258 PLANT PHYSIOLOGY 



causes chlorosis in many plants, the iron salts having been 

 '^hanged to an insoluble form. 



60. Methods of Determining the Nutritive Value of Soils. 

 Methods of Pot and Field Experiments. — The complexity of 

 relations accompanying the absorption of mineral nutrients from 

 the soil does not permit one to determine exactly the degree of 

 its fertility solely on the basis of chemical analysis. Upon 

 complete analysis, there will usually be found even in the poorest 

 soils such quantities of the necessary elements as will not only 

 satisfy but be in excess of all the needs of a plant. An analysis 

 does not give an answer to the question as to whether these 

 elements are present in available or unavailable form. It gives 

 an idea, however of the potential amount of mineral nutrients 

 that can be made available to the plant through the processes 

 of weathering. An analysis furnishes sufficient data only on 

 nitrogen, as this constituent of the soil is comparatively easily 

 and rapidly transformed into substances accessible to plants. 

 In peat-bog soils, however, even nitrogen is so firmly bound that 

 it is unavailable to the roots of plants, and chemical determina- 

 tion of nitrogen does not give a true picture of the soil's fertihty. 



Results of a complete analysis usually give an exaggerated 

 idea of the quantity of available mineral substances that are 

 present. On the other hand, analysis of soil extracts or of 

 artificially prepared water solutions does not give a correct idea 

 of the nutritive qualities of a soil. The value thus secured 

 usually is too small, for the roots have a higher capacity of dis- 

 solving substances than pure water. 



In order to learn more exactly the nutritional capacity of the 

 soil, it 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 

 available phosphates. Data are thus secured that more closely 

 correspond to availabihty of nutrients from the soil, but never- 

 theless these are only approximations. This is particularly the 

 case when it is considered that the various plants possess different 

 abilities to absorb nutrients. 



These defects in chemical methods of soil analysis have led 

 to a search for other methods, in which the plant itself would 

 serve as an indicator of soil fertihty. The method of pot cultures 

 is valuable in this respect. It has been worked out by investi- 



