80 TEXTBOOK OF PLANT PHYSIOLOGY 



affects the availability of the insoluble substances. It has been 

 observed that an excess of calcium salts causes chlorosis in many- 

 plants, the iron salts having been changed to an insoluble form 

 Such "calcium" chlorosis is especially marked in lupines and some 

 other leguminous plants which are unable for this reason to grow- 

 on alkaline soils. 



28. Means of Determining Soil Fertility. Methods of Pot 

 and Field Experiments. — The complexity of relations accompany- 

 ing the absorption of mineral nutrients from the soil does not per- 

 mit one to determine exactly the degree of its fertility solely on the 

 basis of chemical analysis. Upon a complete analysis, one will 

 usually find even in the poorest soils such quantities of the neces- 

 sary elements as will not only satisfy but will be in excess of all the 

 needs of a plant. An analysis does not give an answer to the ques- 

 tion as to whether these elements are present in available or un- 

 available form. It gives us an idea, however, of the potential 

 amount of mineral nutrients which can be made available to the 

 plant through the processes of weathering. An analysis furnishes 

 most often sufficient data only on nitrogen, as this constituent of 

 the soil is comparatively easily and rapidly transformed into sub- 

 stances accessible to plants. In peat-bog soils, however, even 

 nitrogen is so firmly bound that it is unavailable to the roots of 

 plants. 



Results of a complete analysis usually give an exaggerated 

 idea of the quantity of available mineral substances that are pres- 

 ent. On the other hand, analysis of soil extracts or of artificially 

 prepared water solutions does not give a correct idea of the nutri- 

 tive qualities of a soil. The value thus secured usually is too small, 

 for the roots have a higher capacity of dissolving substances than 

 pure water. 



In order to learn more exactly of its nutritional capacity, the 

 soil is often extracted with 1 per cent hydrochloric acid, or with 

 various concentrations of citric acid or ammonium citrate. This 

 method is especially useful for determining the amounts of avail- 

 able phosphates. Data are thus secured that more closely corre- 

 spond to the feeding power of the soil, but nevertheless these are 

 only approximations. This is particularly so when it is considered 

 that the various plants possess different nutrient requirements. 



These defects in the chemical methods of soil analysis have led 

 to a search for other means in which the plant itself would serve 



