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2003
Table of Contents |
Part (a) – Lower disturbance direct seeding systems
Introduction
To maximize crop yields from no-tillage, including in adverse conditions, farmers
must:
The manner in which no-tillage drilled slots are created can have a profound effect on both factors, particularly in relation to the amount and nature of the disturbance that takes place during drilling.
This paper examines low-disturbance slot systems and their effects on:
Slot cover
In-slot micro-environment
Carbon dioxide and moisture loss from slots
In-slot soil moisture content
In-slot soil temperature
Seed germination
Seedling survival and emergence
Seed-to-soil-contact
Smearing and crusting
Root development
Infiltration into the slot zone
Hairpinning of residues
Fertilizer placement
Soil erosion
Pests, diseases and allelopathy
Openers and their disturbanceAll statements are backed by internationally peer-reviewed research, unless otherwise stated.
Tillage in perspective
Aiming to create mini-tilled strips during no-tillage has been an obvious objective
of no-tillage machinery designers since modern no-tillage was first discovered
in the 1960’s. The objective has been to re-create, on a localised slot-scale,
the same tilled soil conditions that have been germinating seeds for centuries.
But unwittingly all that this has done is reduce the germination ability of
no-tilled slots to no-better-than tilled soils, when in fact they have the potential
to be superior.
Once non-residual, environmentally friendly herbicides had been developed, no one has ever been able to advance a good reason for regularly tilling or disturbing the soil. It is well known that tillage has mainly negative effects on soil in general. If this is so, the question then shifts to whether or not zone tillage within a no-tillage slot has negative or positive effects?
Residue management
(micro- and macro-)
In the main, debates about crop residues have revolved around their macro-management
on a field scale. Only in one regard (hairpinning) has the debate revolved around
their micro-management on a slot scale. And yet successful micro-management
of surface residues, in several respects can have a greater influence on the
performance and profitability of individual crops than macro-management (Baker
et al, 1996).
Micro-management of surface residues relates to how much, and in what manner the residues are disturbed at planting. Disturbance at planting might not alter macro-management (after all the residues still remain on the field, whether disturbed or not). But it can have a profound affect on micro-management and therefore crop establishment.
Crop residue close to and over the slot is a key factor that affects most other variables within the slot. In-slot tillage removes residue from covering the slot. At best it incorporates the residue into the covering soil. At worst it casts it aside. While loosening the soil may have a positive effect on seed-to-soil contact, if achieving this prevents residues being used as the main covering medium over the slot, the “trade-off” is that it destroys the slot’s ability to retain soil humidity, which is one of the most precious attributes a no-tillage slot can otherwise have.
Classifying
slot cover
It is important to focus on the different properties of various slot covers,
which were classified by Baker in 1976 and refined by Baker et al in 1996:
Class 1 cover consists of little or no loose soil or residue covering the seed (essentially open slots).
Class 2 cover consists of mostly loose soil covering the seed with up to 30% of residues mixed in or over the loose soil.
Class 3a cover consists of loose soil with 30-60% of residues over the loose soil.
Class 3b cover consists of loose soil with 30-60% of residues mixed into the soil.
Class 4 cover consists of loose soil with 60% or more residues (and even dust mulch) over the slot.
Extensive experiments have shown that there is a clear relationship between these classes of slot cover and seedling emergence in dry soils (Baker, 1976; Baker et al 1996; Wilkins et al, 1992). Further experiments also showed that there could be a similar relationship between the classes of cover and seedling emergence in wet soils even if the causes differed between wet and dry soils (Baker et al, 1988).
In-slot microenvironment
All untilled soils contain water vapour in the macropores that has an equilibrium
relative humidity (RH) of 100% (Scotter, 1976). This is referred to as vapour-phase
water. The equilibrium RH remains close to 100% throughout the entire “available
soil moisture range” (i.e. between “field capacity”
(FC) and “permanent wilting point” (PWP)). Even at PWP
the RH is still 99.8% but by then the soil is too dry to sustain plant life
anyway and plants wilt permanently and die.
By contrast, tilled soils become so aerated and exposed to the atmosphere (regardless of whether or not residues are also incorporated) that their equilibrium soil RH is usually well below 100% unless it is actually raining or the soil is being irrigated at the time. The inability of tilled soils to retain high levels of RH is a major difference between such soils and their untilled counterparts and provides a resource that most no-tillage machinery designers have not yet learnt to harness. But even in untilled soils it is a resource that is largely controlled by the level and nature of disturbance of surface residues close to (and preferably over) the sown slot during no-tillage seeding (micro-management of residues).
