LAND IMPRINTING SPECIFICATIONS FOR REDUCING STORMWATER RUNOFF AND TOTAL MAXIMUM DAILY LOADS (TMDL'S) FROM WATERSHEDS
AWRA 2003 Conference
November 3-6, 2003
San Diego, CA
Robert M. Dixon, PhD and Ann B. Carr, MS
On the following pages the Imprinting Revegetation System (IRS) is contrasted with an older method relative to:
1) Soil Action
2) Mulching Action
3) Furrow Stability
4) Seed Placement
5) Seed-Soil Contact
6) Seedbed Firming
8) Infiltration Control
9) Erosion Control
10) Rainwater Conservation
11) Seedbed Formation
12) Seedling Cradle Formation
13) Brushy Land
14) Weedy Land
15) Rocky Land
16) Steep Land
18) Seedbed Functions
19) Seedling Cradle Functions r
20) Clayey Land
21) Sandy Land
22) Soil Crusting
23) Organic Matter
24) Soil Structure
25) Soil Compaction
27) Seeds and Seeder
The Imprinting Revegetation System was developed to help reverse global land desertification by providing a better revegetation system for sustainable agriculture and ecological restoration of degraded land. Historically, agricultural crops have been annuals (wheat, corn, soybeans, etc.) which like all early seral species respond favorably to excessive tillage. This is because excessive tillage oxygenates the soil, mineralized the organic matter, and limits the rate and depth of rainwater infiltration, all of which favor the rapid growth of short-lived annuals. However plant materials are buried leaving the barren soil surface vulnerable to raindrop impact, smoothing, and sealing, thereby causing low infiltration rates and accelerated runoff and erosion. Thus, the topsoil is mined for short-term profit, but this agricultural practice is not sustainable in the long run. Excessive tillage, as accomplished with heavy farm equipment, is a type of soil slash and slow burn, not unlike the slash and burn practiced by third world farmers the world over. Excessive tillage and the resultant accelerated mineralization of plant materials is one of the two major causes of global desertification with the other one being overgrazing.
Overgrazing, like excessive tillage, denudes the land, thereby causing soil surface smoothing and sealing with resultant decreased infiltration and increased runoff and erosion. Again overgrazing is a non-sustainable practice, which mines the aboveground plant communities and kills the soil ecosystem that depends on plant life for food. The death of the soil ecosystem destabilizes the soil making it even more vulnerable to accelerated wind and water erosion. While excessive tillage is limited to arable (tillable) land, overgrazing has occurred on much of the remaining land surface of the earth at one time or another - - anyplace the hooves of domestic livestock can reach even the steep canyon walls of mountainous areas.
Recognizing that excessive tillage degrades cropland, cultural practices evolved toward less and less tillage, especially after 1960. First, there was minimum tillage, then reduced tillage and now no-till methods for seeding and planting of which IRS is one. No-till methods do not involve any tillage prior to seeding, however some no-till methods may involve considerable tillage at the time of seeding and the use of herbicides or chemical tillage for weed control.
Imprinting or imprintation is the making of an impression or indentation through downward acting forces. Imprinter types include hand, roller, crawler-tractor tracks, wheel tracks, shoe/footprints, and hoofprints. Generally, the comparisons on the following pages will pertain to roller imprinters having standard specifications.
The Imprinting Revegetation System has three principal components: (1) Seeding Imprinter, (2) Seed Mix, and (3) Rainfall or Precipitation.
The foregoing list of 28 items relating to revegetation of degraded land can serve to detail the design features of land imprinters. These features are thought to be consistent with the needs of sustainable agriculture and ecological restoration.
1. Soil Action: Soil is disturbed through downward acting forces just enough to make imprints but without soil inversion, killing of existing plants and covering of plant material. Surface microroughness and macroporosity are restored to control infiltration rates and volumes at much higher levels.
2. Mulching Action: Teeth chew/chop aboveground plant material into a mulch while partially imbedding it in the imprinted surface. Most shrubs resprout from their crowns after having their tops mulched. Perennial grasses are injured very little by the imprinting process.
3. Furrow Stability: Imprinted furrows, about one per square foot, are staggered with respect to each other and have a 2-inch dam around their outer perimeters for safe storage of ponded water. They are also mulched for added stability where aboveground plant material is available. The soil firming action of imprinting adds to furrow stability. Imprint life is about 5 years.
