262 HARMFUL AGENTS IN SOILS AFFECTING FERTILITY 



and which may prove injurious to certain crops and fruit trees 

 if allowed to accumulate. Recent investigations seem to show 

 that most soils are so constituted that through good drainage, 

 proper tillage and the presence of sufficient lime the injurious 

 action of these toxins may be entirely or in a large 

 measure prevented. 



Other Harmful Agents. Among other harmful agents that 

 have been named in certain instances to explain low yields or fail- 

 ures are: (a) An excessive amount of aluminum and iron in soils 

 renders some of the plant-food elements unavailable. The effect 

 of soluble phosphates on certain red, tropical soils is of short dura- 

 tion because of the presence of excessive amounts of iron and 

 aluminum; (b) too much magnesium in proportion to calcium 

 may disturb the normal function of plant cells; (c) too much lime 

 in some soils may prove harmful to some crops, as grape-fruit, 

 pineapple and certain lupines; (d) certain mineral acids may make 

 soils sour; (e) it is advocated by some that certain soil micro- 

 organisms accumulate to such an extent that they greatly retard 

 the activity of the helpful soil organisms. 



Collect specimens showing the work of insects which attack roots of crops in 

 any way. Specimens of these insects may be collected and preserved according 

 to directions given in many good insect books. 



Diseased root specimens of field crops may be brought to class and kept 

 for study. v 



Projects in growing crops in spite of insects and diseases should be con- 

 ducted, using the most approved methods in each case. 



QUESTIONS 



1. What negative factor should be considered in diagnosing a case of infertility? 



2. What may some of these agents be? 



3. Discuss infertility due to worms which feed on plant-roots. 



4. Name methods of control. 



5. What is the corn root-louse? 



6. Name some plant diseases caused by certain fungi and bacteria that live 



in soils. 



7. How may a farmer avoid crop failure due to these diseases? 



8. Explain how too much water within soils may prove a harmful agent. 



9. Suggest a remedy. 



10. Illustrate by diagram. 



11. Discuss the relation of alkali to soil fertility. 



12. What is the toxin theory of the infertility of many soils? 



13. What are toxins? 



14. According to this theory, what is the action of fertilizers? 



15. Do any poisonous substances form in soils? How? 



16. What do recent investigations seem to show as regards these toxins? 



17. What harmful agents in soils do you know from your own experience 



or observation? 



