Soil Water Storage and Use
A.L. Black, USDA – ARS
Northern Great Plains Research Laboratory
Mandan, NB
Much too often, producers plant a crop without knowing how much water is in the soil, how much is available and to what depth is soil water present. Even when producers fallow for 21 months in a spring grain-fallow rotation, the assumption is made that complete soil water recharge of the soil profile has occurred whether it did or not. The fact is, a producer will now know whether specific weed control or crop residue management decisions are increasing water conservation and economic returns unless they know how much soil water is being stored during non crop periods in annual or crop-fallow cropping systems.
Producers need to sharpen their soil water conservation and seed zone soil moisture management skills. All to often producers are reminded just how unreliable mother nature is in providing precipitation at the time it is needed for crop establishment and optimum growth. Producers can only rely on their own management skills to provide adequate soil water storage and seed-zone soil moisture to reduce weather related risk factors.
Fall-Over winter Soil Water Storage:
Fall weed control and crop residue management to conserve and hold precipitation in place (particularly snow) are the principle factors involved in water conservation and storage in the soil. Soil water storage actually begins when the previous crop nears maturity and ceases to use soil water. The crop may stop using soil water, but a weedy crop may continue to use soil water through and after harvest because of the weeds present. Therefore, good weed control in the previous crop helps to conserve precipitation that falls near crop harvest time. Some producers think that fall precipitation is so low that there isn’t any use to try to conserve it as soil water. Yet, these same producers have weeds and volunteer grain flourishing through September or October and using soil water that should have been saved for the next crop.
Aside from obtaining weed control in the previous crop, there are two other management decisions that effect potential soil water storage that must be made at time of harvest, and following harvest, particularly with cereal crops. First, the cutting height of the swathing or combine platform must be adjusted for the stubble height most effective for snow trapping. The combine should also be equipped with straw and chaff spreaders that spread the crop residue over the platform width from which it originated. Second the choice of methods of controlling weeds and volunteer grain must be made and the decision implemented. A dense population of volunteer grain can remove 1.5 to 3.0 inches of soil water from mid-August to soil freeze-up time.
Total snow fall in the northern Great Plains averages 30 to 36 inches and comprises 20 to 30% of the annual precipitation received. However, fall plus over winter soil water gains account for 60 to 100% of the total water stored during 14 or 21 months of fallow.
Researchers of the northern Great Plains of the United States and Canada have documented the additional soil water conservation aspects of standing stubble compared to use of all tillage systems that flatten or bury the stubble. Over 45 location-year studies shown in Table 1, the average over winter soil water gain was 1.54 inches greater for upright stubble (10-17 inches tall) than when the stubble was flattened or incorporated by tillage.
Bauer and Black (1991) have shown that the grain yield per inch of water used in evapotranspiration after the initial yield point is reached, averaged 5.1 bushels per acre per inch of water for wheat. In a 12 year study, Bauer and Black (1991) showed that short (2 inch), medium (8 to 10 inch) and tall (13 to 15 inch) stubble conserved 1.2, 1.8 and 2.8 inches more soil water than bare fallow which should provide wheat yield increases of 6.1, 9.2 and 14.3 bu/acre over the yield level of a no surface residue tillage system.
Summer Fallow:
Crop residue management during summer fallow to conserve water already stored and to gain additional soil water and control soil erosion requires an estimate of how much crop residue is present in the spring. If residue production from the previous crop was less than 2500 lbs/ac, a producer should consider using only a chemical fallow program. If crop residue production was in the 2500 to 4000 lb/ac range, then a producer should consider a combination of minimum tillage and herbicide sprayings.
Three criteria must be met before an increase in soil water storage during the summer period of fallow can occur as a result of residue management or tillage. First, the soil must not already be at or near the water storage capacity of the soil root-zone after the first fall-over wintered period. Secondly, precipitation must occur in sufficient quantity and frequency to effect soil water movement deep enough into the soil (about 4 to 5 inches) to limit its availability to evaporative heat forces at the soil surface. Third, the quantity of crop residue available to suppress evaporation rates must be greater than 2500 lbs/ac (60 to 75% cover).
Second Over winter Fallow Period:
In a spring wheat-summer fallow cropping system, the second winter of the 21 month fallow period is of major concern from the stand point of maintaining sufficient residue cover to protect the soil from wind erosion. In this respect, minimum-and no-till fallow methods have the potential to reduce relative soil wind erodibility about 10-fold (Black and Power, 1965).
The amount of precipitation stored in the soil during the second winter of fallow averages about 10% of the total quantity stored during 21 months of fallow. Researchers have shown that soil water storage ranges from a net loss of 0.8 inches to a net gain of 1.2 inches after severe drought years. Theses results are readily understood if one considers that the soil water stored is near the maximum storage capacity of the soil profile during this period which provides little or no opportunity to store additional soil water. In addition, soils freeze during this period and fallowed soils frozen at a high water content in the upper two feet virtually prevents infiltration of snowmelt or rain while in the frozen state. This is unlike the first over winter period when standing stubble fields are usually dry in the upper 2 foot profile and snowmelt or rainfall can infiltrate frozen soils in the dry condition.
