240 SOILS AND FERTILIZERS 



It is probably where special crops are produced that green- 

 manures will reach their greatest usefulness. Their use in 

 orchards is well established. For this purpose they are 

 plowed under in the spring and planted in midsummer. 

 Potato-growers and even market-gardeners are using green- 

 manures in increasing quantity. ^ 



299. Handling green-manure crops. — The stage of growth 

 at which green-manures should be plowed under has a rather 

 important bearing on their effect on the soil. In order that 

 they shall decompose readily, they should be succulent when 

 incorporated with the soil. If plants that have fully ripened 

 are plowed under, they decompose very slowly and interfere 

 with the formation of nitrates. An acid soil is unfavorable 

 to the decomposition of green-manures and to the formation 

 of nitrates ; hence it is desirable that lime be applied before 

 planting the manure crops unless the soil is already well 

 supplied with lime. 



QUESTIONS 



1. Describe what is meant by green-manure crops. 



2. State four ways in which they may be beneficial to the soil. 



3. What two substances are prevented from being leached from 

 soil in large quantities by the growth of green-manure crops ? 



4. How do legumes differ from other green-manures in con- 

 tributing to soil fertility ? 



5. In what two ways is there a transfer of plant nutrients brought 

 about by the use of green-manures, and how do they benefit the soil ? 



6. Name five leguminous green-manure crops and state the time 

 of year in which they are generally planted in your locality. 



7. Give the same information regarding five non-legumes. 



8. What is the disadvantage of plowing under green-manure 

 crops when they are fully ripe ? 



LABORATORY EXERCISES 



Exercise I. — Study of green-manure in the field. 

 Plan a field trip to some farm where a crop is being turned under 

 for green-manure. Determine whether the time is most favorable 



