162 PLANT GROWTH 



refuse plant material and have an almost perfect soil for all 

 kinds of garden planting. It is an especially desirable soil 

 for planting seeds, if the seedlings must be transplanted. 



The living organisms in the soil are numerous, but, fortu- 

 nately most of them are beneficial. The harmful ones, in 

 most cases, belong to the group of insects that live in the 

 atmosphere and lay eggs in the soil. These develop into 

 larvae of various sorts in the soil. The Japanese beetle be- 

 longs in this class and damages roots of grass or other plants. 

 These can be destroyed by poison sprays on the leaves they 

 eat, or by fumigating, or by poisoning the soil. If the soil is 

 poorly aerated, bacteria which can change nitrates and set 

 the nitrogen free may increase in number. This is discussed 

 in more detail in Chapter 23. 



The beneficial organisms should always be encouraged 

 by the proper cultural methods. Nature will maintain a bal- 

 ance of organisms and the food for them. Therefore if we 

 add organic matter and keep the soil aerated the organisms 

 will increase in numbers. Many of the bacteria play a part 

 in making nitrogen available to the plants, as explained 

 under the topic of the nitrogen cycle in Chapter 23. 



Soil erosion has destroyed many millions of acres of farm 

 land and has reduced the yields of millions more. The prob- 

 lem is of less importance to gardeners than to farmers but if 

 we are soil-erosion conscious we shall see many soil-covered 

 sidewalks and many little gullies at the edges. Such un- 

 sightly conditions indicate the loss of at least three valuable 

 constituents: water, soil, and mineral salts. The loss may 

 not be great but in most cases the remedy is equally simple: 

 prevent the run-off. If the garden has a slope of more than 

 4 per cent. Government soil erosion literature should be 

 consulted. 



The National Soil Conservation Service has found a num- 

 ber of ways to prevent run-off. Most astonishing is the fact 

 that the rate, and therefore the amount of percolation is 



