[Chap. V LOCAL PLANT COMMUNITIES 45 



inert matter and sold on the market under the erroneous name of "plant 

 foods." This misleading use of a name is one of many examples in which 

 a scientific truth is ignored and denied for alleged commercial advantage. 

 We shall learn later that the food used by plants is the same as that used 

 by animals. 



A change in the proportion of salts containing nitrogen, phosphorus, 

 and potassium may affect the composition of a plant community. For 

 example, soluble nitrogenous fertilizers may result in an increase in the 

 amount of bluegrass and a decrease in the amount of clover, while phos- 

 phate fertilizers may result in a marked increase in the amount of clover. 

 In the North Central States many pastures are dominated bv poverty 

 grass and weeds; bluegrass and clover are present but not abundant. 

 Hundreds of demonstration plots in these states have shown that the 

 addition of 300-400 pounds of acid phosphate per acre results in a 

 marked change in the composition of this pasture community. They also 

 showed that this change will occur without the addition of lime except 

 when the soils are very acid. At first there was a rapid increase in the 

 amount of clover, followed later by an increase of bluegrass. Within two 

 years these pastures had become dominated by clover and bluegrass, 

 and by the close of the fourth year they were bluegrass pastures. For the 

 farmer this procedure increases the value of the pasture from 200 to 

 300 per cent, and for others there is the pleasure of resting the eye on 

 luxuriant green pastures. 



There is also an interesting biological background to this sequence. 

 The increase in clover followed the application of phosphate and re- 

 sulted in conditions becoming favorable to bluegrass. The practice of 

 using clover and other legumes as a means of enriching the soil is as 

 old as the history of agriculture, but it was not known until near the 

 close of the last century that the enrichment is due to the process of 

 nitrogen fixation by bacteria that inhabit the nodules of the roots of 

 legumes. 



Animals, like plants, are dependent upon compounds containing cer- 

 tain kinds of mineral elements. They obtain nearly all of these com- 

 pounds either directly or indirectly from plants. We, for instance, obtain 

 them when we eat vegetables, meat, and dairy products. In diis way 

 some of the phosphate added to the pasture is gradually removed by 

 the grazing animals. Some of it also disappears from the land by erosion 

 and in drainage water. After several years the phosphate supply in the 

 pasture gets so low that clover and bluegrass begin to decrease in 