Soil Water and Cropping Strategies:
If the available soil water supply has already been replenished in the root-zone depth of water removal of the previous crop during the fall and over winter period, summer fallowing is not a viable option. Summer fallowing under such conditions is the major contributing factor to the development and expansion of saline seeps (Black et al.,, 1981). Summer fallow should only be used when soil water levels in the soil profile are low enough to arrant its use. A flexible cropping system should be employed based on depth of moist soil whenever there is sufficient available soil water in the profile to justify planting a crop.
Improving crop production per unit of cropland through improved water conservation strategies and precipitation use efficiencies, requires a thorough evaluation of the necessity of summer fallow in existing or flexible cropping systems. The arbitrary practice of summer fallowing a given field ever other year restricts a farmer to a fixed cropping system with limited flexibility for adjusting cropping patterns to fit available soil water supplies. This involves the use of short or long season, and shallow or deep-rooted, crops as needed for different fields. The selection of alternate cropping strategies must be based on a knowledge of the total supply of water available in the soil profile at any given time, the specific water requirements and rooting depth of various crops as needed for different fields. The selection of alternate cropping strategies must be based on a knowledge of the total supply of water available in the soil profile at any given time, the specific water requirements and rooting depth of various crops adaptable to the area and expected growing season precipitation.
Cropping decisions should be based on available soil water present at time of seeding and expected precipitation during the growing season. Available soil water can be estimated by knowing the depth of moist soil plus having a general knowledge of soil texture. Coarse textured sandy soils hold only about one-inch of available water per foot of soil profile; medium textured loam soils, about 2.0 to 2.2 inches; and fine textured silty-clay or clay soils, about 1.8 inches.
To reduce risks of obtaining uneconomical yield levels of a given crop, producers should use fallow only when stored soil water levels are less than three inches (18 inches of moist soil depth) or when the sum of the stored soil water plus average growing season precipitation would not be sufficient to produce an economical yield. As an example, if a producer has a moist soil depth to 36 inches or greater (6 or more inches available soil water) than en economical yield of deep rooted crops (sunflower, safflower or winter wheat) and all shallow rooted crops (spring wheat, barley, oats, canola) is assured with average growing season precipitation. However if the moist soil depth is 18 to 36 inches (3 to 6 inches available soil water) then only shallow rooted crops should probably be grown. Recropping with less than 18 inches of moist soil (3 inches available soil water) is not recommended even if shallow rooted crops are used in most of the Northern Great Plains unless growing season precipitation is 10 inches, or more.
Experience and research has shown that a moist-soil depth of 3 feet, providing about 6 inches of available water, greatly reduces the risk factor in making cropping decisions and obtaining economical yield levels. Crop failures (less than 500 lb/ac) have occurred on the Area IV SCD-USDA, ARS Research Farm at Mandan, ND in annual cropping with sunflower (1990, 1991) when no subsoil moisture was present in the 18 to 36 inch zone and on the spring wheat (1988, 1989) when no subsoil moisture was present in the 12 to 36 inch zone. Spring barley has been planted after sunflower, in a winter wheat-sunflower-barley rotation on the Research Farm since 1984, and grain yields have averaged about 40 bu/ac through 1992 with one crop failure of zero yields in 1988. Winter wheat seeded in annual cropping systems fallowing spring wheat on the Research Farm has been very successful using no-till or direct seed methods 9 our of 10 years and averaged 36.5 bu/ac.
In 1988, when winter wheat yields in no-till annual cropping was only 6.0 bu/ac, spring wheat grown on fallow that same year only yielded 8.0 bu/ac. Conventional disk-seed winter wheat was not successful in 3 out of 10 years (winterkill in 1985 and 1989) and averaged 23.8 bu/ac, 12.7 bu/ac less than no-till (Table 3).
The guidelines for making the decision to seed winter wheat or not are based on having adequate soil moisture in the 0-1 foot soil depth by about September 20 or obtain good seedling emergence and plan populations. The critical factor is crop residue management. A stubble height of 8 to 12 inches is needed to trap snow and protect the winter wheat plants from winterkill and to utilize snow as a soil water resource to recharge subsoil moisture. If a producer plans ahead for winter wheat after spring wheat, or spring barley and seeds the spring cereal crops as early as possible, harvest the crop as soon as possible, and controls weeds from harvest to seeding of winter wheat, then sufficient soil water should be present most years to establish a winter wheat crop. By following these concepts and guidelines, we have not failed to establish winter wheat in any year since 1983, even though fall precipitation has been below normal in about half of those years.
Achieving Water Use-Efficient Crop Production Systems:
Since minimum and no-till residue management systems enhance soil water conservation, grain and straw yields are improved and the additional crop residue produced will sustain a higher potential to store more soil water and protect the soil from erosion than conventional till systems. When soil water conservation is enhanced, crop yields and crop residue production are enhanced, thus making the system somewhat self-perpetuating as adequate levels of crop residue can then be sustained to enhance soil and water conservation.
To develop water-use efficient cropping systems, a producer should use a decision making logic that progresses in order through the following eight steps:
Reference Cited