WHY CALIFORNIA SOILS ARE GOOD. 25 



It is clear then that both groups of characters are essential to 

 success with the plant, from a horticultural point of view and that 

 the two groups are closely inter-related. 



The Physical Characters. All the physical characters of the soil 

 are horticulturally best when they occur in moderation and worst 

 when they occur in extremes. Two extremes in soils, for instance, 

 are clay and sand and intermixtures of the two, in connection with 

 various forms of organic matter, give all the degrees of variation in 

 soil texture. If the soil contains much more than 25%' of clay it 

 becomes a heavy clay soil and physically less fit for most horticultural 

 uses. If it contain less than 4% of clay it becomes almost clear 

 sand. The best horticultural soils are silty loams which have pro- 

 portions of clay within the limits stated with sand and finer particles 

 sometimes called rock powder, and organic matter to form the bulk 

 of the soil. Fortunately California has this most suitable mixture 

 largely predominating in her soils naturally and this constitutes one 

 feature of the splendid horticultural adaptation of California. 



There are various reasons why such a physical condition of the 

 soil promotes the most satisfactory growth of plants. First, water 

 capacity: which in such soil is ability to hold water equal to from 

 30 to 40% of their bulk, and they also have a coefficient of hygroscopic 

 moisture (moisture which cannot be taken away by air drying) of 3 

 to 7%. This assures about the right degree of moisture retentiveness 

 for the best plant growth. 



Second, permeability: the association of fine and coarse particles 

 being such that air and water enter freely and without such sufficient 

 access there is less thrift to the plant and less bacterial action to 

 furnish food for it. 



Third, penetrability: the same mixture of soil particles renders it 

 easy for plant roots to extend freely and deeply to render strong sup- 

 port to the plant mechanically and to enable it to reach supplies of 

 plant food and moisture. Too much clay prevents free root growth, 

 too little clay prevents moisture retention and is apt to bring the 

 plant into distress unless the most exacting measures are taken to 

 supply moisture constantly in the right amounts. 



After the condition of the soil in the foregoing respects is found 

 to be suitable for horticultural uses, the next physical character is 

 the depth of soil available to the plant. Depth means not only that 

 the plant shall have room for root extension and a large amount of 

 plant food within reach but depth is also directly concerned in 

 moisture retention as a sufficiently retentive soil with frequent surface 

 cultivation acts as a subterranean reservoir. Depth is directly related 

 to the growth habit of different plants. Our common garden plants 

 use from two to four feet of soil though they may thrive on less, and 



