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Capillary Intake Rate of Water and Soil Structure¹

ABSTRACT

Rate of capillary absorption of water in soils was measured and compared with some other physical measurements as a possible index of structure. Disturbed and undisturbed samples were used. The disturbed samples were treated in two ways. Part were incubated in the presence of moisture and nutrient solution; part were mixed with a synthetic conditioner. After the disturbed samples were treated, they were air-dried and screened to the size range 0.25–2.00 mm. Only this range of aggregate sizes was used in subsequent determinations. The undisturbed samples were cubes, I inch on an edge, sawed from larger, air-dry clods, taken originally from moist, but not wet soil profiles, some in good structure, some in poor.

The structure measurements, which were used for the disturbed samples to adjudge the value of the capillary absorption rate as a structure index, were hydraulic conductivity and water stability of aggregates. For the undisturbed samples the comparison measurements were capillary porosity and aeration porosity.

For those disturbed samples which were incubated, and for those undisturbed samples which were taken in the A soil horizon, no significantly consistent relationships were found between the capillary absorption rate and the comparison measurements. For those disturbed samples treated with the synthetic soil conditioner, and for those cube samples taken below the A-horizon, there were found significant or highly significant correlations with the comparison measurements. The action of organic matter on the wetting angle of the soil is believed to have caused the non-significant results for the incubated samples and for the samples from the A-horizon.

It is concluded that for soils containing variable amounts and kinds of organic matter, the capillary intake rate is not a reliable index of structure; but that for soils containing constant amounts and kinds of organic matter the capillary intake rate should be satisfactory.

An idealized capillary tube model of soil is presented as an aid in interpreting results. The model shows that with a constant wetting angle, the capillary absorption rate should be an index of structure. But the same model shows that hydraulic conductivity should be a better index than the capillary absorption rate. The last statement is in agreement with the experiments.

The existence of tables is reported for determining other statistical levels than the usual 1% or 5%. These tables are utilized to calculate specific levels for some of the data.

¹ Journal Paper No. J-2326 of the Iowa Agr. Exp. Sta., Ames, Iowa. Project 739. Presented before Section I of the Soil Science Society of America, Cincinnati, Ohio, Nov. 20, 1952.

² Graduate assistant, graduate student and Professor. The second author, who was on leave from the Soils staff of the Agricultural College, Ghent, Belgium, when his contribution to this paper was made, expresses thanks to the Belgian Institute I.r.s.i.a. and to the United States government, through the Fulbright Act, for financial aid.

Received for publication June 11, 1953.