174 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



same type of soils located in different sections, and even to differ- 

 ent types of soil from those under experiment. 



The analysis of a soil with strong hydrochloric acid does not 

 show exactly what fertilizer to apply to it, but it does give indica- 

 tions (a) as to the wearing qualities of the soil, (b) what elements 

 are likely to become deficient first under a given system of 

 cropping, (c) what deficiencies already exist in the soil, or will 

 soon be brought out. 



Virgin soils containing high percentages of plant food are 

 highly productive unless improper physical conditions interfere 

 with the welfare of the plant. Low percentages of plant food do 

 not necessarily indicate low production when first put in cultiva- 

 tion. To use Hilgard's illustration, suppose a heavy alluvial soil 

 of high plant food content is diluted with its own weight of sand. 

 By improving the physical condition of the soil an increased crop 

 will very likely result, though the percentage of plant food has 

 been reduced by half. The root system of the plant in the diluted 

 soil will probably be better developed and in more intimate contact 

 with the soil particles, than in the undiluted soil. We may con- 

 tinue the dilution, using say 4 parts of sand to one of soil, 6 parts 

 of sand to one of soil, and so on. At some point, the size of the 

 crop will begin to decrease on account of the difficulty of secur- 

 ing plant food. How far the dilution may be carried depends 

 upon the plant and on the soil, and is a subject worthy further 

 study. 



Standards for Interpretation. Varying methods of analysis 

 for soils, and varying standards of interpretation are used accord- 

 ing to the method of analysis and the individual opinions of the 

 analysts as to what constitutes a good soil. The standards of 

 Hilgard 1 are based upon a large number of analyses and wide 

 observation, and appear well adapted to American conditions. 

 These standards give best results when applied to virgin soils. 



Phosphoric Acid. Hilgard states that phosphoric acid is 

 seriously deficient in virgin soils when below 0.05 per cent., unless 

 accompanied by a large amount of lime. In heavier virgin soils, 

 1 Tenth Census of the U. S. 



