10 BACTERIA AND SOIL FERTILITY 



in fertility and that legumes differ from other plants in their 

 nutritive requirements. Less ancient, however, is the knowledge 

 that uncropped soil may gain in nitrogen or that legumes with 

 the aid of bacteria may get their nitrogen from the soil. 



In the middle of the nineteenth century Boussingault wrote: 

 "Vegetable earth contains living organisms — germs — the vitality 

 of which is suspended by drying and re-established under favor- 

 able conditions as to moisture and temperature." He also hinted 

 at the fact that the microorganisms may take nitrogen from the air 

 and so change it that it may be used by the growing plant. He 

 spread out thinly 120 gm. of soil in a shallow glass dish and for 

 three months moistened it daily with water free from nitrogen 

 compounds. At the end of the time he found that it had gained 

 nitrogen but had lost carbon. Thirty years later Hellriegel and 

 Wilfarth solved the apparently hopeless problem of the nitrogen 

 nutrition of leguminous plants. Bacteria live in little nodules on 

 the roots of legumes and manufacture nitrogen compounds which 

 they give to the higher plants in exchange for carbohydrates and 

 other nutrients needed by bacteria. Since then we have learned 

 much concerning the relationship of plants to free and combined 

 nitrogen of the air and the soil. 



In 1 90 1 Beijernick discovered large yeast-like microorganisms 

 which live free in the soil and take from the atmosphere nitro- 

 gen and build it up into complex compounds within their bodies. 

 Every year since this date the number of microorganisms which 

 have been found to possess this property has been added to, until 

 today scores of organisms possessing this important property are 

 known. 



Terrific battles are being waged in the soil between the micro- 

 scopic plants and the microscopic animals. This is the conclusion 

 reached by Russell and Hutchinson in 1909. Since this date each 

 year has added new knowledge concerning organisms already 

 known or else added new ones to the list of beneficial organisms, 

 until today we know that the soil is teeming with organisms most 

 of which are beneficial. Some of them, however, are without 

 significance, and a few of them are injurious. They not only 