4. Seed Placement: Seeds are dropped on top of the imprinting roller where they are carried forward, dropped on the ground just ahead of the roller and then imbedded in the surface of the imprints. Each subsequent rainfall moves seed from imprint ridges to imprint furrows along with rainwater, plant litter and topsoil where these resources can work in concert to germinate seeds and establish seedlings. The firmed, smooth-walled imprints make for efficient resources funneling. Imprint stability helps to prevent excessive covering of the seeds and greatly extends the time-window of opportunity for successful establishment of the species of complex seed mixes commonly used in ecological restoration projects. The mix commonly includes both cool and warm season germinators, thus the need for seedbed stability. Also in arid regions where rains are infrequent, imprints and their imbedded store of seeds may have to wait several years before enough rain occurs for vegetation establishment.
5. Seed-Soil Contact: The heavy imprinting roller presses seed in good contact with the faces of the imprint while partially imbedding some of them. The seed-soil contact is excellent in untilled soils that are initially firm to hard. The soil firming inherent in the imprinting process reduces pore size, thereby improving upward capillary flow of soil moisture to seeds following rainfall. Seedbed firming is especially needed in sandy and cloddy clayey soils to improve capillary wicking action.
6. Seedbed Firming: The heavy roller firms the imprinted seedbed while impressing seed into the soil surface. This firmness provides imprint stability under the forces of wind, water and gravity; makes the resource funneling function of imprints more efficient, improves seed-soil contact, and enhances capillary moisture flow upward to the seeds. No deleterious effects of excessive firming have been observed after more than 20 years of imprinting. Design standards call for a maximum of 1000 pounds of static imprinting force per foot of roller length with half of that amount being add-on weight.
7. Interseeding: Because of minimal soil and vegetation disturbance, imprinting rollers are excellent for interseeding missing components into overgrazed rangeland such as perennial grasses. Ranchers have interseeded grasses into some 50,000 acres of degraded rangeland in southern Arizona since 1980. Existing vegetation may be initially stunted by imprinting, but ultimately thrives because of greatly increased infiltration and soil moisture. The initial stunting of imprinted vegetation allows the seeded grasses enough time to become established before regrowth becomes excessive. Even the cryptogamic crusts are not destroyed by imprinting, but are merely folded into the V-shape of imprints.
8. Infiltration Control: The design of the imprinting roller is based on infiltration research conducted at a number of geographic locations under a variety of conditions. This research showed that manipulation of soil surface microroughness and macroporosity provides an order-of-magnitude control over rainwater infiltration. Thus rolling imprinters are designed to convert the smooth sealed surface of degraded or desertified land to the rough open condition that will infiltrate rainwater rapidly, thereby accelerating revegetation processes and the reversal of land desertification. Not only are rates of infiltration controlled but also the location. The V-shaped imprints literally funnel rainwater into the seedbed and seedling cradle where it is needed for seed germination and seedling establishment. The inherent stability of imprints allows them to control infiltration until vegetation becomes established, at which time the vegetation itself maintains the rough, open soil surface and high infiltration rates.
9. Erosion Control: Imprinting provides excellent control over soil erosion through infiltration control and revegetation. By increasing infiltration rates to levels higher than most rainfall rates, runoff and erosion ceases to be a problem. Revegetation, accelerated by imprinting, is the permanent solution to infiltration and erosion control.
10. Rainwater Conservation: Imprints conserve rainwater by reducing surface runoff, by increasing infiltration and deep moisture penetration, and by suppressing evaporation. The deep penetration of rainwater makes it less available for surface evaporation, any mulched vegetation suppresses surface evaporation, and quick establishment of seeded vegetation converts surface evaporation into transpiration. That rainwater saved from surface runoff and evaporation is then used productively to grow the seeded vegetation.
11. Seedbed Formation: The imprinting process is a relatively precise method for forming a stable seedbed whose geometry matches that of the imprinting roller. The staggered imprinted furrows, each of which is about one-foot square in size, are V-shaped and thus can funnel resources together where they can work in concert to germinate seeds and establish seedlings. The firming action of imprinting smoothes and stabilizes the sides of the vee furrow, thereby increasing the efficiency of the funneling process and its longevity.
12. Seedling Cradles: After germination and seedling emergence, the imprint serves as a seedling cradle-a safe place with a microenvironment where seedlings can grow rapidly largely protected from the stresses of the macroenvironment. The V-shaped imprints partially shade seedlings from direct sunlight while slowing the desiccating winds. The imprints collect plant litter that serves as a mulch to cool the soil and shade the seedlings. On mineralization, the mulch enriches the soil, thereby making nutrients available to the growing seedlings. By the time seedlings grow up out of the vee imprint, their roots have penetrated deeply into the moist soil below thereby permitting the seedlings to cope with the harsh macroclimate.
