Agronomy Library > Winter Annuals/Other

Minimum Versus Maximum Slot Disturbance in No-Tillage: How Much is too Much?
Author: Dr. C John Baker
Date Created: February 22, 2007
Last Reviewed: February 22, 2007

While it is well accepted that the practice of no-tillage is synonymous with residue retention on a macro-scale, less attention has been paid to the effects of residue retention on a micro-scale, especially over the sown row. This paper examines the role of residues close to and over the sown slot in relation to 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, and openers and their modes of action. 

Data are available showing that in relation to almost all of the listed criteria, controlled minimum-disturbance produces superior and more predictable results compared with maximum-disturbance, which otherwise traces its origins to an assumption that tilled soils (even within a row in the form of strip tillage) create optimum environments for seeds and seedlings. In reality untilled soils contain greater potential for seed germination, and survival than tilled soils, but man has been slow to harness this potential.

Introduction: Crop residues are the lifeblood of no-tillage. Indeed they are the lifeblood of sustainable agriculture itself. In the past, debates about surface residues have mostly centred on their macro-management; the percentage of ground that is covered by residues in relation to erosion control, surface sealing, shading and the ability of machines to physically handle them. Recent emphasis has been on reducing the amount of residue disturbance during drilling because of the erosion protection that greater amounts of ground-cover offer. Micro-management of residues centres on the influence that residues have on seed, seedling and plant performance in individual rows, all of which ultimately affect crop yield.

One aspect relates to soil erosion. The other to crop yield. Is one more important than the other? Unless crop yield is maintained, no-one is going to undertake no-tillage anyway and the soil erosion benefits would then become irrelevant. Therefore it could be said that micro-management of surface residues should be the first objective in any no-tillage system. But sadly, history shows that that has seldom been the case. Then again, minimum slot disturbance means different things to different people. For example, an allowable limit of 30% slot disturbance means that the disturbed zone in 150 mm wheat rows can only be 45 mm wide - a tall (but achievable) order for many no-tillage openers. But 30% disturbance in rows of maize (corn) or cotton sown in 750 mm – 1 metre rows represents 225 - 300 mm of disturbance - a much more generous objective. So the development of no-tillage openers for wheat and other narrow-row crops may take a very different course to those for wide-row crops. But since there is twice as much wheat sown in the world as the next most common crop, the constraints on openers for narrow-row crops are going to provide the most challenges for machinery designers. Some of the design and functional trade-offs are outlined below (Baker
et al, 1996):

Minimum disturbance no-tillage is created by the following conditions: 
• Heavy and/or wet ‘plastic’ soils. 
• Residues that are evenly distributed over the surface of the ground.
• Openers that disturb the surface of the ground as little as possible, retaining at least 70% of the surface residues intact after their passage. Maximum disturbance no-tillage is created by the following conditions: 
• Dry and/or light and friable soils. 
• Low levels of surface residues, especially root residues that otherwise hold the soil together. 
• Openers that either burst the soil aside or deliberately till a strip at least 50 mm wide.

To maximize crop yields from no-tillage (regardless of the conditions) farmers must:
(a) Establish the target population of plants, and
(b) Feed these plants.
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 - Opener designs.

Slot cover: 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. But once non-residual, environmentally friendly herbicides had been developed, no one has advanced a good reason for regularly tilling or disturbing the soil even on a localised slot-scale. It is well known that tillage has mainly negative effects on soil and this applies as much to the slot zone as anywhere else on a field. No-tillage slot cover can be classified according to the amount and nature of the covering medium (Baker,1976, 2003; Baker et al 1996): 
• Grade 1 = no cover (eg. double disc openers operating in damp soils) 
• Grade 2 = some loose soil (eg. hoe type openers in friable soils) 
• Grade 3a = loose soil mixed with residue (eg. hoe type openers with covering sweeps in friable soils; power-till type openers)
• Grade 3b = loose soil covered by 30-60% residue (eg. angled disc openers in friable soils) 
• Grade 4 = loose soil covered with at least 70% residue (eg. inverted t type slots in residue-covered soils).

Seed and seedling performance has been shown to increase markedly as the grade of cover increases, especially as conditions become sub-optimal. Loosening the soil within the slot (maximizing disturbance) produces Grade 3a cover in dry conditions and Grade 2 in damp or plastic conditions. Minimizing slot disturbance can create either Grade 1 or Grade 4 slot cover, depending on the design of opener. The objective is to produce Grade 4 cover in all conditions. Some opener designs achieve this. Others are unable to achieve it in any conditions. In-slot microenvironment Within the “available” moisture range, all untilled soils contain water vapour in their macropores with an equilibrium relative humidity (RH) close to 100% (Scotter, 1976). By contrast, tilled soils become so aerated and exposed to the atmosphere that their equilibrium soil RH seldom approaches 100% unless it is actually raining or the soil is being irrigated at the time. The ability of untilled soils to retain high levels of RH is a major resource that most no-tillage machinery designers have not yet learnt to harness (Choudhary and Baker, 1981). Many seeds are capable of germinating in a soil atmosphere of 90-100% RH with minimal (and even no) physical contact with soil or liquid soil water (Martin and Thrailkill, 1993, Wuest, 2002). Since the only zone that is 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 within the slot. Because Grade 4 cover encourages retention of vapour-phase water, seed-to-soil-contact in such slots is not important. With lower grades of cover seed-to-soil-contact assumes the same importance as it does in tillage. 

