

 These harmful products accumulate and finally prevent the growth 

 of the same crop although other crops are not so much affected 

 by them and may even be stimulated by them. 



Another theory of crop rotation is the sanitation theory whose 

 followers insist that the cause of crop deterioration is the presence 

 in the soil of plant diseases. These microorganisms, which are 

 more or less specific for each crop, ultimately break down the 

 resistance of the plant growing on any particular plot of land and 

 cause a constant diminution in the yield. Other plants may grow 

 on that soil, however, without being affected. 



All three of these causes doubtless work together to make a 

 rotation advisable. In such rotations some legumes should al- 

 ways be included because, of all crop plants, they are the only 

 ones which ever leave the soil richer in any element than they 

 find it. 



QUESTIONS 



1. Why are some plants " heavy feeders" on the soil and others "light 

 feeders"? 



2. Why should different plants use different materials from the soil? 



3. If plants lack chlorophyll, what may be the cause? 



4. Which requires more potash, — lettuce or wheat? more nitrogen? 



5. Why are the necessary elements among the most common ones in the 

 earth's crust? 



6. What are toxins? 



7. Why must the 'T " be omitted in the mnemonic system for remembering 

 the twelve essential elements? 



8. Can plants use pyrrol salts in place of iron? See Am. Jour. Bot., 13:276 

 1926. 



9. Why is sulphur returned to the earth in rain? See Plant Physiol, 1 :77, 

 1926. 



10. What is meant by a "4:4:3 fertilizer"? 



11. Only recently has it been possible to get iron free from impurities such 

 as manganese. Of what significance is this? 



12. Johnston calculated that when a plant is nourished by one part of boron 

 in two million of solution, the nutrient solution is as concentrated as pea soup, 

 made up of one pea in 132 gallons of water. Was his calculation correct? 