13. Brushy Land: In the Desert Southwest, overgrazing by cattle converted perennial grasslands into scrub/shrubland often referred to as brush. Cattle grazed out the grasses leaving the land to be colonized by relatively unpalatable woody shrubs and cacti. Ranchers recognize the value of some of the leguminous shrubs for browse, however many feel their cattle would benefit from more forage as provided by perennial grasses. Thus they have imprinter interseeded perennial grasses into their brushy rangelands over tens of thousands of acres in southern Arizona . Heavy large diameter imprinting rollers mulch the shrubs as they roll over them while dropping seeds into the mulch. Most of the mulched shrubs regrow after perennial grasses have established with the help of mulch. Regrowth of the leguminous shrubs provides nutritious forage in addition to that provided by the newly established grasses. Where the imprinting roller is towed by a crawler tractor equipped with a dozer blade, shrubs with basal diameters up to four inches can be easily mulched.
14. Weedy Land: In the Great Basin region, overgrazing by sheep converted perennial grasslands into annual grasslands. Shrubs did not colonize after the perennial grasses were grazed out because sheep by nature are browsers as well as grazers. The exotic cheatgrass (a cool season annual) is the main component. It matures in the spring and often burns off from dry summer lightning storms leaving the land barren and vulnerable to wind and water erosion. Rolling imprinters are well suited for restoring perennial grasses, using the dead cheatgrass as a mulch. Fall imprinter seeding before the cheatgrass germinates is the best way to displace the annual grass with perennials. In land areas infested with Russian thistle, fall imprinter seeding is again recommended because the lush weed growth makes a beneficial mulch. The bottom of V-shaped imprints favors the germination of perennials because of deep soil moisture at this location. Perennial seeds are rewetted daily by upward capillary flow causing them to germinate eventually-usually within a week. Thus, imprinting easily converts an annual weed problem into a beneficial mulch which then helps push the secondary succession in the direction of seeded perennials.
15. Rocky Land: Rough rocky (nonarable) land poses no problem for rolling imprinters. One-foot rocks can be rolled over quite easily with a roller having a core diameter of two feet or greater because the rocks become partially imbedded in the soil as the heavy roller climbs over them especially if the soil is moist. Typically, large boulders are partially imbedded which makes them easier to roll over. For rocky land the core should be fabricated from ½-inch thick steel plate with the angular teeth being of equal thickness to avoid the possibility of bending on impacting large boulders. A boulder may lift one end of the roller out of the ground, but this loss of imprints is offset somewhat because the other is driven deeper than normal into the soil.
16. Steep Land : Recently, standard imprinting rollers have been developed for imprinting slopes up to 2:1 in steepness. These rollers have curvilinear angular teeth. The roller frame clamps to the dozer blade of a crawler tractor. Crawler tractors equipped with triangular self-cleaning pads also make excellent track-type imprints when driven up and down steep slopes with relatively soft soil. The soil firming action of imprinting is especially important on steep slopes where successful establishment of vegetation depends to a large extent on seedbed stability.
17. Wetlands: Standard rolling imprinters can be used to revegetate wetlands when and if the wetlands dry enough to prevent soil sticking to the imprinting teeth. Many wetland areas in the arid and semiarid regions are dry much of the year, becoming too wet for imprinting only at times during the rainy seasons. Otherwise crawler tractors with self-cleaning tracks and triangular pads make excellent imprints in moist soils. A self-cleaning imprinting roller is currently under development which could be used to imprint the space between the tractor tracks. These imprinters will have standard imprinting teeth and patterns except that teeth will be mounted on short legs to facilitate self-cleaning.
18. Seedbed Functions: The principal function of imprinted seedbeds is to funnel resources (such as seed, water, topsoil, and plant litter) together into the bottom of the imprints where they can work in concert to germinate seeds and establish seedlings. Typically these seedbeds are stable enough to remain functional for years or until such time adequate rainfall occurs. They also increase infiltration and reduce runoff and erosion.
19. Seedling Cradle Functions: The principal function of imprinted seedling cradles is to provide a favorable microclimate for establishing seedlings that have been germinated through the resource funneling function of the imprint. Specifically, functions include resource funneling, partial shading and wind breaking which, in turn, work together to cool and humidify the seedling cradle. By the time the seedling tops grow out of the cradle, it's roots are deep into moist soil below, thereby permitting the seedling to cope with the drying stresses of the macroclimate.
