SOII< PHYSICS, CHEMISTRY AND MICROBIOI.OGY 



I413 



Tabi,e IV. — Relation oj wilting^ coefficient to moisture intake by seeds 



Soil types 



Percentage 



of hygroscopic 



moisture 



Percentage 



of wilting 

 coefficient 



Percentage 

 of soil H,0 



Percentc^e 

 of seed intake 



1. Sand (coarse) 0.205 0.73 ^ 0.02 | 0.65 



2. l<oam 3-130 12.93 it 0-05 12.66 



3. Sandy loam (very fine). ^ 1.836 8.33 J:; o.oS 7.86 



4. 1,0am 2.280 12.41 -^ 0.02 13-30 



5. Clay loam | 3.820 16.12 ± o 01 i 16.01 



6. Clay loam ; 5.210 ' 16.34^0.02 I 17.78 



7. Fine sand 0.750 3.21 -^ 0.03 i 3.19 



8. Sand (coarse) 0.218 0.83^0.01 0.80 



9. I<oam . , .1 2.30 I 10.82 i 0.06 1 10.51 



34-44 

 49.02 

 48.38 

 49.01 

 49-49 

 47-31 

 49-77 

 40. 98 

 50.42 



CONCI.USIONS. 



i) The iorce with whk h the seeds of Xanthium pennsylianicum absorb 

 water has been measured by two methods : {a) osmotic solutions, and (6) 

 vapour pressure equilibrium. The osmotic method is at present the more 

 reliable. 



2) The air-dry seeds of Xanthium show an initial attraction for water 

 of nearly i coo atmospheres. 



3) The attraction which exists at any moisture content of the seed 

 between air-dry and saturation can be approximated. See Table I. 



4) The seeds have in turn been used to measure the complex moisture- 

 holding forces of soils, with the following results : 



a) The air-dry subsoil of the Oswego silt loam holds its hygroscopic 

 moisture with about the same force as an air-dry seed, that is, about i 000 

 atmospheres. 



b) As the moisture content of the soil increases, the surface force 

 decreases rapidly. When about 3.5 per cent of water has been added to 

 the air-dry soil, the force remaining is about 395 atmospheres. When the 

 soil moisture reaches 6 per cent above air-dry in this soil, the moisture is 

 held with a force of 130 or more atmospheres. At. 11 per cent above air- 

 dry the holding power has fallen to 22.4 atmospheres. 



c) At the wilting coefficient of the soil (13.3 per cent above air-dry 

 in the Oswego silt loam subsoil) the " back pull" of the soil particles a- 

 mounts to not more than that of a o.i M Na CI solution, that is, not more 

 than about 4 atmospheres. This is shown to hold true for a number of 

 types of soil with widely varying wilting coefficients. 



5) This water-holding power of soils at the wilting coefficient is less 



