26 THE MONTHLY BULLETIN. 



fjreatly the liiimns content. We have found at Riverside, for example, 

 that the annual application of manure, coupled with plowing under a 

 leguminous cover crop each year for the past eight years, has not 

 resulted in any appreciable increase in the humus content of the soil. 

 Yet it is obvious that the soil is in an improved condition as compared 

 with the virgin soil alongside, and the trees show excellent vigor and 

 thrift. Likewise at both the Pennsylvania and Indiana stations the fre- 

 quent application of barnyard manure for thirty or more years has 

 resulted in only a small increase in the humus content of the soil. Our 

 soils would certainly be benefited by a larger humus supply, but the 

 matter of increasing the actual humus is one of great difficulty and slow- 

 ness ; so from a practical standpoint it is not to be expected that great 

 gains in the humus content can be obtained. 



In any event, however, humus is the stable residue left behind after 

 tlie ])acterial decompositions have become arrested. It represents the 

 more resistant and slowly decomposable organic residue that is formed 

 from organic matter, cover crops and manure. We are coming to believe 

 that it is the processes involved in the decomposition of fresh organic 

 matter, the transitory products that are formed and the physical, chem- 

 ical and biological effects produced, rather than the humus residue left 

 behind, that constitute the great value derived from adding organic 

 matter to soils. As one of the eastern experiment station men recently 

 expressed the matter, "It is the current of organic matter flowing 

 through the soil rather than the humus residue formed that constitutes 

 the chief value of green manuring." 



Following this line of reasoning it is easy to see why it is necessary to 

 make frequent applications of organic matter. As Dean Himt has said, 

 the chief object in soil management should be to keep the soil virgin. It 

 is w^ell known that virgin soils generally contain more or less organic 

 matter in a partially decomposed condition. In the state of nature the 

 grasses, weeds and leaves fall and become incorporated with the soil, 

 thus continually adding fresh supplies of organic matter. In its decom- 

 position the soil is kept in the state we call virgin, and so far as soil 

 organic matter is concerned the closer we imitate nature b}^ frequent 

 applications of organic matter, the better. 



During recent years much effort and thought have been directed 

 towards supplying deficiencies of the soil by the use of commercial fer- 

 tilizers and, to some extent, of cover crops. It is outside my present 

 ])urpose to discuss the wisdom of these practices further than to point 

 out that cover cropping with a legume is fundamental to soil main- 

 tenance in fruit culture in California. In the larger aspects of the 

 question, however, when we consider the soil of the State as a whole, the 

 systems now being generally employed are only temporary so far as 

 maintaining the soil is concerned ; fertilizers have played, and certainly 

 will continue to play, a prominent part; cover cropping and mulching 

 are likewise valuable, but inherently the great agricultural need of Cali- 

 foi-nia, both from the standpoint of soil maintenance and economic 

 stability, is aiversiiicaiion. 



At the present time only a small percentage of the arable land of Cali- 

 fornia is devoted to fruit culture, and the use of organic materials and 



