BENEFITS
OF ZERO TILL AND ROTATIONS IN THE NORTH AMERICAN GREAT PLAINS |
G.
A. Peterson and D.G. Westfall
Colorado State University
Zero till has been an increasingly useful technique over the last 10-15 years for Great Plains farmers. Scientists first began studying chemical fallow", a form of zero till, circa 1960, and most found it to be beneficial in terms of soil erosion control and improved water storage. But for the most part, it was not economically feasible. Herbicidal weed control was too expensive compared to tillage, and wheat yields were not improved enough to warrant its use. The introduction of herbicides such as glyphosate at moderate price levels greatly stimulated the number of acres under zero till. Producers are now benefiting from zero till in increasing numbers. It is quite obvious that they view zero till as economically advantageous or its use would not be spreading. In fact organizations such as this one would not exist if zero till was not considered the wave of the future.
My presentation will cover the overall benefits of zero till as stated in the title, but specifically it will focus on some of the more subtle benefits and hopefully supply reasons why zero till has these effects.
BACKGROUND:
Producers manage systems NOT crops or soils or animals. Thinking of your operation as a system is key to understanding the benefits of zero till and key to increasing the benefits derived from zero till. Farming systems generally are complicated because they have so many components, but for our purposes we will look only at the more obvious components. The most necessary outcome of any farming system is that it must be profitable to the producer. If not profitable in the short-term, the producer will not likely use it, even though it may have potential to become profitable in the long-term. Secondly the sustainability of profit is critical to the producer. Beyond this, the producer is looking for systems that enhance the future land productivity and profit potential.
All systems have feedbacks that are either negative or positive. A "feed back" in a system can be so subtle that it is not noticed in the short-term. If it is negative in terms of system productivity, failure to recognize it or failure to counteract it, will cause the eventual ruin of the system. An example of a negative feedback in a farming system is soil erosion promoted by a particular tillage practice. The tillage practice may have advantages in terms of seedbed preparation and establishment of good stands, but at the same time it may be leaving the soil surface bare and open to erosion forces. In this case failure to counteract the erosion will result in eventual productivity losses to the system, and/or the need to purchase more fertilizer to offset the soil loss. The long-term use of moldboard plowing in wheat-fallow systems is an example of a practice that had a negative "feedback".
Systems also can have positive "feedbacks", and these are what farmers and scientists are in search of. Positive "feedbacks" eventually improve the productivity and potential profitability of the system. Zero till has the possibility of providing several positive "feedbacks", but sometimes these are quite subtle and may take years to express themselves in terms of increased productivity. Now let's consider both the direct benefits of zero till in terms of profitability, and the subtle benefits via positive "feedbacks".
PRODUCTIVITY AND PROFITABILITY:
Zero till wheat-fallow has not been a profitable system in much of the Great Plains, at least in the areas where winter wheat is grown. Dhuyvetter, et al. (1996) summarized the economic outcome of cropping Systems for the Great Plains and concluded that zero till wheat-fallow was not as profitable as reduced till and conventional till systems. For example, return to land, labor, capital, and management in wheat-fallow was reduced 28% by converting to zero till from conventional tillage in northeastern CO (Table 1), and in southeastern CO zero till reduced profit by 36% compared to a reduced tillage system (Table 2) (Peterson, et al., 1993).Table 2. Relative return to land, labor, capital, and management on a 1200 acre farm basis as affected by cropping system and tillage choice during fallow preceding wheat planting in southeast Colorado.