Some seeds (e.g. corn) will germinate in a soil atmosphere of 90-100% RH without having any physical contact with soil or liquid soil water at all (Martin and Thrailkill, 1993). Other seeds are capable of getting some of their requirements from liquid-phase water and an increasing amount from vapour-phase water as the soil dries. But since tilled soils seldom retain vapour-phase water approaching even 90% RH, vapour-phase water seldom plays any part in crop establishment with tilled soils. Instead tillage farmers the world over have become used to seeking darker, cooler, damper soil in which to plant seeds so as to be assured of germination from the uptake of liquid-phase water.
Since the only zone that is (or should be) disturbed in no-tillage is the sown slot, the nature of slot disturbance and micro-management of surface residues close to the slot influence how much vapour-phase water is available for seeds. If the slot shape encourages retention of vapour-phase water, seeds with poor seed-to-soil-contact, or are suspended in a dry hairpin, will not suffer greatly. If it does not, these factors will have a negative effect.
Carbon dioxide
and moisture evaporation from slots
While measurements of moisture vapour retention within the slots created by
various no-tillage openers are compelling enough, verification has also come
from aboveground measurements of emissions into the atmosphere. Reicosky (1996)
and Reicosky et al (1996) have made extensive studies of carbon dioxide and
evaporation losses in the USA as a function of tillage and no-tillage methods.
Limited data (D. C. Reicosky, unpublished data, 1996) suggested that in corn residue, slots with grade 4 cover (Cross Slot™ opener) lost less moisture vapour and carbon dioxide to the atmosphere than slots with grade 2 cover (double disc opener). But when both openers drilled into sod, the cover produced by the double disc opener improved to grade 4 resulting in no noticeable differences in moisture or carbon dioxide emissions between the openers. This illustrates the role that slot cover may play in retaining both moisture and carbon dioxide within a technique (no-tillage) that offers major advantages over tillage in these regards anyway (Reicosky, 1996).
In-slot liquid
moisture content and temperature
While large differences have been recorded between slots and covers in terms
of vapour-phase water retention, little or no differences have been recorded
in terms of liquid-phase water (Baker 1976: Baker et al, 1996). It seems that
neither slot shape nor cover alter the amount of liquid-phase water present
in the undisturbed soil alongside, or the disturbed soil within the slot. This
might also account for the fact that only minor in-slot temperature differences
have been recorded between various slot shapes in dry soils.
The absence of temperature differences in dry soils contrasts with wet soils where residues covering the slot are known to reduce the rate of drying and thereby slow down the rate of rise of in-slot temperatures during spring warming.
Seed-to-soil contact,
smearing and early root development
In no-tillage it is important to distinguish between germination and emergence
since both are affected in different ways by micro-management of residues. It
is also important to recognise that in no-tillage there is a slot wall between
the seed and the undisturbed soil alongside, whereas in tillage any slot walls
that do exist are indistinct and non-restrictive due to the general tillage
process (Baker et al, 2001).
In dry soils seeds sown by double disc openers might germinate because they have adequate (even point) contact with the slot walls. But after germination, the embryonic roots often have difficulty penetrating the slot walls to seek water, especially if the slot walls are nearly vertical. As a consequence many seedlings die before emergence. This problem is greatest in open slots (class 1 cover) where transpirational demand on seedlings occurs almost as soon as they germinate.
With openers that create loosening in a dry soil, even good seed-to-soil contact may not provide sufficient liquid-phase water for germination because loose soil does not readily transport water. But seeds often survive until rainfall provides the necessary liquid-phase water.
In soils that are initially wet but experience drying conditions following drilling, some disc and non-disc openers will cause the slot walls to become smeared. Smears are usually non-restrictive so long as they (a) are not so thick as to form a compacted layer instead, and (b) remain moist due to good slot covering. On the other hand if the cover is not good and those same slots then dry rapidly (windy spring climates are particularly troublesome in this respect) the smears may dry to become internal crusts, which can be quite restrictive. Even a covering of loose soil (class 2 cover) or soil and limited residue (class 3 cover) will prevent in-slot drying of most smears and transpirational demand will also diminish. But seedlings are still at risk until rain arrives.