Carbon dioxide and moisture loss: Limited data (D. C. Reicosky, unpublished data, 1996) suggest that slot shape and residue retention may also have minor affects on the ability of no-tillage slots to retain carbon dioxide. No-tillage offers major advantages over tillage in this regard anyway (Reicosky, 1996; Reicosky et al, 1996) but differences in no-tillage slot disturbance may also be important when it comes to allocating carbon credits to farmers practicing no-tillage.

In slot moisture and temperature: Some studies have shown that slot shape and residue retention have minimal short-term effects on the liquid-phase soil water content and temperature within the sown slot even although they are both affected on a macro-scale over the longer term by residue retention (Baker, 1976). On the other hand it has become common practice in some countries to remove residues from over the slot in order to increase soil temperature in the slot zone during spring warming. This begs the question whether seeds sown shallow beneath a grade 4 residue canopy, for example (which provides water for germination at shallow sowing depths) experience any lower soil temperatures than seeds sown deeper in less-moisture-friendly slots with grades 1, 2 or 3a covers?

Soil-to-seed-contact: Smearing and compaction In dry soils, even good seed-to-soil-contact in maximum-disturbance slots may not provide sufficient liquid-phase water for germination because loose soil does not readily transport water. In no-tillage, unlike tillage, there is a distinct slot wall between the seed and the undisturbed soil alongside. In dry soils the embryonic roots of seeds that do germinate in vertical slots often have difficulty penetrating these slot walls to seek water and the plants die before emergence. The young roots of seeds sown into horizontal (or inverted T-shaped) slots (grade 4 cover) are able to negotiate the horizontal slot walls (wet or dry) without difficulty (Baker et al, 1996). In wet soils the slot walls may become smeared. Smears are usually non-restrictive so long as they (a) are not so thick as to form compacted layers, and (b) remain moist due to good slot covering. The greatest and most sustainable effect on slot aeration in wet soils is from earthworms and other soil fauna (Baker et al, 1987, 1988; Chaudhry and Baker, 1988).

Surface-feeding earthworms respond strongly to where surface residues lie. If they lie over the slot (inverted T slots, grade 4 cover) earthworms will colonize the slot zone. If they lie beside the slot (hoe-type or angled disc openers that push residues aside, grades 2 or 3b cover) earthworms will colonize the zones alongside the slot, but not necessarily the slot itself. In wet soils that subsequently dry, slot shrinkage can expose both seeds and seedlings to drying, which is a common cause of no-tillage failure. On the other hand seeds that have been placed under a soil flap to one side of the central slit (as occurs with inverted T slots) are seldom troubled by slot shrinkage. Infiltration into the slot zone The effects of soil fauna and slot shape can increase infiltration within the slot zone with inverted T-shaped slots compared with all others slot shapes (Baker et al, 1987). This contributes to aeration and crop yield but depends 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 differences between openers are minimal. 

Hairpinning of residues:The most quoted negative effect from residues overlying the slot zone is hairpinning or tucking of residues into the slot. Decomposing residues in wet soils create acetic acid that can kill seeds that are touching the residue. In dry soils seeds suspended in hairpins have difficulty accessing water. All disc-type no-tillage openers hairpin residues at least some of the time but no one has yet designed an opener that can physically handle surface residues in closely spaced rows without the assistance of a disc. But some disc openers physically separate seeds from direct contact with hairpinned residues and thus avoid the problem. Cross Slot™ openers achieve this better than any other known disc opener design by placing the seed to one side of the central slit where it is removed from direct contact with any residue that is hairpinned by the central disc.

Fertilizer placement: 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). The effect is partly because soluble nutrients broadcast on the soil surface, often flow down undisturbed bio-channels in untilled soils and largely bi-pass small new root systems (Kanchanasut et al, 1978) and partly because micro-organisms temporarily lock up soil nitrogen as they decompose residues. Banding of fertilizer close to, but not touching the seeds at seeding becomes vital if maximum crop yields are to be obtained (Fink, 2000, 2002). Some designers achieve this by combining two openers together, which increases row spacing and surface disturbance, or by using “skip-row” planting. Others use separate fertilizer openers altogether that increase slot disturbance markedly. But there are openers that have been purpose-designed with no sacrifice of row spacing or surface disturbance (Baker, 2003).

Soil erosion: Since retention of surface residues is the most effective mechanism for controlling soil erosion, the more of the surface that remains covered with residues after seeding, the better. 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. In early trials with no-tillage, poor crop results were often attributed to toxic exudates from dying residues (allelopathy). But as scientists have come to understand what really affects seed germination and seedling emergence during no-tillage (particularly the effects of slot disturbance and residue retention) examples of true allelopathy have become difficult to find. 

Effects of opener design: Low-disturbance opener designs include winged openers based on a central disc (e.g. Cross Slot®, Grade 4 cover, Baker et al, 1979a,b); double disc openers in non-sticky soils (Grades 1 and 2 cover, Karonka, 1963), some narrow knife openers operating in low residue conditions; some angled disc openers operating at slow speeds on flat ground and in non-friable soils. High-disturbance openers include most hoe, sweep and shank-types (eg Grades 2 and 3a cover, Anderson, 2003); angled discs operated at high speed and/or on hills (e.g. John Deere, Moore, Case, Grade 2 cover); double or triple disc openers in sticky soils; dished disc type openers; powered-till. At slower speeds, angled disc type openers might best be classified as medium disturbance (Grade 3b cover).

Conclusion: While most no-tillers 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 (Baker et al, 2001). Future incentive-programmes aimed at encouraging the adoption of no-tillage seem destined to begin specifying maximum allowable surface-disturbance levels in order to qualify for incentive benefits. If the no-tillage industry does not take notice of such criteria it will be doing itself a dis-service.

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