Improving Water Infiltration in Frozen Soil

Chapter 3 – Residue Management, No. 13, Summer 1989

Don Wysocki

Most major runoff and erosion events in the dryland farming region of the Pacific Northwest occur over frozen soil. In a frozen condition soil can be very impermeable, consequently water added to the surface begins to run off as soon as surface storage is exceeded. In addition thawed soil above a frozen layer is typically saturated and easily eroded. To alleviate this situation it would be beneficial to increase surface storage and/or provide avenues for water to enter soil through a frozen layer. Of various options available, tillage is the most practical method of providing such avenues.

Ideally, tillage should open numerous large cracks into the soil and increase surface roughness. Large cracks will drain off water and remain ice free even when adjacent soil is frozen and provide channels for water to enter the soil. Collectively the interconnected network of cracks and large voids are called macropores. One of the objectives of STEEP research is to study soil conditions created by tillage and to develop techniques and strategies for creating, maintaining and enhancing conditions that are effective in erosion control. In recent work at the Columbia Plateau Conservation Research Center near Pendleton, Oregon, Soil Scientist, Joseph Pikul Jr. studied water infiltration into soil fractured by a tillage shank.

Chisel Groove Study

A experimental site (Table 1) 103 by 123 feet was prepared in June by mowing and removing clippings to leave only standing stubble. A single chisel was pulled through the site at 1 mile/hr on 20-ft spacings at an 8-inch depth. Water content in the upper 6 inches was 20 percent by weight. Infiltration rates were measured on areas of soil bisected by a tillage groove and on equal areas of undisturbed soil in July 1988, December 1988 and February 1989. Frost depth was 5 inches in December and 14 inches in February. A rainfall simulator was used to apply water at a constant rate typical of winter storms.

Table 1. Cultural and soil condition on the experimental site.

Cumulative average infiltration for the July, December and February measurements are shown respectively in Figs. 1, 2 and 3. Fig. 1 shows that infiltration was similar for chisel and unchiseled treatments when soil was not frozen. Chiseling did not improve infiltration because the unchiseled soil could accept water at a rate equivalent or greater than the rate of application. In other words all water being applied was entering the soil even on the unchiseled soil. When the soil was frozen to a depth of 5 inches the infiltration rates were contrasting (Fig. 2). In this situation the frost layer acted as a barrier to water movement into the unchiseled soil and this treatment had an infiltration rate of nearly zero. The infiltration rate of the chisel soil remained the same as had been measured in the unfrozen condition. The chisel groove provided a channel for water to enter the soil below the frozen layer.When soil was frozen to a depth of 14 inches the infiltration rates of both treatments were zero. The chisel fracture zone was less than the frost depth and water could not enter the soil.

Conclusions

The results of Pikul’s work are logical and straight forward. Let’s apply this information to tillage options for runoff and erosion control. Two obvious conditions must be satisfied for chisel tillage to reduce runoff and erosion on frozen ground: (1) chisel marks must be open to the surface, and (2) chisel fracturing should penetrate below the frost depth. The first of these conditions can be satisfied if chisel marks am not closed by soil slumping or subsequent tillage. Unfortunately frost depth cannot be anticipated or controlled. Therefore the second condition cannot always be guaranteed.

Strategies for fall tillage to control erosion should include: tilling on the contour, leaving tillage grooves open to the surface and tilling as deeply as feasibly possible. No recommendations on shank spacing have been developed, but common sense indicates pulling fewer shanks that penetrate more deeply would be preferred. Commonly in a wheat-fallow rotation, chiseling is performed the fall following harvest. Chisel grooves remain open the first winter but fallow tillage operations close them. Recently, creating chisel marks and/or tillage reservoirs in fall seeded cereals after or during seeding has gained interest. This has the advantage of creating avenues for water infiltration in fields that typically have the severest erosion.