Seeds sown into horizontal (or inverted T-shaped) slots with class 4 cover (e.g. Cross Slot openers) may have no better seed-to-soil contact than most other covered slots and may also experience smearing. But the entrapment of 90 -100% RH ensures germination will take place regardless. Equally importantly, sub-surface seedlings remain in an atmosphere of high RH while their roots negotiate the slot walls, which are largely horizontal and therefore easier to penetrate anyway; and are prevented from drying to form crusts by the high RH. Grass seedlings have been known to survive beneath the soil surface in horizontal slots in a dry field for eight weeks before rain eventually fell and completed their emergence.
In the light of the ability of certain no-tillage slots to retain vapour-phase water, some plant physiologists have reasoned that even when stand counts between contrasting no-tillage openers are identical in dry-to-optimum soils, greater yield potential may exist in those seedlings that emerge less-stressed from an atmosphere of high soil humidity than those seedlings that emerge somewhat stressed from having to survive on the ability of their young roots to negotiate through the slot walls. Thus slot shape alone may have an early influence on yield, regardless of observed stand counts.
In dry soils, micro-management of residues should therefore aim to return as much as possible of the residues over the soil that covers the slot (class 4 cover) especially in autumn seeding. Residues are nature’s way of entrapping humidity within the top layers of soil while still allowing the soil to breathe. This was illustrated in an experiment where plastic strips were used to cover otherwise exposed no-tillage slots. The strips certainly raised the humidity levels in the slots but also led to fungal growth because of poor aeration. A covering of residue raises the humidity level without creating anaerobic conditions (Choudhary and Baker, 1981; Baker et al, 1996).
In wet soils the problem is aeration. Not only do residues allow air (but not water vapour) exchange to take place between slots and the atmosphere, they also influence the temperature gradients that cause such exchanges to take place. In-slot tillage creates even greater aeration but the effect is temporary until the damaged soil puddles and re-seals itself. The greatest and most sustainable effect from no-tillage is from earthworms and other soil fauna that colonize the slot zone, sometimes within 24 hours of seeding. Surface-feeding earthworms respond strongly to where residues lie. If they lie over the slot (class 4 cover, horizontal slots) the earthworms colonize the slot zone. If they lie beside the slot (hoe-type or angled disc openers that push residues aside) earthworms colonize the zones alongside the slot, but not necessarily the slot itself.
If the slot walls are compacted, regardless of whether or not residues lie over the slot (e.g. double disc openers) earthworms may avoid the slot zone altogether (Chaudhry and Baker, 1988).
Infiltration into
the slot zone
The effects in wet soils are seen not only as increased seedling emergence but
also increased infiltration within the slot zone (Baker et al, 1987). Both effects
contribute to greater aeration and crop yields, but of course they also depend
on the presence of earthworms and other soil fauna, which are often a medium-term
rather than “instant” benefit of no-tillage. In the absence of earthworms,
residue placement and disturbance have minimal direct effects on slot aeration
but remain important in relation to hairpinning.
In a soil of optimum moisture content, disturbance within the row may therefore do no harm. Whether or not it does any good is debatable. But certainly in a soil that is too dry or too wet, excessive slot disturbance may literally mean the difference between success and failure.
Hairpinning of
residues
The most quoted negative effect from residues overlying the slot zone is hairpinning
or tucking of residues into the slot. All disc-type no-tillage openers hairpin
residues at least some of the time. None are entirely guiltless, although some
are worse than others. Hairpinning occurs when a disc fails to cut through all
of the residue and instead folds a portion of it in two and pushes the folded
residue into the soil in the shape of a hairpin. It occurs mainly when residues
are pliable (often damp) which makes them difficult to cut, and the soil is
soft and provides little or no anvil-effect for discs to cut against. One designer
tried to align straw in the direction of the disc so as to avoid hairpinning,
but this proved impractical.
The conundrum is that while discs are the main cause of hairpinning, no one has yet designed an opener that can physically handle surface residues in closely spaced rows without the assistance of a disc. The question then shifts to whether or not the disadvantages of discs and the effects of hairpinning can be counteracted? With thoughtful design they can be.
In wet soils, especially those that are not well aerated (i.e. are anaerobic) rotting residues may produce acetic acid or other fatty acids (Lynch, 1977, 1978) that kills seeds. But beneficial soil bacteria also break the fatty acids down rapidly.
Seeds that end up embedded in hairpinned residues may suffer early death from direct contact with acetic acid. Seeds that do not actually contact the residues (even removal by less than half an inch is often sufficient) may survive because soil bacteria neutralize the acid before it reaches the seeds.
