ACID SOILS AND ALKALI SOILS 115 



ing about an acid condition. In the case of this fertilizer 

 the ammonia is absorbed either directly or indirectly and 

 most of the sulfate, which is an acid, remains in the soil. 

 Probably no other fertilizer is so active in producing acidity, 

 but it is possible that sulfate of potash or muriate of potash 

 or gypsum may, -in less degree, have the same tendencj'. 



The use of free sulfur for combating fungous diseases may 

 also lead to the formation of a sour soil. 



140. Effect of green-manures on acidity. — In soils defi- 

 cient in lime the incorporation of green-manure crops has 

 been thought to produce temporarily an acid condition. 

 It is during the early stages of fermentation in the soil that 

 the acids are formed. When further decomposition pro- 

 ceeds, the acids are broken up and acidity disappears. This 

 condition has been noticed mainly in the South Atlantic 

 states. Where it has been found to occur, there is some ad- 

 vantage to be gained from plowing under the green-manure 

 as long as possible before planting the next crop. 



141. Weeds that flourish on sour soils. — Whether a soil 

 is acid or not will make a great difference in the kinds of 

 plants that will thrive on it. Certain weeds will generally 

 be found growing on sour soil and the presence of these in 

 large numbers may be taken as evidence that the soil needs 

 lime. Weeds that appear to flourish on acid soils may do 

 so either because they are physiologically adapted to an 

 acid condition, or because other vegetation does not thrive, 

 and hence these particular weeds have less competition on 

 this soil. The weeds that in one part of the country or 

 another may be considered to indicate an acid soil are as 

 follows : 



Sheep sorrel Corn spurry 



Paintbrush Wood horsetail 



Daisy Plantain 



Horsetail rush Goose-grass 



