434 UTILIZATION OF MINERAL SALTS 



to maturity in sand or solution cultures has led to attempts to use this as a 

 method for the commercial propagation of crops, a procedure for which the 

 name hydroponics has been proposed. Some investigators have used solution 

 cultures (Gericke and Tavernetti, 1936) for this purpose, others sand cultures 

 (Biekart and Connors, 1935) and still others gravel cultures (Withrow and 

 Biebel, 1936). The use of such methods for the commercial production of 

 plants is still in the experimental stage, but it seems probable that the methods 

 of sand and solution cultures can be adapted to the commercial production 

 of at least certain greenhouse crops. 



Discussion Questions 



1. The addition of inorganic nitrogen to soil in which legumes and grasses 



are growing together often results in the disappearance of the legumes. 

 What are some possible explanations? 



2. The growing of sorghum has frequently been found to be injurious to crops 



planted later in the same soil. Suggest some possible explanations of 

 this effect. 



3. Why can corn plants grow to maturity with their roots in solution cultures 



while flooding a corn field results in death or serious injury of the plants? 



4. Small quantities of copper greatly increase yields of certain crops upon many 



muck soils. Does this prove that copper is an essential element for these 

 crops? 



5. What is a "fertilizer"? What elements are most commonly supplied to plants 



as fertilizers? List several reasons for the addition of fertilizers to soils. 



6. Do you consider it advisable to use the term "plant food" as a synonym for 



"fertilizer"? Discuss. 



7. How does the list of essential elements for plants compare with that for 



animals? 



8. Why are salts of copper and other heavy metals toxic to plants in lower 



concentrations in solution cultures than in soils? 



9. Why does nitrogen deficiency become evident more quickly in plants than 



sulfur deficiency? 



10. List the important ways in which conditions in a solution culture differ from 



those in a loam soil. Similarly contrast conditions in a sand culture with 

 those in a loam soil. 



11. In what parts of a plant would you expect "salt injury" (injury due to 



over-fertilization) to appear first? Why? 



12. Would an overdose of fertilizer be more injurious to plants growing in the 



bright light or to similar plants growing in the shade? Explain. 



13. Can the mineral salt deficiencies of a soil be determined by analyzing the 



ash of plants growing on that soil? 



14. When an Increase in yield of a crop is produced by the addition of certain 



mineral elements to the soil as fertilizers does this prove that those ele- 

 ments were deficient in that soil? 