In a wet residue-covered soil, disc openers that physically separate seeds from direct contact with decaying residues should therefore be used. This precludes double disc and angled disc openers, which create vertical or slanted slots and implant seeds directly into hairpins. But it does leave the door open for other designs such as Cross Slot that create horizontal slots on either side of a vertical disc slit and implant the seeds on these shelves away from direct contact with any residues that may be hairpinned vertically by the central disc. Experiments have shown positive emergence responses from Cross Slot openers to the presence of surface residues over the slot in a wet soil while double disc openers created a negative (or at best neutral) response to residues (Baker et al 1988).
In dry soils, hairpinning suspends seeds in the residue and interrupts soil-to-seed contact. This is important if soil-to-seed contact is the only (or predominant) mechanism by which seeds derive water from the soil. In many slot shapes it is. But in some (e.g. Cross Slot) it is not.
Fertilizer placement
and residues
Nutrient uptake can be markedly affected by opener design and performance, particularly
by an opener’s ability (or not) to band fertilizer separately from the
seed at the time of planting (Baker and Afzal, 1986; Saxton and Baker, 1990).
While micro-management of residues has little direct effect on the uptake of nutrients by no-tilled plants (although it will provide some nutrients as it decomposes) it has some interesting indirect effects, again largely through its interactions with earthworms. In this instance the effects are, in a way, negative. Bio-channels created by earthworms, other soil fauna and decaying roots under a no-tillage regime provide ready conduits for preferential infiltration of soluble nutrients such as nitrogen and potassium (Kanchanasut et al, 1978). Often these nutrients, when applied as surface applications of inorganic fertilizers, bypass young root systems altogether resulting in poor crop responses to fertilizer and inferior final yields compared with tilled fields where all bio-channels in the tillage layer are destroyed, leading to uniform infiltration of surface-applied nutrients.
The problem is exacerbated
under no-tillage because microorganisms tend to temporarily lock-up soil nitrogen
as they decompose residues and there is reduced mineralization of organic nitrogen
anyway; making applied nitrogen all the more important.
Under these circumstances banding of fertilizer close to, but not touching the
seeds at seeding becomes vital if maximum crop yields are to be obtained. Few
no-tillage machines provide this capability, and even fewer openers have genuine
double-shoot capabilities that avoid compromising micro-management of surface
residues or row spacing. Many double-shoot openers are in fact combinations
of two openers strategically joined together to create separate seed and fertilizer
slots close together. The price paid for providing double-shooting in this manner
is to create even greater physical disturbance of the surface residues close
to the slot, with all of the negative effects that this entails (Baker et al,
1979a).
Further, some double-shoot openers occupy so much physical space that close-row drilling is sacrificed at a time when the moisture conservation advantages of no-tillage are encouraging closer (rather than wider) rows to gain the agronomic benefits available.
Cross Slot openers not only create horizontal slots (Baker et al, 1979b) they also provide genuine double-shoot capabilities with a single opener in close rows while at the same time ensuring surface residues are returned over the slot to provide class 4 cover.
Then there is the issue of deep banding versus horizontal separation of fertilizer and seed. Any banding is better than broadcasting especially with spring drilling. But deep banding below the seed inevitably involves increased surface disturbance while horizontal banding appears to give superior yield responses compared with vertical banding anyway (Baker and Afzal, 1986). Most evidence (Saxton and Baker, 1990; Baker et al, 1996; Finck, 2000, 2002) also supports using close separation distances (10-12 mm or 3/8 – ½ inch) and horizontal banding in no-tillage rather than larger separation distances. Deep banding (2 - 3 inches below the seed) therefore seems to have less relevance in no-tillage than it does in tillage.
Soil erosion
Since retention of surface residues is the main mechanism of erosion control,
the more of the surface that remains covered with residues after seeding, the
better will be the erosion protection. Certainly disturbance from many opener
types exceeds the 70% minimum cover that has defined no-tillage in the USA for
many years. More advanced designs are capable of 90% residue retention.
This author was once berated for driving a car across an unsown long-term no-tillage field in USA. Apparently the tire marks could become a source of rill erosion. If this is so, what does it say about the potential for high-disturbance tine-type no-tillage openers to initiate localised soil erosion?
Pests, diseases
and allelopathy
Early predictions of uncontrollable residue-related pest and disease problems
attributable to no-tillage in general, and slot conditions in particular, have
proven to be exaggerated if not in most cases, groundless. Certainly this author
is not aware of confirmed reports showing any grade of slot cover to be more
or less susceptible in this respect to any other grade.
