JOMAL OF AGRlOimm ISEARCH 



Vol. XII Washington, D. C, February ii, 1918 No. 6 



WATER EXTRACTIONS OF SOILS AS CRITERIA OF 

 THEIR CROP-PRODUCING POWER 



By John S. Burd, 

 Chemist, California Agricultural Experiment Station 



THE PROBLEM . V vuKfe 



Given sunlight, suitable moisture and physical condition, the limi- ^'-^f ANiCAl 

 tation on the power of soils to produce crops is variously ascribed to the ^^^Oi^, 

 following general causes: 



1. Lack of capacity of the soil to supply the plant roots at all times 

 with watery solutions of the essential elements in proper concentrations, 



2. Presence of toxic substances. 



3. Lack of physiological balance in dissolved soil constituents. 



It is clear that any one or a combination of these conditions may be 

 the cause of low production, even though the remaining conditions may 

 be entirely satisfactory. The instances where the second and third in- 

 hibitive causes operate to limit production are perhaps quite numerous. 

 It may even be that, because of frequent occurrence, they are of as much 

 or more practical importance than incapacity of the soil to supply the 

 plant with proper nutrients. Nevertheless we are disposed to regard 

 such instances as special cases and to lay greater stress on the capacity 

 of the soil to deliver up solutes to the growing plant. Studies involving 

 this capacity are obviously more fundamental, in that they have to do 

 with an important function of all soils. 



WATER-EXTRACTABLE SUBSTANCES IN SOILS 



When we consider this function, we naturally turn to water as the 

 logical agent for the removal and determination of the soluble substances 

 in soils. The application of water as a solvent requires the use of a suffi- 

 cient amount in proportion to the soil to cause it to come into equili- 

 brium with the true soil solution and thus to insure the complete removal 

 of all dissolved matters when the soil suspension is filtered. Water in 

 excess unquestionably removes not only the substances already in solu- 

 tion but additional quantities somewhat proportional to the amount of 

 water used.^ Furthermore, v\e are told that the absorption of solutes 

 by plants is related to the concentration of the soil solution and not to 



1 HOAGLAND. D. R. THE FREEZING-POINT METHOD AS AN INDEX OF VARIATIONS IN THE SOn, SOLUTION 



DUE TO SEASON AND CROP GROWTH. In Jour. AgT. Research, v. 12, no. 6, pp. 369-395, 9 fig. 1918. 



Journal of Agricultural Research, Vol. XII, No. 6 



Washington, D. C. Feb. n, 1918 



Ik Key No. Cal.— 15 



(297) 