| Table 1. Relative return to land, labor, capital, and management on a 1200 acre farm basis as affected by cropping system and tillage choice during fallow preceding wheat planting in northeast Colorado. | |||
| Tillage Preceding Wheat Planting | WF (1) | Cropping System WCF | WCMF |
| Conventional | 100% | 140% | 127% |
| Reduced Till | 92% | 136% | 120% |
| Zero Till | 72% | 125% | 113% |
| (1) WF = Wheat-fallow; WCF = Wheat-corn-fallow; WCMF = Wheat-corn-millet-fallow:(2) Net income base was $41,768 | |||
| Table 2. Relative return to land, labor, capital, and management on a 1200 acre farm basis as affected by cropping system and tillage choice during fallow preceding wheat planting in southeast Colorado. | |||
| Tillage Preceding Wheat Planting | WF (1) | Cropping System WSF | WSHF |
| Conventional | - | 101% | 82% |
| Reduced Till | 100%(2) | 93% | 76% |
| Zero Till | 64% | 74% | 71% |
| (1) WF = Wheat-fallow; WCF = Wheat-sorghum-fallow; WCMF = Wheat-sorghum-hay-fallow:(2) Net income base was $23,312 | |||
This happened because gains in wheat yields using zero till have been small in relation to the cost of converting from stubble mulch fallow tillage to herbicidal weed control. Halvorson, et al.(1994) reported no significant wheat yield increase with zero till compared to conventional stubble mulch tillage, yet production costs for zero till were about $8.00/A more for zero till than for stubble mulch. Norwood and Dhuyvetter (1993) did report a yield increase of 4 bu/A with zero till compared to conventional tillage, but the cost increased about $11.00/A with zero till.
Since we know that zero till increases soil water storage and simultaneously provides excellent protection against erosion, how can it be made profitable? Colorado data, along with that of many other scientists in the Great Plains, indicates that the key to increased profits with zero till is linked to increased cropping intensity. Zero till saves more water compared to tilled systems and thus permits change to cropping systems with 2 crops in 3 year, and 3 crops in 4 years. Our three-year systems are winter wheat-corn or sorghum-fallow and our four-year systems are winter wheat-corn or sorghum-millet or hay or sunflower. In Colorado annualized grain yields have increased from about 1000 lbs /A/year with wheat-fallow to over 1750 lb/A/year with 3- and 4-year systems (Figure 1). This is a 75% increase over wheat-fallow.Figure 1. Annualized grain yield as affected by cropping system at three Colorado sites (1988-1995).
Expressed on a water use efficiency basis (WUE) the 3- and 4-year systems are producing 30-35% more grain per inch of rainfall than the 2-year winter wheat-fallow. Norwood and Dhuyvetter (1993) reported an increase of 90% in annualized grain yield going from a zero till wheat-fallow system to a wheat-sorghum-fallow system, which represented a 34% increase in WUE. Obviously the zero till in association with intensified cropping is causing some large and profitable increases in grain yield. How does zero till increase WUE?
WATER STORAGE EFFICIENCY:
Zero till greatly enhances the efficiency of water capture, especially when the soils have been depleted of their water by a crop. For example in the Great Plains after wheat harvest the soils usually are totally depleted of plant available water. Zero till in this warm period is especially effective in capturing the rainfall. Furthermore the standing stubble enhances snow capture which further improves water storage in the soil. Soil water contents the following spring are very high compared to tilled systems or compared to situations where no weed control is practiced after wheat harvest. Thirty years ago Smika and Wicks (1968) demonstrated the value of zero till for improving water storage efficiency Table 3). With the moldboard plow tillage, the soil lost available soil water from July harvest until freeze up in November, but with zero till they stored 24% of the rainfall. Over-winter soil water storage was greatest with zero till, which had maximum standing stubble for snow capture.
By April of the following year, the system storage efficiencies for the fallow period were: plow = 16%, reduce till 40%, and zero till = 60%. Actual amounts of available water in each system by 18 April were plow = 2.2 in., reduced till = 5.5 in., and zero till = 8.3 in. If water is stored early in the fallow season, storage becomes less efficient in the latter part of the fallow phase. From 1 July to wheat planting, the plow treatment stored 31% (2 in.) of the precipitation, reduced till stored 11% (0.7 in.), and zero till stored 2% (0.2 in.).