Rhizoctonia and certain other fungal diseases have the potential to be troublesome under no-tillage because tillage is known to partially control the fungus by rupturing subterranean mycelia. Therefore the problem under no-tillage was expected to be sensitive to opener design. Openers that caused least disruption to the soil and residues during seeding were expected to do less rupturing of mycelia than others with more vigorous actions. But in reality disturbance of surface residues has not been a good indicator of opener-susceptibility to Rhizoctonia.
In non-sticky soils for example, double disc openers cause about the same (minimal) disruption to surface residues as Cross Slot openers. But the vertical penetration of double disc openers is limited to the seeding depth, whereas with Cross Slot openers the disc penetrates up to 3 inches (75 mm) below seeding depth. Whether or not this is sufficiently deep to rupture mycelia and thereby reduce this opener’s susceptibility to Rhizoctonia is unknown, but the point is that in relation to Rhizoctonia, no-tillage openers cannot necessarily be judged alone on the effects they have on residues near the surface or their lateral zones of disturbance.
A relevant question might therefore be - are openers that create surface disturbance but no under-seed disturbance any better or worse than openers that create under-seed disturbance but little surface disturbance? It is doubtful if anyone truly knows.
In any case, strategic and innovative crop rotations probably have more potential to circumvent pest and disease problems in no-tillage than opener design. This has certainly been the case in New Zealand.
In early trials with no-tillage, poor crop results were often attributed to toxic substances that some resident species of plants exude as a protection against competition (know as allelopathy). But as scientists have come to understand what really affects seed germination and seedling emergence during no-tillage, it has been difficult to find any genuine cases of allelopathy. And it is certainly not the widespread problem many expected it would be. The effects of acetic acid burn from decaying hairpins might (debatably) be classified as allelopathy. But in this regard more advanced opener designs appear to be free of the affliction anyway.
Openers and their
disturbance
Low-disturbance openers include Cross Slot and double disc, although the latter
can cause considerable disturbance in sticky soils from soil sticking to the
outside of the discs. High-disturbance openers include most hoe or shank-types
and angled discs run at speed or on hillsides. At slower speeds, angled disc
type openers might best be classified as medium disturbance. Zone tillage, of
course deliberately causes disturbance and pre-drilling shanking of nitrogen
creates disturbance that no opener can rectify at seeding time. Often pre-drilling
disturbance (including wavy pre-discs ahead of otherwise conventional double
disc openers) does no more than create a loosened zone to offset the undesirable
wedging and compacting action of the double disc openers that follow.
Conclusions
While no-tillers have all leant to accept the need to macro-manage surface residues
and avoid tillage on a field scale, it is now time to learn to micro-manage
those same residues and reduce in-slot disturbance if they want to maximize
the responses to, and benefits from no-tillage both on hills and flat land.
Recent advances in opener design such as Cross Slot, and strategic use of crop
rotations mean it is no longer necessary to compromise micro-management and
disturbance to achieve other objectives, such as seed-to-soil contact, disease
control, fertilizer placement and residue handling.
No-tillage incentive schemes such as Nebraska’s “Lower Elkhorn NRD Notill Incentive Program”, despite discouraging soil disturbance between crops, often allow up to 25% disturbance at planting. Imagine how much more could be gained if the allowable disturbance at planting was further reduced using minimum-disturbance methods and technologies.
Part (b): The Cross Slot minimum-disturbance no-tillage opener
The science reported above underpinned a completely new approach to no-tillage
opener design in New Zealand. The objective was to eliminate as many of the
negative factors as possible. Not only did the 30-year project achieve this,
it also contributed several new opener design concepts that have had repeatable
positive effects on crop stand and yield. The opener was designed to fulfil
all of the scientific principles discussed above.
Cross Slot no-tillage openers and air drills have the following unique features:
The most pleasing aspect
of the Cross Slot machines that have been operating in New Zealand, Australia
and the USA since 1995, is that the extensive science that underpins their design
has been regularly verified in practice. The consistency and predictability
of crop performance have been outstanding. Surveys of New Zealand owners over
a 4-year period covering a total of 100,000 acres in 6,000 separate fields sowing
a wide range of crops in an equally wide range of soil and residue conditions
in both autumn and spring, gave Cross Slot drills a failsafe ranking of 99%
and the overall crop success rate was 90-95%. The latter figure also took account
of non-drill-related impaired crops from factors such as inadequate weed and
pest control and operator error (Baker, 2001).
In recent years, crops sown by Cross Slot drills in New Zealand have won numerous
independent regional and national crop yield competitions involving both tillage
and no-tillage. This is the first time that no-tillage has even achieved parity
with tillage in New Zealand, let alone superiority.