20. Clayey Soils: The rolling imprinter operates the best in clayey soils when they are just dry enough to not stick to the imprinting teeth. Dry clay soils should be ripped prior to imprinting and the imprinter should be loaded fully to smash clods. Standards require 1,000 pounds of imprinting force per foot of roller length for a fully loaded imprinter. Clay soils on steep slopes can probably be imprinted the best with a crawler tractor equipped triangular-padded, self-cleaning tracks. The dynamic traction effect on the tracks helps with soil penetration and clod breaking. Good seed soil contact is impossible without breaking down the large clayey clods. Capillary return flow of moisture to seed also depends on clod size reduction.
21. Sandy Soils: Sandy soils are best imprinted soon after they are wetted by rainfall. Since sandy soils are inherently soft, rolling imprinters are operated at their base weight without any loading. Base imprinter weight is about 500 pounds per foot of imprinter length. Imprinter firming of sandy soil is especially beneficial as it stabilizes the soil and improves capillary moisture flow to seeds to promote germination.
22. Soil Crusting: Soil crusting seldom if ever presents an emergence problem in full-depth imprints. This is because shrinkage cracks typically form in the bottom of the V-shaped imprints to allow seedling emergence. Also, capillary flow of moisture upward to the bottom of imprints softens the soil enough for emergence. Furthermore, any plant litter funneled to the bottom of imprints serves as a mulch to prevent crusting. Then too, the imprint is formed in the topsoil without soil surface inversion where organic matter content is the highest and where crustibility is the lowest.
23. Organic Matter: Because imprinting does not invert the soil and cover plant material, organic matter is retained at the soil surface and even tends to increase as plant litter decomposes. Also, imprinting firms the soil instead of loosening it, thereby reducing the oxygen available for organic matter mineralization. Further, increased infiltration improves the soil moisture regime and thusly accelerates biomass production. On decomposition, this biomass adds to the store of organic matter in the topsoil.
24. Soil Structure: Soil structure is improved and maintained by imprinting and the resultant accelerated biotic activity. Soil structure improves with the microbial activities associated with organic matter formation. Also the thread-like hyphae of soil fungi tie soil particles together into aggregates. See the preceding no. 23 for further detail on organic matter.
25. Soil Compaction: At first glance, excessive soil compaction might be a major drawback to imprinting, especially because compaction is a major problem in croplands farmed for many years with heavy equipment and often operated at moisture contents conducive to severe and deep compaction. However, the soil firming that results from imprinting has many beneficial aspects which far outweigh any detrimental compaction which may or may not occur. The firming action stabilizes imprints making them function better in the funneling of resources while making them last longer. The firming action of imprinting also improves seed-soil contact and capillary flow of soil moisture up to the seed. Firming is also involved in the imbedding of plant materials in faces of the imprints. Finally, the foregoing adds up to better control over infiltration and water and wind erosion. Once the imprint has served its purpose and vegetation has been established, the surface firming resulting from imprinting is quickly undone by biological activities that loosen the surface soil.
26. Herbiciding: Herbiciding and other methods of weed control are usually not needed in conjunction with imprinter seeding for restoring perennial grasses in degraded rangelands and ecological restoration of natural areas. This is because the imprinter teeth will mulch both woody and herbaceous weeds-a process that accelerates revegetation while reducing moisture and light competition from the weeds. Imprinter mulching conserves soil moisture for the seeded seedlings by suppressing weed transpiration and surface evaporation. Regrowth of the woody weeds is a relatively slow process compared with the growth of seeded perennial grasses. Thus, more often than not, weeds serve as an ecological "stepping stone" to the interseeding of perennial grasses or a later stage in the secondary succession. Finally, both native and exotic weeds are quite beneficial in accelerating revegetation when imprinter mulched. More destructive methods of weed control are seldom justifiable. Such disturbances often push the secondary succession back to weedy species, and hold it there.