| Table 3. Water storage efficiencies during the summer fallow phase of a winter wheat-fallow system by time periods as influenced by tillage system. (Adapted from Smika and Wicks, 1968) | ||||||||
| Time Period in the Fallow Phase | ||||||||
| Wheat harvest to Freeze 3 Jul. to 15 Nov | Over winter 15 Nov. to 18 Apr. | Early Summer 18 Apr. to 1 Jul. | Late
Summer to Planting 1 July to 15 Sept. | |||||
| System | Precip.(1) Stored
% | Amount Stored In. | Precip.(1)
Stored % | Amount Stored In. | Precip.(1) Stored % | Amount Stored In. | Precip.(1) Stored % | Amount Stored In. |
| Plow | -27 | -2.5 | 80 | 3.5 | 35 | 3.1 | 31 | 2.0 |
| Reduced Till | 12 | 1.2 | 100 | 4.4 | 50 | 4.4 | 11 | 0.7 |
| Zero Till | 24 | 2.3 | 100 | 4.4 | 50 | 4.4 | 2 | 0.2 |
| (1) Percent of precipitation received in this period that was stored in fallow. | ||||||||
Data collected across the Great Plains substantiates the great inefficiency of water storage during the late portion of zero till fallow systems. Peterson et al. (1996) summarized data from North Dakota to Texas demonstrating the inefficiency (Table 4). The early fallow in Texas has a zero till storage efficiency of 18% (3.2 in of water stored), while an additional 90 days of fallow reduced the overall efficiency to 10% (3.0 in of water stored). Obviously the water received in the summer, late fallow, was lost to evaporation. Kansas and Colorado data were similar. In spring wheat-fallow systems in North Dakota, early water storage efficiencies were 56 to 59% (3.6 to 4.9 in. of water stored), but efficiencies were only 25 to 36% after a full 21-month fallow period (4.4 to 4.6 in. of water). The additional 12 months of fallow resulted in a loss of 0.3 in. of water at Minot and a gain of only 0.8 in. at Williston.
| Table 4. Water storage efficiencies and quantities of water stored in the early fallow phase [wheat harvest to spring wheat or corn or sorghum planting] compared to water storage in the total fallow period for zero till wheat-fallow systems [wheat harvest to wheat planting] in the Great Plains. | ||||||||||||
| Location | ||||||||||||
| Bushland, TX(1) | Garden City, KS(2) | Stratton,CO(3) | Sterling, CO (3) | Williston, ND(4) | Minot, ND(4) | |||||||
| Fallow Period | Stor. Eff. % | Water Stored In. | Stor. Eff. % | Water Stored In. | Stor. Eff. % | Water Stored In. | Stor. Eff. % | Water
Stored In. | Stor. Eff. % | Water Stored In. | Stor. Eff. % | Water Stored In. |
| Wheat harvest to spring wheat or corn or sorghum planting in CW or WCF or WSF | 18 | 3.2 | 46 | 6.9 | 50 | 5.4 | 39 | 4.4 | 59 | 3.6 | 56 | 4 .9 |
| Wheat harvest to wheat planting in WF | 10 | 3.0 | 23 | 5.4 | 23 | 6.0 | 20 | 4.6 | 36 | 4.4 | 26 | 4.6 |
| (1) Jones personal community; (2) Norwood (1994); (3) Peterson and Westfall (1994); (4) Deibert, et al. (1986) | ||||||||||||
These data vividly illustrate that with zero till it is most efficient to terminate the fallow and plant a spring crop. Continuing the fallow period saves little water and requires expensive weed control measures. If high cost zero till inputs are made to improve water storage, then that water must be used efficiently to obtain a profit. Planting a spring crop that can utilize both the stored water and the summer rainfall is the key to increased yield and profit. In zero till intensified cropping systems, the summer rainfall is used by the crop instead of being lost to evaporation during the second summer of fallow.
The efficient early fallow is a positive feedback in the zero till system. The removal of tillage during the hot weather after wheat harvest, the peak residue cover present after harvest, and the standing stubble that traps winter snow provide the feedback in terms of improved early water storage. The early water storage makes it possible to grow the spring crops, which in turn improves productivity and profitability.
ORGANIC MATTER CONSIDERATIONS:
Soil organic matter is an important component of soils for its nutrient content of course, but it also greatly affects soil structural stability. During residue decomposition the soil organisms synthesize polymeric compounds like polysaccharides that stabilize and strengthen soil structure. Stable soil structure is critical to maintaining open pores at the soil surface and to diminish soil crusting problems. Stable aggregates allow for stable porosity and maximum water infiltration. Management techniques that foster residue accumulation on the soil surface and that promote slower and more uniform residue decomposition times increase the opportunity for stable structure. Zero till provides an ideal environment for the continuous production of the polymeric stabilizing compounds and simultaneously maximizes soil cover via crop residue for protection of soil aggregates from water and wind.