The driving force behind the Cross Slot project continues to be a belief that ultimately nothing will influence the widespread uptake of no-tillage anywhere, more than crop yield!
References
Baker, C.J., 1976. Experiments relating to the techniques of direct drilling of seeds into
dead turf. Journal of Agricultural Engineering Research 21(2), 133-145.Baker, C.J., Badger, E.M., McDonald, J.H. and Rix, C.S. 1979a. Developments with
seed drill coulters for direct drilling: 1 Trash handling properties of coulters. New Zealand Journal of Experimental Agriculture 7, 175-184.Baker, C.J., McDonald, J.H., Seebeck, K., Rix, C.S. and Griffiths, P.M. 1979b.
Developments with seed drill coulters for direct drilling: III An improved chisel coulter with trash handling and fertilizer placement capabilities. New Zealand Journal of Experimental Agriculture 7, 189-196.Baker, C.J. and Afzal, C.M. 1986. Dry fertilizer placement in conservation tillage: seed
damage in direct drilling. Soil and Tillage Research 7, 241-250.Baker, C.J., Chaudhry, A.D. and Springett, J.A., 1987. Barley seedling establishment
and infiltration from direct drilling in a wet soil. Proceedings of the Agronomy Society of New Zealand 17, 59-66Baker, C.J., Chaudhry, A.D. and Springett, J.A., 1988. Barley seedling establishment by
direct drilling in a wet soil. 3 Comparison of six sowing techniques. Soil and Tillage Research 11, 167-181.Baker, C.J., Saxton, K.E and Ritchie, W.R. 1996. No-Tillage Seeding: Science and
Practice. CABI publication, 258 pages, (ISBN 0851991033).Baker, C.J., Choudhary, M.A. and Collins, R.M. 2001. Factors affecting the uptake of
no-tillage in Australia, Asia and New Zealand. Proceedings 1 World Congress on Conservation Agriculture (“Conservation Agriculture: A Worldwide Challenge”), Madrid, Spain, Volume (1), 35-42.Chaudhry, A.D. and Baker, C.J., 1988. barley seedling establishment by direct drilling in
a wet soil: 1 Effects of openers under simulation rainfall and high water-table
conditions. Soil and Tillage Research 11, 43-61.Choudhary, M.A. and Baker, C.J. 1981. Physical effects of direct drilling equipment on
undisturbed soils: II Seed groove formation by a “triple disc” coulter and seedling performance. New Zealand Journal of Agricultural Research 24, 183-187.Fink, C. 2000. Nutrient management. Farm Journal, April 2000, pages 12-14.
Fink, C. 2002. On target. Farm Journal field tests help you hit the mark with starter
fertilizer. Farm Journal, February 2002, pages 14-16.Kanchanasut, P., Scotter, D.R. and Tillman, R.W. 1978. Preferential solute movement
through larger soil voids: II Experiments with saturated soil. Australian Journal of Soil Research 16, 257-267.Lynch, J.M., 1977. Phototoxicity of acetic acid produced in an anaerobic decomposition of
wheat straw. Journal of Applied Bacteriology 42, 81-87.Lynch, J.M., 1978. Production of a phytotoxicity of acetic acid in anaerobic soils
containing plant residues. Journal of Soil Biology 10, 131-135.Martin, D.L. and Thrailkill, D.J., 1993. Moisture and humidity requirements for
germination of surface seeded corn. Applied Engineering in Agriculture 9(1), 43-48.Reicosky, D.C., 1996. Impact of tillage on soil as a carbon sink. Proceedings National No-
Tillage Conference, St Louis, MO, USA, pp 106-109.Reicosky, D.C., Kemper, W.D., Langdale, G.W., Douglas, C.L. Jr. and Rasmussen,
P.E., 1996. Soil organic matter changes resulting from tillage and biomass production. Proceedings National No-Tillage Conference, St Louis, MO, USA, pp 97-105.Saxton, K.E. and Baker, C.J., 1990. The Cross Slot drill opener for conservation tillage.
Proceedings of the Great Plains Conservation Tillage Symposium, Bismark, North Dakota, USA, pages 65-72Scotter, D.R. 1976. Liquid and vapour phase transport in soil. Australian Journal of Soil
Research 14, 33-41.Wilkins, D.E., Bolton, F. and Saxton, K.E. 1992. Evaluating seeders for conservation
tillage production of peas. Applied Engineering in Agriculture 8(2), 165-170.
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