27. Seeds and Seeder: As major components of the Imprinting Revegetation System, seeds and the seeder are as important as imprints and the imprinter. Good imprints and seedling cradles are useless unless adapted seeds are properly placed in them. Species should be included in seed mixes for ecological restoration that are adapted to the climate and the degraded soil including the soil seed bank. For ecological restoration early, mid, and late seral species should be included in the mix. Seeding rates of 10 to 15 pounds of pure live seed per acre with about one-fourth being early seral species are appropriate in most cases. Seeds should be mixed with an equal volume of red flaky wheat bran to prevent segregation in the seed box. The standard seedbox has a flexible 4-vane agitator which keeps the bulk of the seed stirred as it is metered through the adjustable discharge ports. Discharged seed falls to the top of the imprinting roller where it is carried forward to become imbedded in the faces of the imprint.
28. Rainfall: The third component of the Imprinting Revegetation System is rainfall or precipitation. Although the land manager has minimal control over rainfall, he/she can minimize the amount of precipitation required for plant stand establishment by making good imprints, using a good seed mix, and imprinter seeding at a favorable time of the year. Although seeding time is not critical for imprinting, probably the best time for the Desert Southwest is during November when the weather becomes dependably cool and just before the onset of the winter rains. The rains are generally relatively gentle and will stabilize the imprints while germinating cool season annuals for even greater imprint stability. The secondary succession is now off to a good start. The winter annuals complete their life cycle in the spring and then serve as a nurse crop to help establish warm season germinators during the summer monsoon.
In this section imprinting is briefly compared to pitting for each of the 28 revegetation features listed previously. In the United States , there are no established standards for pitters and pitting. This may be because the rate of success has been so low that standardization did not seem warranted. The best pitters are probably those developed in Australia and used there on large land areas. Pitters dig holes in the soil surface in varying sizes, shapes, depths and patterns using disks or plowshares. The brief comparisons, which follow, are necessarily quite general lacking standard specification for pitters and pitting.
1. Soil Action: Compared with imprinting, pitting disturbs the soil much more because of the digging action. The pit is formed by cutting into the topsoil and lifting a chunk of topsoil upward and inverting it next to the pit. Thus much topsoil is damaged in the pitting process, either dug up or covered up. The damaged topsoil is unstable and highly vulnerable to water erosion. Thus the pitted surface, unlike the imprinted surface, is a hazardous place for seeds and seedlings.
2. Mulching Action: Any mulching that occurs from pitting is entirely by accident and not design. While tractor tires or tracks may mulch some vegetation, the pitting action eliminates any existing plant litter on the soil surface by the digging and covering action, leaving a soil surface that is much more barren than before.
3. Furrow Stability: The pits or furrows are highly unstable relative to imprints because the soil surface is barren with loose piles of inverted topsoil next to the pits. This loose soil is quickly eroded back into the pits by heavy rainfall. In summary the pitted surface is barren and loosened, two conditions conducive to erosion; whereas the imprinted surface is covered and firmed, conditions which resist erosion.
4. Seed Placement: Seeds are dropped onto the pitted surface by various mechanisms as is the case with imprinting, but are not pressed firmly in contact with the soil surface unless press wheels are added to the pitter with sufficient flexibility to follow the rough pitted surface.
5. Seed-Soil Contact: Relative to imprinting seed-soil contact is very poor as the seed is merely scattered on the pitted surface. Following the pitter with flexible press wheels would improve the seed-soil contact.
6. Seedbed Firming: By design, pitting does not firm the seedbed. Although there may be some plowsole effect, the general action of pitting is to loosen rather than firm the soil.
7. Interseeding: Unlike imprinting, pitting is too destructive to existing vegetation to be very useful for interseeding. However where thinning is acceptable it could be used for interseeding if seedbed formation were adequate.
8. Infiltration Control: Whatever infiltration control results from pitting is somewhat accidental and temporary at best. Pitting makes the soil surface highly unstable by exposing barren subsoil with little structure and loosening this soil so that it can be easily detached and eroded during rainfall. Infiltration may be increased initially until the barren surface seals at which time rates fall rapidly. Infiltration time is increased some by the water stored in pits. The funneling of resources by pits is very inefficient relative to imprints.
9. Erosion Control: Pitting destabilizes the soil surface and therefore increases rather than decreases erosion. However, the pits do make excellent sediment traps. Thus, pits fill rapidly with the loose soil removed from them with each rainfall event-not a very safe environment for seeds and seedlings. Seeds often become covered with sediment too deeply for emergence or small seedlings become submerged in sediments. In contrast, imprints are relatively stable and therefore provide safety for seeds and seedlings.
