Food from the Soil 129 



the soil would make it yet more easily dissolved by water, 

 as well as more easy for roots to penetrate; nor must it 

 by any means be forgotten that while the land lay idle the 

 worms were busy, turning it over, also, grain by grain, 

 and enriching it as already described. And though the 

 farmer did not sow it, seeds were sown by the wind, and 

 a crop of weeds was certain to spring up, whose half- 

 decayed leaves the worms would drag into their burrows. 

 The whole wild crop, too, would be plowed in, and with 

 the roots of the farmer's last crop, would be gradually 

 decayed. The soil when damp would absorb ammonia 

 from the air, and ammonia would also be brought down 

 by rain and snow, and converted into nitric acid, which 

 would combine with some of the minerals already men- 

 tioned; and in this way, as well as by the decay of the 

 organic matter left in, or added to, the soil, a fresh sup- 

 ply of nitrates would be prepared. 



QUESTIONS FOR REVIEW 



1. Illustrate the fact that plants absorb mineral food in 

 very weak dilutions. 



2. Why does the farmer have to vary his crops? 



3. How differently is the soil affected by the various ways 

 in which the crops are disposed of? 



4. What substance is it which plants have to get from 

 organic matter? 



5. How do some plants secure this from living matter? . 



6. How is the soil provided with nitrates? 



7. What causes the decay of organic substances? 



8. What processes go on in a fallow field? 



