Measurement of the Surface Forces in Soils

Abstract

1. The force with which the seeds of Xanthium pennsylvanicum absorb water has been measured by two methods: (a) osmotic solutions, and (b) vapor 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 1000 atmospheres. 3. The attraction which exists at any moisture content of the seed between air-dry and saturation can be approximated. Table III gives the data. 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 1000 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 375 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 amounts to not more than that of a 0.1M. NaCl 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 than the osmotic pressure of the root hairs of many kinds of plants, as shown by Hannig and others. 6. The wilting of plants at the wilting coefficient of the soil cannot be due to lack of moisture in the soil, nor to lack of a gradient of forces tending to move water toward the plant. 7. The view is held, therefore, that the wilting at this critical soil moisture content must be due to the increasing slowness of water movement from soil particle to soil particle, and from these to the root hairs, the rate of movement falling below that necessary to maintain turgidity of the cells of the aerial parts, even under conditions of low transpiration.

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