320 GENERAL SCIENCE 



the soil will be well supplied with nitrogen. Third, by 

 covering the soil with manure which is composed of 

 animal excreta and waste from the stems of plants, and 

 plowing it under. Every kind of decaying organic matter 

 has nitrogen in it. Fourth, by buying nitrates in the form 

 of commercial fertilizer and sowing it on the soil; this 

 is usually done by sowing it with grain seed in order 

 that it may help to produce a crop without much waste. 

 The other elements of plant food that are often bought 

 as fertilizers are potassium and phosphorus. Phos- 

 phorus is purchased in the form of phosphoric acid and 

 potassium as potash or muriate of potash. Nitrogen is 

 sometimes purchased in the form of ammonia. The 

 nitrogen, potassium, and phosphorus compounds are 

 usually mixed in known proportions and sold as com- 

 mercial fertilizer. 



QUESTIONS AND EXERCISES 



1. Give the history of the formation of the soil. 



2. Name the parts of good soil. 



3. How does the weather affect rock lying on the surface? 

 Does rock contain food for plants? Explain. 



4. What is meant by soil water? How can it be retained for 

 plant use? 



5. How much of the soil is air by volume? Prove your answer 

 by an experiment. 



6. Is there any decaying material mixed with the soil? Of 

 what use is it? 



7. Of what use are living organisms in the soil? Could plants 

 grow without them? Explain. 



8. Which plants protect soil bacteria? 



9. Which method of keeping the soil supplied with nitrogen 

 is the cheapest? Why? 



