Loss of Seed Zone Water Before Fall Seeding

Chapter 3 – Residue Management, No. 8, Spring 1988

Don Wysocki

Adequate seed zone water is critical to the germination and emergence of fall seeded cereals. In the dryland areas of the Pacific Northwest, most producers use a summer fallow tillage system to conserve water and maintain seed zone water for fall-seeded grains.

Generally summer fallow tillage consists of plowing, disking or chiseling, one to two secondary tillages such as spring toothing and rod weeding three to five times during the summer. This tillage system, culminating with rod weeding, creates a dry, low bulk density surface layer over a firmed moist soil. Assuming weeds are not a problem, evaporation is the primary mechanism for water loss from the soil beneath the mulch. The rate of vapor movement through the mulch layer depends upon both soil and environmental conditions.

Factors such as density and thickness of the mulch layer, soil temperature, air temperature, wind velocity, relative humidity, solar radiation and residue cover can influence the rate of vapor movement. Hot and dry conditions can frequently dry the soil below seeding depth, forcing producers to seed in dry soil or wait until fall rains.

A situation that is sometimes observed by producers is an apparent accelerated loss of seed zone water in late summer and early fall before seeding. Seed zone moisture seems to be adequate into August and then, over a short period, moisture is depleted from the seed zone. Reports such as this prompted Joseph Pikul, USDA-ARS soil scientist at Pendleton, to investigate this phenomenon.

Theory of Late-Season Seed Zone Water Loss

To explain accelerated late-season water loss from the seed zone, we must use a mechanism of loss. Pikul proposed that late-season water loss might be related to the changes in heating and cooling of the soil surface that take place during the late summer fallow period. During the summer the soil continues to gain heat until mid or late August, In mid to late August the heat balance shifts and soil begins to lose heat as fall cooling takes place. The soil warms and cools in both seasonal and daily cycles.

In late August the soil begins to cool from a seasonal maximum, but this heat loss takes place through daily variations of heating and cooling. As cooling takes place at night, heat is lost from the soil. During the day the soil may regain some heat but the net effect is a gradual cooling until the process reverses some time in spring. The situation that exists most nights during the late season and fall is cool dry soil at the surface and warm moist soil below. This condition should favor vapor movement upward. As an analogy, think of a cold beverage can placed in a warm room. Water vapor will move to the can and condense on its surface. With this concept in mind, Pikul conducted two field studies on late season water loss, near Heppner, OR.

Field Studies

These studies were conducted from August 1 to November 1 on summer fallowed Ritzville soil series, in a 10-inch rainfall area. Fields were prepared by chiseling 3,000 to 4,000 pounds/acre standing wheat stubble early in the spring and rod weeding once each in June and July. Daily measurements were taken at selected intervals and depths in the profile of soil temperature and soil water content, potential evaporation and rainfall. Comparing water content at various depths and times reveals the gain or loss of soil water or its movement in the profile. A loss of soil water without movement deeper into the profile indicates loss by evaporation. Potential evaporation is a measure of the theoretical amount of water that could be lost if there was an unlimited supply and no barrier to evaporation. It is directly related to the amount of heat a soil receives.

The results of these investigations can be summarized as follows:

1. Approximately 40 percent decline in daily potential evaporation was observed from the beginning to the end of this study (August 1 to November 1) (Table 1). This decline results from shortening of daylength and decline in air temperature as the season progresses.
2. The rate of daily soil water evaporation did not decline with daily potential evaporation. The study found that the soil lost about the same amount of water by evaporation in October as it did in August (Table 1).
3. Soil temperature and daily warming and cooling of the soil was characterized. Selected soil temperature data are shown in Table 2. The variation between maximum and minimum daily temperature is greatest at the surface and diminishes with depth. Damping depth is the point where this variation is about one-third of that at the surface. Damping depth is influenced by both the water content and bulk density of the soil. Daily heat flow at several depths in the soil is illustrated by four dates in Fig. 1. The units for heat flow (flux) have been omitted from this figure for simplicity. Suffice to say that the soil is warming when the curve is above the zero line, and it is cooling when the curve is less than zero. If the area of the curve above the zero line is greater than the area of the curve below the zero line, the soil has a net gain in heat for the day (it is warmer than the previous day). If the reverse is true, the soil has had a net heat loss for the day (it is cooler than the previous day).
4. Precipitation during the study period compacted and dampened the mulch layer and prevented a direct comparison of evaporation and soil heat characteristics for early and late season. Consequently, no clear relationship between heat flow and water loss could be described.
5. Rain that fell during this study did not contribute to stored water. The small amount of precipitation that fell dampened only the surface few centimeters and was lost by evaporation.

Table 1. Rainfall, change In soil water content, evaporative loss and potential evaporation averaged1 for various times in a summer fallow soil (Pikul, USDA-ARS, Pendleton).

Table 2. Selected daily soil temperature and water data in a summer fallow soil at various dates (Pikul, USDA-ARS, Pendleton).

Interpretations

From a producer’s perspective, an important point of this study is that evaporative water loss from the seed zone of summer fallowed fields is similar from August through October even though potential evaporation declines. In other words, even though temperatures become cooler and the number of daylight hours less (i.e. there is less energy available for evaporation), the amount of water lost by evaporation remains about the same in October as it is in August. The reason for this appears to be a combination of the daily and seasonal soil cooling that occurs during this season and a decrease in the effectiveness of the mulch layer brought about by fall rains. This implies that seed zone moisture should be monitored carefully in September and October. If deterioration of seed zone moisture is observed or evaporative water loss is imminent, appropriate adjustments can be taken, such as seeding slightly earlier, seeding deeper or with deep furrow drills or delaying seeding until rains again wet the seed zone.