Tillage Effects -Tillage enhances soil organic matter decomposition; both of the native soil organic matter and of the organic matter formed when crop residues decompose. Tillage has the same effect on soil organic matter as the effect you experience when you give a furnace more air. Oxidation rate is increased. In addition tillage breaks the existing soil structural units into smaller and smaller units. Surface area is increased and organic matter that was hidden from the microorganisms is now available to them for decomposition. It has been documented multiple times that our prairie soils, once rich in organic matter, now often contain less than half the amount present in their native condition. Decades of farming with the wheat-fallow system, the dominant farming system in the Great Plains, have accentuated soil C losses. Haas et al. (1957) reported C losses of over 50% at many Great Plains sites after 30 to 40 years of cultivation in a wheat-fallow system with conventional tillage.
Reduction in soil disturbance from conventional, highly disturbed, tillage methods to reduced- or zero till practices slows C losses, and may even increase the amount of C stored in a soil. Data from Nebraska, Colorado, and Texas all support this conclusion. For example at Sidney NE (Table 5), a soil plowed from sod in 1970 lost over 7140 lb/A in the first 12 years under plowed fallow and then lost an additional 2,140 lb/A in the following 8 years. In this same time frame, soil under zero till management lost about 3570 lb/A in the first 12 years, and then remained stable over the next 8 years. Conversion to zero till conserves soil C and may even allow it to accumulate relative to stubble mulch and plow tillage.
| Table 5. Effects of tillage system on soil C content in the surface 8 inches at various times after sod breaking at Sidney, NE. (See Lyon, et al., 1996). | ||||
| Year | Tillage System | |||
| Native Sod | No-till | Stubble Mulch | Plow | |
| Lb/A | ||||
| 1970 | 36900 | - | - | - |
| 1982(1) | 37300 | 33200 | 32000 | 29500 |
| 1990(2) | 37900 | 33300 | 31200 | 27300 |
| (1) Lamb, et al., 1985; (2) Cambardella and Elliott, 1992. | ||||
Cropping System Effects
The added water conserved in zero till Systems has made it possible to use more intensive cropping systems relative to wheat-fallow. Hypothetically, intensifying cropping Systems relative to wheat-fallow should add more C to soils because more C is photosynthetically fixed in these systems. Coupled with the minimum disturbance of zero till soils, the overall effects of intensification on C storage in soils should be significantly positive.
Black and Tanaka (1996) compared spring wheat-fallow versus continuous cropping (spring wheat-winter wheat-sunflower) under three tillage regimes: zero till; minimum till; and conventional till. Minimum till was a combination of sweep operations and herbicides for weed control. Conventional tillage weed control used sweeps and tandem discing with no herbicidal weed control. With conventional tillage, continuous cropping increased grain yield about 30% compared to alternate crop-fallow (Table 6). With zero till, grain yield in the continuous cropping treatment was increased by more than 45% compared to spring wheat-fallow. Residue production, the major source of C returned to the soil, was increased by 54, 72, and 88% by conversion to continuous cropping compared to alternate crop-fallow for conventional, minimum, and zero till treatments, respectively (Table 6).
| Table 6. Annualized grain and residue yields as affected by cropping system and tillage system at Mandan, North Dakota. (Mean of 8 years of data) (See Black and Tanaka, 1996). | ||||||
| Cropping System | Grain Yield | Residue Yield | ||||
| Conv. Yield | Mulch Till | Zero Till | Conv. Yield | Mulch Till | Zero Till | |
| Lb/A | ||||||
| SW(1)-F(2) | 1035 | 1050 | 1040 | 1630 | 1600 | 1600 |
| SW-WW(3)-Sunflower | 1340 | 1460 | 1545 | 2510 | 2760 | 3010 |
| Change due to Cropping Intensification | +305 | +410 | +505 | +880 | +1160 | +1410 |
| (1) Spring wheat; (2) Fallow; (3) Winter wheat | ||||||
By 1991, under conventional tillage, continuous cropping had 2600 lb/A less soil C than did alternate crop fallow (Table 7). However, under minimum and zero till, continuous cropping had 8270 and 13990 lb/A more soil C than did alternate crop fallow.Table 7. Changes in soil carbon content from 1983 to 1991 as affected by cropping and tillage systems at Mandan, North Dakota. (See Black and Tanaka, 1996)
| Table 7. Changes in soil carbon content from 1983 to 1991 as affected by cropping and tillage systems at Mandan, North Dakota. (See Black and Tanaka, 1996). | ||||
| Depth | 1983 Soil Carbon | Change due to Cropping Intensification by 1991 | ||
| Conv. Till | Mulch Till | Zero Till | ||
| Inches | Lb/A | Lb/A | ||
| 0-3 | 16480 | +1220 | +2290 | +3870 |
| 3-6 | 18260 | -710 | +10 | +3130 |
| 6-12 | 24740 | -3110 | +5970 | +6990 |
| Total | 59480 | -2600 | +8270 | +13990 |
Peterson and Westfall (1996) compared effects of cropping systems on soil C. Their cropping Systems increased in intensity from winter wheat-fallow, to winter wheat-corn or sorghum-fallow, to winter wheat-corn or sorghum-proso millet or sorghum or forage-fallow, to continuous cropping (Opportunity), all with zero tillage. A perennial grass treatment (mixture of cool and warm season types) was included as a reference.
