132 



THIRTY-FOURTH FRUIT-GROWERS' CONVENTION. 



but it brings in contact with the tied up or unavailable plant food the 

 soil solvents like carbonic acid, water soluble inorganic acids, and 

 organic acids exuded from roots of plants or generated by plant decay, 

 making these plant foods immediately available in small but constant 

 quantities. So great is the body of soil thus affected by this process in 

 the perfect soil that little exhaustion is felt in any particular portion 

 or within an appreciable length of time. 



Departures from this ideal condition require a careful study of ferti- 

 lizers, depending upon such factors as the restriction of water passage 

 b}^ the heavy texture of the soil or the rapid leaching of soils from too 

 porous a texture. In both these extremes, the capillary power in soils 

 is greatly decreased or restricted, permitting the exhaustion without 

 replacing the available plant foods in the soil thus cropped. Such 

 soils often need fertilizing, but the question is. What kind of fertilizers 

 shall be applied; how can the greatest benefit be derived from their 

 use ? The quantity of plant foods available in soils is extremely difficult 

 to determine, ancl soil chemists are not at all agreed on the proper 

 chemical methods to determine this. It is very likely that no chemical 

 method will ever be devised to accurately determine the plant foods 

 in soil available for crops. Each plant seems to possess a different 

 povrer of extracting these plant foods from soils, and I am inclined 

 to agree with Professor Wickson. who once said that the best analyst 

 of the available plant food in soils was the plant itself. Such a view 

 renders uncertain the determination of exact application of fertilizers 

 to soils, except as tested by the actual application to crops grown upon 

 each soil. This is likely to be a slov»' and uncertain process, but is in 

 accordance with the most successful fruit growing. Fertilizers should 

 not be applied at all until every other practical available method of 

 maintaining or increasing the soil fertility has been thoroughly tried. 

 Such methods include better and deeper cultivation, application, and 

 conservation of moisture in the soil and attention to such features as 

 drainage, aeration of soil and subsoil, and the determination of dele- 

 terious substances like alkali, etc. 



But more important than all these is the preservation and incorpora- 

 tion of humus, or decayed organic matter, in the soil. California soils 

 appear to depend for their fertility upon this perhaps more than any 

 other single factor. In this hiunus is held the major part of the avail- 

 able plant foods. As humus is found in the upper part of the soil 

 column, and to a much lesser extent in the lower depths, or subsoil, 

 it will be seen that the presence of humus is most important in the 

 growing of crops. Nitrogen, which is more rapidly exhausted than 

 any single plant food, is held almost exclusively in that part of the 

 soil containing the humus. As this humus is found mainly in the upper 

 part of the soil column, the nitrogen is therefore exposed to greater 

 exhaustion, not only by the growth of crops themselves, but by the 

 action of the elements and the burning out by the summer sun. This 

 burning out of humus and consequent reduction of nitrogen has been 

 one of the prime factors in the exhaustion of grain soils, especially 

 those systematically smmner-fallowed and left exposed during the whole 

 of the long summer season. The clean culture in orchards and vine- 

 yards closely resembles this summer-fallowing of grain lands, and 

 permits of the same evils. To such cleanly tilled soils commercial 



