I08 FERTII.ITY AND FERTILIZER HINTS 



thoroughly oxidize (by exposing it to the air for a long time 

 in heaps mixed with earth) in which case the injurious com- 

 pounds are changed so that they are not harmful. Sometimes it 

 is put on the land before being oxidized to get rid of insects and 

 if so it should be applied a long time before planting.* 



Gypsum. — This product is sometimes called land plaster. The 

 lime is as sulphate in this compound. Gypsum does not contain 

 as much lime as good limestone. It is a good fertilizer for 

 leguminous crops as clover, alfalfa, etc. The use of gypsum in- 

 stead of lime (CaO) is not to be recommended as the real value 

 of gypsum is in liberating locked-up potash. Super-phosphates 

 contain gypsum and when they are used it would not be necessary 

 to apply gypsum. Gypsum seems to keep the soil moist in dry 

 weather by absorbing moisture from the air or conserving it in 

 the soil. On soils low in potash, gypsum does not seem to be 

 beneficial and when soils fail to respond to gypsum an application 

 of potash may be needed.* 



Green Manures. — Any crop that is grown and plowed under in 

 order to benefit the soil is called a green manure. A green 

 manure may help the soil in any of the following ways : 



1. By keeping up the humus supply by furnishing organic 

 matter. 



2. By improving the texture of soils, by making heavy soils 

 lighter and sandy soils more retentive. 



3. By utilizing the soluble plant food that would otherwise be 

 lost if the land was left bare. 



4. By ridding the land of many weeds and thus serve as a 

 cleaning crop. 



5. By bringing up plant foi^l from the subsoil to the surface 

 soil. 



6. By using a leguminous crop the nitrogen content of the soil 

 may be increased, by utilizing the nitrogen from the air. ' 



7. By preventing the washing of soils, or erosion. 



Classes of Green Manures. — There are many crops used as green 

 manures and the section of the country determines to a great ex- 