Cropping system intensification increased annualized grain production, compared to WF, by 70% in all climatic areas (Peterson et al., 1993). The 3- and 4-year rotations and opportunity cropping have increased residue production by only 12, 25 and 17%, respectively compared to WF. On a weight basis this is a residue increase, relative to WF, of about 220, 450, and 270 lb/A annually for the 3-year, 4-year and continuous cropping rotations, respectively. For a 10 year period it is a total increase of 2230 to 4460 lb/A of residue, which is only about 980 to 2010 lb/A of actual C. (Residue = 40-45% C) Considering, however, that this amount is being placed into the surface 2 in. of soil, which presently contains about 4460 lb/A, it represents a significant opportunity for C accumulation in that soil layer. Compared to the total of 17850 lb/A of C in the surface 8 in. of soil, this amount is small. Because most of the residue C will be oxidized to CO2 within a few months, measurable changes in total soil C will be slow to occur in any soil layer, and especially at deeper depths. Seeding a soil to perennial grasses increased soil C level by 21% compared to the original 1986 levels (Sherrod et al., 1995) (Table 8).
Increasing cropping intensity under zero till management has slowed C losses, but has not stopped them. All cropping systems, even WF, have maintained or increased the C level of the 0-1 in. soil layer (Table 8). The most intensive systems, WC(S)MF and Opportunity cropping, have maintained C levels even in the I -2 in. soil layer.
| Table 8. Soil C in 1986 and in 1993 as affected by cropping system in eastern Colorado. Data averaged over climate areas and soil positions. (See Sherrod, et al. 1995) | |||||||||
| Depth | Cropping System | ||||||||
| WF | WC(S)F(2) | WC(S)MF(3) | |||||||
| 1986 | 1993 | + | 1986 | 1993 | + | 1986 | 1993 | + | |
| Inches | Lb/A | ||||||||
| 0-1 | 2930 | 2920 | -10 | 2890 | 2870 | -20 | 2830 | 3040 | 210 |
| 1-2 | 2510 | 2270 | -240 | 2630 | 2 340 | -290 | 2450 | 2460 | 10 |
| 2-4 | 5480 | 4370 | -1110 | 5690 | 4 620 | -1070 | 5620 | 4800 | -820 |
| 0-8 | 10920 | 9560 | -1360 | 11210 | 9830 | -1380 | 10900 | 10300 | -600 |
| Opportunity(4) | Grass(5) | ||||||||
| 1986 | 1993 | + | 1986 | 1993 | + | ||||
| Inches | Lb/A | ||||||||
| 0-1 | 2590 | 2960 | 370 | 2700 | 3750 | 1050 | |||
| 1-2 | 2410 | 2320 | -90 | 2450 | 3320 | 870 | |||
| 2-4 | 5380 | 4550 | -830 | 5120 | 5400 | 280 | |||
| 0-8 | 10380 | 9830 | -550 | 10270 | < TD>124702200 | ||||
| (1) W-F = Winter wheat-Fallow: (2) W-C or S-F = Winter wheat-Corn or Sorghum-Fallow; (3)W-C or S-M-F = Winter wheat-Corn or Sorghum-Proso millet or Forage-Fallow; (4) Opportunity = Continuous Cropping; (5) Grass = Mixture of perennial warm and cool season grasses seeded in spring 1986. | |||||||||
Peterson, et al. (1997) chose to do a more complete evaluation of the C budget of each system by considering the quantity of C contained in residues on the soil surface in addition to the soil C. Figure 2 shows the soil C loss for each system from the initiation until 1993 (black bars), and the amounts of residue C on the soil surface in 1993 (gray bars). Residue was considered to be 45% C based on laboratory analyses. Note that the 3-year, 4-year, and opportunity cropping Systems all had residue C levels (gray bars) that exceeded the soil C losses (solid bars). Only WF decreased in soil C more than the amount of residue C. The grass reference treatment increased in soil C from initiation to 1993. No residue C is shown for the grass reference because we harvest the grass annually and there is little residue present. In summary, the soil organic C plus surface residue C indicates that all systems, except WF, are approaching the grass C levels (Figure 2). All systems, other than WF, have increased in C content relative to the 1986 initial levels.Figure 2. Change in soil C after 10 years and residue C present in 1993 as affected by cropping system in eastern Colorado.
