92 PHOSPHORUS AS A SOIL PRESERVER 



ing plowing and cultivation, and drainage. It is obvious 

 that plowing to a depth of four inches will not supply 

 the soil with the amount of air which plowing to twice 

 the depth will. It is likewise clear that when a soil is 

 water-logged, or partially so, or, in other words, when 

 its pores are filled wholly or partly by water instead of 

 air. we cannot expect that a sufficient supply of oxygen 

 will be maintained there for crop production, and these 

 facts hold true for bacterial development in soils. Our 

 study therefore of the oxygen needs of soil bacteria 

 serves to emphasize more clearly the necessity for 

 rational methods of tillage and drainage. 



No farm should be without its experiment plot, for it 

 has been by experimental work only that anything in agri- 

 culture has become known. Knowing the history of a 

 soil, the plot or field experiment, supplemented, in some 

 instances, by chemical and physical analyses, tells the 

 farmer the best plan to follow with the particular soil 

 to restore it to full power. The ratio of straw to grain 

 tells its story to the critical eye. If for several years the 

 straw production is abnormally high and the grain pro- 

 duction is low, these facts point to phosphorus being 

 needed. If the leaves of the grain are long, loose, hang- 

 ing and fluttering and the stems too long for their thick- 

 ness, the soil probably requires calcium. A bright green 

 to yellowish colored foliage with the tips of the leaves 

 brown or reddish in color, indicates want of nitrogen. 



Broad-leaf plants, like burdock and nettles, indicate 

 moisture, while narrow-leaf plants indicate dryness. 

 Nitrogen is abundant where chickweed and red pimperel 

 grow, while lack of nitrogen is indicated by jagged chick- 

 weed, field chickweed and vernal whitlow-grass. 



Soil that is rich in nitrate of soda is indicated by the 

 presence of goose foot, oraches and burning nettle. Fox- 

 gloves, spurry and corn marigolds indicate calcium. 



