194 Fruit-growing in Arid Regions 



Humus : What it is and what it Does 



Popular writers on horticultural topics have woefully 

 confused humus with decaying organic matter, and have 

 implied that all organic material is humus as soon as it is 

 mixed with the earth and decay has set in. The fact 

 is, humus is the final product of organic decay, and as 

 such has entirely different effects on soils than have 

 organic materials that are undergoing the processes of 

 decomposition. 



When green-manure is plowed into the soil, various 

 low forms of plant life, including fungi, yeasts, and bacteria, 

 attack it, thus inducing decay. Fraenkel 1 " found in 

 the cultivated soil of Liebefeld 5,750,000, in meadow 

 land 9,400,000, in a manure pile 44,500,000, bacteria per 

 cubic centimeter. These figures seem high for so small 

 a quantity of material, but taking the average size of 

 a bacterium, a cubic centimeter might readily contain 

 six hundred millions." 



Other forms of bacteria begin to multiply as soon as 

 fermentation sets in. Different organisms have different 

 and important functions in promoting chemical activities 

 in the soil; plant-food elements are set free, changed 

 and combined into substances that plants can use. No 

 less than five different acids are generated by the processes 

 of decay, carbonic acid being among the most important. 



Sackett 2 found that clover taken in full bloom in June, 

 when ground and mixed with soil at the rate of 10 tons 

 per acre, gave off at the end of three days carbon 



1 Hilgard, "Soils," p. 143. 



8 Sackett, W. G., unpublished notes. 



