SOII< PHYSICS, CHEMISTRY AND MICROBIOLOGY 



1409 



CROPS AND CUIyTlVATION. 



1059 - Measurement of the Surface Forces in Soils, — shull, Charles albert, in 



The Botanical Gazette, Vol. I,XII, No. i, pp. 1-31, 8 tables, 5 figs. Chicago, July 1916. 



A contribution to our knowledge of the mechanics of soil moisture and 

 the relations of this latter to plant growth. The main purpose of the work 

 was to find some means of measuring the force with which particles of 

 soils of varying fineness retain moisture at different degrees of dryness and 

 to obtain some more definite knowledge concerning the amount of " back 

 pull " occurring in soils when the total moisture content is so low as to be 

 unavailable to glowing plants. A number of experiments were carried out 

 on the relation of seeds to soil moisture, an aspect of the question which 

 hitherto has not received the attention it deserves. 



The seeds of Xanthium were chosen for the experiments owing to the 

 rapid re-establishment of moisture equilibrium relations after disturbance. 

 They were derived from 119 plants derived in their turn from the seed of 

 a single plant of X. pennsylvanicum ; individual variations should therefore 

 be reduced to a minimum. 



The soils used in the major portion of the work were i) the subsoil oj 

 the Oswego silt loam — a heavy clay — and, as a contrast to this, 2) a fine 

 quartz sand manufactured from quartz rock and, finally, 3) various other 

 soil types, details of which appear in table IV. The average composition 

 of No. I as determined by mechanical analysis was as follows : 



SOIL PHYSICS, 



CHEMISTRY 



AND 



^UCROBIOLOGY 



The moisture equivalent was 35.2 per cent and the wilting coefficient 

 1 9. 1 per cent. 



No. 2 was a very pure quartz sand, the average diameter of the parti- 

 cles being very close to o.io mm. The moisture equivalent was 2.41 per 

 cent and the writing coefficient 1.3 per cent. 



Methods. — While the internal forces of Xanthium seeds have been 

 approximated by osmotic means, many seeds lack semi-permeable coats. 

 For such seeds a vapour pressure method has been used which gives results 

 which are in a way comparable to the osmotic measurements. It consists 

 essentially in measuring the vapour pressure equilibrium of the air-dry seeds 

 over sulphuric acid of varying strength and calculating the internal pressure 

 of the seed from the vapour pressure of the solution over which it was found 