10. Rainwater Conservation: Water conservation by pits is a mixed bag. Pits store some rainwater that might have otherwise runoff the surface. However because of the destabilized soil surface, these pits fill rapidly with sediment thereby losing their capacity for water storage. Thus, runoff control is temporary. Surface evaporation is increased relative to the unpitted surface because of a reduction in vegetation and an increase in surface area. Unlike imprinting, the very slow vegetative response to pitting maintains the high evaporative loses for several years following pitting.
11. Seedbed Formation: Relative to imprinting, pitting produces a poorly shaped and highly unstable seedbed. Thus vegetative response to pitting is relatively slow. Pitting may produce a thin stand of vegetation after 10 years, whereas imprinting often results in a thick stand in one year, dramatically reflecting the superiority of the imprinted seedbeds. Pitted seedbeds are often too cold when wet and too hot when dry-like a miniature lakebed.
12. Seedling Cradle Formation: Again, relative to imprinting, pitting produces a poorly shaped and highly unstable seedling cradle. Small seedlings often become covered with sediment from erosion of the loose soil removed from the pits. Pits often function like miniature lakebeds being too cold for seedlings when wet and too hot when dry. They go from one extreme to the other not allowing enough time for the seedling to adapt.
13. Brushy Land: Pitters cannot be operated in brushy land unless the brush is thin enough to allow the operator to steer between individual shrubs. Topgrowth becomes entangled in the pitter mechanism and crowns and roots break the disks and plowshares that dig the pits. In contrast brush presents no problem for imprinters which roll over them creating a beneficial mulch in the process. The best stands of seeded vegetation arises out of the shrub mulch. Most desert shrubs resprout from the crown after topgrowth is imprinter mulched. But regrowth is a slow process and in the meantime seeded vegetation becomes established.
14. Weedy Land: Weedy land refers to standing herbaceous exotic and native plants and the soil seed bank for these plants. By conventional agricultural wisdom, pitting would be superior to imprinting because it kills existing vegetation, excavates and inverts the seedbank soil and places seeds in the bottom of pits where seedlings will be relatively free of weed competition. However, by the ecological wisdom of sustainable agriculture and ecological restoration, weeds are viewed as beneficial pioneer (early seral) plants that can greatly accelerate the secondary succession toward the desired goal of a stable productive plant community. Thus, this new wisdom for sustainable land management puts imprinting well ahead of pitting. In conjunction with imprinting, weeds serve beneficially as cover, nurse, mulch and green manure crops to hasten the establishment of later seral species. Early seral species are added to seed mixes to insure the foregoing benefits and to compete with less desirable exotics in the soil seed bank.
15. Rocky Land: Unlike imprinters, pitters cannot be operated in rocky land because of excessive breakage of the pit digging mechanism. Thus pitting is largely confined to arable land. This may be why pitting is used more in Australia than the United States -the relative absence of rocks at least on the soil surface. Imprinting rollers with a 2-foot or greater central core diameter can easily roll over one-foot boulders and larger if they are partially imbedded.
16. Steep Land : Soil stability is essential to controlling erosion on steep land, thus pitting is not an appropriate technology as it actually increases soil instability. In contrast, imprinting provides a firm stable surface with highly functional seedbeds and seedling cradles that resist runoff and erosion temporarily while vegetation is becoming established for permanent control. For a quick cover crop, seed mixes should include annual grasses with fibrous root systems and herbaceous legumes with deep tap roots.
17. Wetlands: Because of the unusually wide spacing of pits with resultant sparse stands of vegetation, both drills and imprinters would probably be superior for use in wetlands that normally can support thick stands of vegetation. Thick vegetation is needed in floodplains for erosion control. Imprinters with self-cleaning teeth are currently under development for wetlands. Otherwise crawler tractors equipped with triangular padded, self-cleaning tracks can be used for imprinting wetlands. When wetlands dry out, a standard dryland imprinter can be used for seeding.
18. Seedbed Functions: The principal function of a seedbed is to germinate seeds. Pitting produces seedbeds that are nearly non-functional as indicated by the very poor vegetative responses. Several years are required by pitting to produce a sparse stand of vegetation, whereas imprinting often produces a thick stand in one year or less. The key to imprinted seedbed success is the V-shaped imprints which funnel vital resources together where these resources can work in concert to germinate seeds and establish seedlings. In dryland areas where soil moisture is the limiting factor this funneling function is especially important. Because of their inherent stability, imprints can remain functional for several years until the rains finally come. Resource funneling by pits is at best highly inefficient and at worst, non-existent.
