142 Humus: Nature and Formation 



necessary for plant growth under field conditions. By eliminating 

 the possibility that minor elements were concerned, the conclusion 

 was reached that exhaustion is associated with loss of organic matter. 

 The possibility was suggested that productiveness of a given soil 

 rises and falls with the percentage of humus present. 



The influence of treatment upon the rate of humus decomposition 

 can be best illustrated when a peat bog is drained. When the bog 

 is saturated with water, humus accumulates, the rate of accumula- 

 tion depending upon the nature of the vegetation, climate, topog- 

 raphy, and chemical composition of the waters entering the bog. 

 When the bog is drained, humus ceases to accumulate and begins to 

 disintegrate. The rate of disintegration of the humus is controlled 

 by the nature of the peat, climatic conditions, and the depth of the 

 drainage system. The rapid rate at which certain peats decompose 

 in warm climates points to the great danger of their rapid disappear- 

 ance by improper handling. 



In recent studies Broadbent and Norman used isotopic nitrogen 

 and carbon compounds. They found that, when decomposable or- 

 ganic matter was added to the soil, a considerable increase took place 

 in the rate of decomposition of the soil organic matter present. In 

 one experiment they added 1 and 2 gm of sudan grass to 100 gm of 

 soil, incubated the soil for 11 days, and measured the COo produced. 

 Since the sudan grass contained 2.67 per cent of the isotopic carbon, 

 they could calculate the proportion of the COo which resulted from 

 the oxidation of the soil organic matter. When the soil alone was 

 incubated, 49 mg CO2 was produced from the soil organic matter; 

 when 1 and 2 gm of sudan grass were added, 216 and 329 mg were 

 produced, respectively, from the soil, in addition to the 418 and 651 

 mg of CO2 from the decomposition of the sudan grass. For every 

 100 mg of CO2 produced from the sudan grass, an additional 40 mg 

 was formed from the soil organic matter. No explanation could be 

 given for the fact that soil organic matter became a better source 

 of nutrient for the soil population when readily decomposable or- 

 ganic matter was added. 



Abundance and Nature of Humus in Different Soils 



The hiunus content of soils varies considerably, from extremely 

 small amounts, as 0.1 per cent in some poor sandy soils, to as much 

 as 20 per cent, in prairie soils; it may be even higher (50-80 per 

 cent) in certain soils which have originated from peat bogs that have 