It appears that the 3-year and Opportunity cropping are not adding the C to the soil pool as rapidly as is the 4-year system. One reason obviously is that the quantity of C added is smaller relative to that for the 4-year system (Table 8). Another contributing factor may be the relative size of the residue pieces. In the 3-year rotation, 46% of the C returned to the soil is in the form of corn or sorghum stalks, which are relatively large pieces compared to wheat or millet residue. In the 4-year rotation the corn and sorghum represent only 32% of the total C returned, which would allow a more rapid turnover compared to the 3-year rotation. Forty percent of the C returned for opportunity cropping is from corn or sorghum stalks. Differences in residue size, coupled with the quantity, may explain why the Systems differ.
Our conclusion is that on a short-term basis, 10 years or less, the effects of cropping system intensification on soil C should not be investigated independent of residue C still on the surface. How fast the surface C accumulation will be incorporated into the soil organic C pool is not known, but the fact that the 4-year system, the most effective system in terms of C, only has affected the surface 2 in. of soil in a 10 year period means this may be a very long-term proposition.
Campbell, et al. (1996) have shown that the effects of zero till and cropping intensification on organic C changes is dependent on soil texture. They report that zero tillage, in combination with less fallow, significantly increased soil C in a medium-textured soil (Campbell, et., 1996a), but gains in soil C were much less in a coarse-textured soil (Campbell, 1996b). The difference appeared to be linked to smaller production of crop residues and greater susceptibility to erosion at the coarse-textured site in contrast to the medium-textured site.
Overall the combination of zero till and cropping intensification certainly results in increased C in the surface soil and on the soil surface. This is ideal in terms of strengthening and protecting soil aggregates. This represents a positive "feedback" to the system because larger and stronger surface soil aggregates promote water infiltration. Enhanced infiltration increases the opportunity for soil water Storage, which can result in improved crop yields and increased profit.
SUMMARY:
Zero till, because it allows maximum water conservation in a given environment, allows cropping intensification with less summer fallow time. This results in increased productivity, increased profit, increased water use efficiency by plants, increased soil organic carbon in the surface soil, and much less erosion potential. The combination of zero till and intensified cropping provides several positive "feedbacks" to the system, which means that the long-termbenefits of zero tilling should be large. Consider the following hypothesis and discussion as you view the future of zero till for your own farm:
Hypothesis
Zero tilling, coupled with intensified crop rotations, is a movement toward an agroecosystem that mimics the Great Plains prairie ecosystem before cultivation began.
Observations
Consider the plant-animal-soil ecosystem in the Great Plains prior to cultivation. The plant community consisted of mixed prairie grasses growing on fertile soils formed in glacial, wind blown, and residual parent materials. These soils had relatively high organic C contents at the surface because of the grass vegetation and tended to be neutral to alkaline in pH. The animal community ranged from the bison to the prairie dog to the thirteen striped ground squirrel. Climate type was continental, as it is today, with cold, relatively dry winter periods and warm to hot summers with most of the annual precipitation occurring in the summer period.
The soils rarely had much stored water because when precipitation occurred there was a warm or cool season grass plant ready to use it. Soils had the most stored water in the spring after snow melt and before cool season plants had an opportunity to use the water. Once plants broke dormancy, water was used about as fast as it was received. Since 80% of the precipitation occurs within the growing season (March - September), this means the native plants used the water efficiently and kept the soil surfaces dry. Little water escaped as runoff or evaporation. Most water was used as the transpiration component of total evapo-transpiration. Furthermore at no time was the soil surface bare, or was the soil stirred to any great extent. Maximum disturbance would be animal hoof action during intense grazing events, and even this disturbance was not annual for most landscapes.