19. Seedling Cradle Functions: The principal function of a seedling cradle is to establish seedlings by providing a favorable microenvironment. The microenvironment provided by pits is somewhat like that of a dry lake. After a rain which ponds some water, the pit bottom is wet and cold, but later it becomes hot and dry. The relatively large pits, unlike imprints, provide little protection from hot sunlight and dry winds. The moist microenvironment of imprints is largely a function of funneling of plant litter and rainwater, but this function is almost non-existent in pits. Thus, both the seedbeds and seedling cradles of pits do not favor rapid establishment of vegetation. The inherent instability of pits is a hazard for both seeds and seedlings. Eroded sediments can cover seeds to deeply for seedling emergence and suffocate those seedlings that do emerge.
20. Clayey Land: In order to get tool penetration in dry clays, ripping should precede both pitting and imprinting. A fully loaded imprinter will break the clods down somewhat for seed-soil contact and capillary return flow, however pitters will not reduce the clod size to any appreciable extent. Thus pitting is inappropriate for seedbed formation in dry clayey soils. In moist clayey soils, prior ripping is not necessary, however as discussed previously pitted seedbeds suffer from the dry-lake-bed effect. Crusting of the bottom of the pits is severe as they dry following rainfall severely hindering seedling emergence.
21. Sandy Land: Sandy soil is inherently soft requiring no weighting of either pitters or imprinters for adequate penetration of the tools. Sandy soil also has large pores unless firmed that makes capillary return flow to the seedbed quite sluggish. Imprinting moist-to-wet sandy soils reduces the pore size to improve capillary return moisture flow to seeds imbedded in the faces of the imprint. As a result of the good seed-soil contact and capillary return flow, seed germination occurs with minimal rainfall. In contrast, pitters do not firm the soil significantly or press the seeds against the sand with the result being little or no seed germination. Imprinter firming of moist sandy soil also creates some silica bonding between adjacent sand particles as the soil dries. This bonding stabilizes the imprint somewhat. With each rainfall event following imprinting, further bonding occurs. Little if any such bonding occurs in conjunction with pitting and the unstable sand quickly fills the pit.
22. Soil Crusting: Pitting presents an especially severe crusting problem because of the lake bed effect. Eroded fines settle out in the bottom of the pit with each rainfall event. The fines form a hard crust as the pit dries through which seedlings cannot emerge. Even without this sedimentation, the bottom of pits are often fine-textured subsoil with low organic matter-a condition favoring formation of durable crusts. In contrast, imprints are formed in the topsoil that is relatively high in organic matter. Furthermore, litter and sand particles gravitate to the bottom of V-shaped imprints, thereby greatly reducing the crustibility of the seedbed. Furthermore, shrinkage cracks often form at the bottom of the vee allowing seedling emergence. After more than 20 years of observations, a crusting problem has never been encountered in connection with imprinting. Crusting remains a major problem in conventional agriculture.
23. Organic Matter: Organic matter makes topsoil black and is the main difference between topsoil and subsoil along with associated soil microbes. It builds soil structure, water holding capacity and soil fertility. Seeding imprinters are designed to conserve and build organic matter by not loosening and inverting the topsoil, by mulching aboveground plant material, by holding rainwater in place, and by accelerating seed germination, seedling establishment and biomass production. In contrast, pitting loosens and inverts topsoil, covers plant litter, thereby accelerating mineralization of organic matter. Seedbed & seedling cradles are relatively poor so vegetal response to pitting and seeding is very slow and organic matter synthesis is equally slow.
24. Soil Structure: Soil structure results from the clumping of soil textural particles into aggregates that exhibit some water stability. Structured soil possesses more stability, organic matter, and biological activity than unstructured soil. Seeding imprinters are designed to conserve and build structure by not loosening and inverting topsoil while impressing seedbeds, by holding rainwater in place, and by greatly stimulating biological activity both above and below the soil surface. In contrast, pitters loosen and invert topsoil, thereby oxidizing the organic glues that bind primary soil particles together. Stimulation of biological activity is only marginal and very slow at best.
25. Soil Compaction: Pitters usually do no compaction or firming by design, however tractor wheels or tracks will compact the soil somewhat and the plowshare type pitters will produce a plowsole on the bottom of the pit. Seedbed firming is non-existent and thus seed-soil contact and capillary return flow are both poor. In contrast, imprinting produces good seed-soil contact and capillary return flow by the firming action of the toothed roller. There is no evidence of excessive compaction.
