NURSERY PRACTICE ON THE" NATIONAL FORESTS. 83 



a substance which is often found naturally in large beds and which 

 can be applied directly as a fertilizer. It is an especially good mate- 

 rial to add to light, sandy soil, on account of the clay contained. 



GREEN FERTILIZERS. 



The use of green fertilizer is the easiest and perhaps the cheapest 

 method of supplying soils with nitrogen, and as it adds humus it 

 improves their physical quality as well. The method consists in the 

 raising of rapid-growing herbaceous plants and plowing them under 

 before the stems become woody. The leguminous plants are pre- 

 ferred for this purpose. They have the ability, with the aid of the 

 microorganisms in the soil to assimilate the free nitrogen of the air 

 and store it in the root tubercles. A number of plants may be used, 

 among which are the lupines, yellow lupine in particular, different 

 kinds of vetch, field peas, cowpeas, beans, soy beans, dwarf beans, 

 frijoles, and serradilla. Engler found that on all soil rich in lime 

 field peas and cowpeas furnish the richest fertilization; on rather 

 moist soil with little lime the yellow lupine is the most suitable; 

 vetch is suitable only for heavy and compact soil and furnishes a 

 cheap but not so rich a fertilizer; in high localities with rigorous 

 climate and later sowing the field pea is recommended. Cowpeas 

 are the most expensive, but they produce the greatest volume and, 

 on account of their strong root development, are especially to be 

 recommended for rough, hard soil or for soil which has long lain 

 fallow. 



Vigorous growth of leguminous plants and the assimilation by 

 them of free nitrogen is dependent upon the presence in the soil of 

 certain fungi which live in a symbiotic relation with the plants. 

 When such fungi are not present — their absence is indicated by the 

 poor growth of the plants — they must be introduced. This is ac- 

 complished by the so-called inoculation of the soil. Surface soil 

 from other fields where legumes have grown in a flourishing manner 

 is collected and sown over the new area at the rate of about 100 

 pounds per acre. The same result can also be secured by the use of 

 inoculating fluids, with which the seed are moistened. 



CHOICE OF FERTILIZERS. 



If only the chemical qualities of the soil need improvement chem- 

 ical fertilizers may be introduced, but if the physical qualities need 

 improvement the addition of humus will be necessary. The chemical 

 fertilizers necessary can best be determined by experiment. Analyses 

 of soil are inconvenient and expensive and give only temporary 

 results, because the contents of the soil change with each year's 

 production of plants. It is preferable not to use fertilizers which 