Contrast the above situation with the wheat-fallow system, which has been the most commonly used farming practice in the Great Plains. We purposely keep plants from growing in an attempt to store water for subsequent crops for periods ranging from 12 to 21 months. In other words, 50-85% of the time there are no plants on the land. No carbon additions are made in the form of roots or plant litter, and the soil is vigorously tilled to control weeds. The tillage has broken open soil aggregates that have been forming over decades to centuries, and has resulted in smaller and weaker aggregates. During fallow we greatly increase the chance for runoff of water, and greatly increase the evaporation component of total evapo-transpiration. Near the end of a fallow period the soil profile is charged with water and surfaces are not dry; thus runoff and evaporation are enhanced. This because the moister soil surfaces have less negative matric potential, and water remains on the surface longer. With larger rain events this means more runoff and in all cases it means extended periods of evaporation because the water remains in contact with the air for longer times. Zero till fallow is not helpful in this case because it stores more efficiently early compared to tilled fallow and thus lengthens the period of time at the end of fallow where water storage is very inefficient.
Based on these observations of the native systems, and the contrast with the wheat-fallow systems, we can now test our hypothesis that zero tilling, coupled with intensified crop rotations, is a movement toward an agroecosystem that mimics the Great Plains prairie ecosystem. First with zero till we minimize soil stirring and mimic the situation where root channels remain intact in the soil and plant litter remains on top of the soil. The soil is covered even when no active plant growth is occurring. The exception to this is in cases where wide row spacings allow bare ground between rows compared to a prairie situation where plants are not in rows. Remember, however, that in some prairie bunch grass communities there also is bare soil between bunches much like our row spacings.
Second with the intensified rotations we are now growing more warm season plants whose water use patterns coincide with our summer rainfall pattern. This means soil matric potentials are low (more negative) because the plants are using water as it is received, and canopy and residue are available to intercept raindrop impact and reduce runoff. We are now maximizing the transpiration component of evapo-transpiration and minimizing the evaporation component. Rotations that still contain fallow, like our wheat-corn-fallow, are only a first step toward the prairie system. Systems that use more summer crops, whose water use patterns coincide with precipitation received, become more and more like the prairie system. Our opportunity cropping, which is an attempt at continuous cropping, is like the prairie in water use pattern and minimal soil disturbance. However, even in that system there are times when no crop is growing and rainfall is occurring. For example, when wheat is harvested in mid to late summer and we intend to plant corn the following spring, there is a period from mid July -September when rainfall can be substantial and no plants are present. Fortunately the matric potential of the soil is very negative at this point because the wheat has exhausted most of the soil water by maturity. This means that the stubble covered zero till situation can capture much of the precipitation. Our data show storage efficiencies of 50-60% in this period.
So when planning zero till rotation systems, the goals are to have plants growing when most of the rainfall occurs, and when that is not possible, to at least have the soil as dry as possible going into the non-crop period to maximize the infiltration rate and minimize runoff and evaporation. It does appear that zero till, in conjunction with cropping intensification, mimics the prairie system.
REFERENCES
Black, A.L. and D.L. Tanaka. 1996. A conservation tillage-cropping systems study in the northern Great Plains of the USA. In: Paul. E. and c.v. Cole (eds.) Soil Organic Matter in Temperate Agroecosystcms. CRC Press, Inc. Boca Raton, FL.
Cambardella, C.A. and E.T. Elliott. 1992. Particulate soil organic matter changes across a grassland cultivation sequence. Soil Sci. Soc. Am. 1. 56:777-783.
Campbell, C.A., B.G. McConkey, R.P. zentner, F.B. Dyck, F. Selles. and D. Curtin. 1996a. Carbon sequestration in a Brown Chernozem as affected by tillage and rotation. Can. 1. Soil Sci. 75:449-458.
Campbell, C.A., B.G. MeConkey. R.P. zeutner, F.B. Dyck, F. Selles. and D. Curtin. l996b. Tillage and crop rotation effects on soil organic C and N in a coarse-textured Typic Haploboroll in southwestern Saskatchewan. Soil and Tillage Res. 37:3-14.
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