26. Herbiciding: The excessive soil disturbance and poor seedbeds produced by pitting means that soil bank weeds can easily overwhelm the seeding making the use of selective herbicides attractive. In contrast the excellent imprinted seedbeds induce a fast response by the seeded species making herbicides unnecessary. Additionally the imprinting action turns weeds into a beneficial mulch that helps the seeded species get off to a rapid start. Weed mulching conserves soil moisture for the seeding by reducing both transpiration and evaporation.
27. Seeds and Seeder: Seeds and the seeder, in addition to the mechanical method of preparing the seedbed, are important components of all revegetation systems. Drop or broadcast seeders of various designs are usually attached to the frame of pitters and imprinters. These seeders should be able to discharge a uniform mix of seeds of up to 15 species of native plants for ecological restoration projects. Seeds of such species vary greatly morphologically ranging from large to very small and from high density smooth to low density fluffy. Many have appendages that can be described by general adjectives such as hairy, cottony, and spiney. The standard seeding imprinter has a seed hopper with a semicircular bottom that surrounds a flexible four van agitator that, in turn, is directly driven by the imprinting roller. The agitator pushes the seed mix through one-inch round discharge ports centered above the imprinting roller. Seeds drop onto the roller and are then carried forward to be pressed into the surfaces of the imprint. The seed mix is mixed with an equal volume of red flaky wheat bran to prevent species segregation in the seedbox. Without bran, smooth dense seeds migrate downward in the mix, whereas light fluffy seeds rise to the top. After seeds are uniformly imbedded in the faces of the imprint, the resource funneling function of imprints begins. This function is especially active during rainfall events during which rainwater, seeds, splash-eroded soil and plant litter are funneled to the bottom of V-shaped imprints for an opportunity for seed germination and seedling establishment. Imprints are sufficiently stable for a wide time window of opportunity-up to five years if necessary. In contrast, pits are highly unstable, seeds are not pressed into the surface of the pits, and any concentration of resources in the pits is highly inefficient and haphazard at best.
Successful revegetation depends on a good matching of well-formed seedbeds and a well-adapted seed mix. The extent of soil degradation has to be considered in determining which species are adapted. More xeric species than those of historic ecosystems should be selected for severely disturbed sites. All seedbeds and seedling cradles fail whether formed by imprinters or pitters if species are selected that are unadapted to the soil and climate. Imprints may induce early establishment of unadapted species but they will not survive in the long run.
Each species included in mixes for ecological restoration has a set of optimal germination and establishment requirements such as depth of seed covering, soil temperature and soil moisture content. Seed dormancy and embryonic development are also important. Thus, seedbeds that may have to germinate up to 15 species need to be capable of providing a variety of germination conditions through time to give each species one or more opportunities to germinate. This in turn requires a seedbed with a good general shape which is sufficiently stable to function several years if need be. Imprints fit this criterion quite well, but pits do not. Contrast this with the seeders and planters of conventional agriculture that are designed to establish a monoculture of annual crops such as wheat, corn and soybeans. Such equipment is ill-suited to providing the variety of germination conditions required by complex seed mixes for ecological restoration.
28. Rainfall: Rainfall or precipitation is the most natural component of revegetation systems as it is the least controllable. Rainfall or precipitation is an essential component because the resulting soil moisture drives both seed germination and seedling establishment in drylands. Rainfall energy can quickly destroy an unstable seedbed without providing enough soil moisture for germination and establishment of the seeded species. Imprints are not only V-shaped for funneling resources to seeds and seedlings, but this shape is relatively stable because it is impressed into the soil surface without soil inversion and covering of plant material. Thus, an imprinted soil surface resists the erosional forces of raindrop impact. In contrast the pitted surface erodes rapidly under rainfall, filling the pits in the process. In dry regions, rainfall is not only limited annually, but amounts vary widely between seasons and from year to year. Thus seedbeds need to be stable enough to remain functional for several years until adequate rainfall occurs. Imprints have passed this test of durability, but pits have not. Finally, imprinting provides excellent control of rainfall and rainwater at the soil surface. Imprinting impresses into the soil surface a staggered pattern of short V-shaped furrows that serve as both seedbeds and seedling cradles whose main function is to funnel raindrops and rainwater to seeds and seedlings. This funneling action concentrates the rainwater precisely where it is needed to make efficient use of this sparse dryland resource in germinating seeds and establishing seedlings.
Citation: Dixon , R.M. 1998. The Imprinting Revegetation System Versus the Pitting Revegetation System. The Imprinting Foundation. Tucson , AZ 21 May.