Getting to grips with the benefits and pitfalls of vertical tillage regimes
Vertical Tillage (VT) tools can run in wet soil and can help warm up cold soil for faster seed emergence - an important benefit in climates with a short growing season, says Mr Pitt. There are other benefits too; the elimination of the soil layers allows roots to probe deeper and access moisture and nutrients which are otherwise unavailable.
Agronomically speaking, vertical tillage (VT) is defined as any tillage operation that does not result in horizontal shearing or disturbance of the soil, which in turn leads to a soil density change within or below the tillage operation.
It is usually a shallow operation undertaken at planting depth, but it can be deeper, explains Mr Pitt, adding that the soil is not inverted and residue is kept on the surface.
Mr Pitt says: “The purpose is to achieve a level seedbed in one pass, effecting soil disturbance near the surface and the breaking up the residue.”
Typical VT tools enter and exit the soil in a vertical plane and include flat coulters, chisels, spikes, tines and in-line rippers, while conventional tillage is more horizontal, typically using sweeps, shares or regular concave disc blades.
“Any confusion regarding the definition of VT comes about either from the claimed objectives, or from the implement companies themselves trying to cash in on a trending market demand,” warns Mr Pitt.
Tools benefitting from this definitional blurring include shallow concave discs and fluted coulters. Such tools are really residue management weapons rather than ‘pure’ VT’. The main difference is in the realm of density reduction – the concavity or the fluting puts horizontal pressure into the soil and the dragging effect causes additional compaction.
VT for corn on corn rotations
The practice of VT probably grew out of the desire of ‘no-till’ farmers to plant more corn on corn, and following this strategy results in large amounts of corn stalk residue to contend with at planting.
“At first it was thought the more residue the better, but consistently high yields also require level plant heights and uniform emergence and plant distribution.”
VT reduces the size of the residue, bringing about a closer relationship with the soil and soil microbes to enhance decomposition. This usage led to the blurring of the definition – the straight coulters certainly avoided the soil density layers, but did not move enough soil or bury the right amount of residue.
The consequence was the introduction of shallow concave discs and fluted or rippled coulters, burying more residue while minimising the soil contact area. However, the greater the emphasis on residue management, the less true vertical tillage occurs, he notes.
“VT really took off with the development of high speed, wider width, hub-mounted straight coulter implements, usually incorporating levelling harrows and baskets. However, the horsepower (hp) required to drag the machines, and the high cost, tends to limit the application to the larger field acreages.
However as no-till, or more accurately min-till gained in popularity the VT implements were called upon to bury more and more residue. This is typically achieved by adding disc blades of varying concavity – the end result being more and more side-to-side soil movement and the consequent creation of horizontal layers.
Concave disc blades do not roll as freely through the soil, and any consequent drag will cause additional compaction. Any shallow VT tools can bury excessive amounts of residue in the seed zone, which in turn can increase disease potential and impair planter performance. Pure VT machines can run into problems with excessive residue, particularly when used with the newer high yielding Biotech (Bt) corn varieties.
More yield brings more residue, which in turn requires more dirt transfer to break it down. More residue also harbours more pests – such as slugs – which can seriously reduce yields.
“Finally, VT is not no-till, and therefore many of the perceived benefits of no-till will be lost,” emphasises Mr Pitt.
The market exists to satisfy a tillage need – and this need can include the breaking up of the various compaction levels within the soil; coping with soil erosion and tire tracks; breaking up residue; breaking up the soil crust etc.
However, VT cannot perform all these tasks, so many manufacturers simply blur the VT definition so that the machines they market can be branded as VT implements, he warns.
“The farmer needs to purchase the machine that best achieves his particular requirements, rather than adhering to any particular artistic license technology.
“For example, the farmer can employ VT deep soil rippers to break up the hard pan; improve aeration and moisture storage deeper in the soil profile, and then conventional tillage, or VT, to manage residue and soil density at the surface.”
Agronomic advances from cover crops
Cover crops can provide a benefit whatever the tillage philosophy of the farmer, says Mr Pitt. Moreover, they can help growers achieve many of the benefits of VT at a much cheaper cost, whilst the application of liquid manure can stimulate residue decomposition.
A cover crop is planted primarily to improve the agro-ecosystem rather than to be harvested and used to feed livestock or be sold, he explains. They range from various clover types and grasses to crops that can be harvested, such as winter wheat. When grown correctly, they can help reduce the need for agrichemical application, helping to protect the environment, while also providing a habitat for wildlife.
The original purpose was probably the prevention of soil erosion as fast-growing cover crops hold the soil in place and reduce crusting. In addition, their foliage protects the soil from falling rain, while below the ground longer term use increases water filtration and reduces runoff.
“VT proponents claim benefits of combining the two – fewer passes; earlier cover crop planting; wider equipment – but these benefits can typically be achieved through other fall tillage practices.
“The planting mechanism for cover crops depends a little on the cover crop chosen, but most commonly on the type of equipment the farmer already has.
“Cover crops can usually be drilled in to shallow depths or broadcast. No- till drilling is probably the most common method – cover crops are an important part of the no-till culture.
“Broadcasting always works best if followed by a light tillage pato incorporate. It will save a lot of time to mount the seeder directly on the VT implement, and will also improve the seed-to-soil contact if the residue is dealt with adequately.
“However, one drawback is that using concave blades may result in burying the seed too deeply. “Cover crops can also be planted simultaneously with corn or soybeans – offset of course – which will help growers to achieve the same time savings claimed from VT.”
Cover crops have further benefits too; for example, they can help slow soil erosion and water runoff, thereby reducing general water table pollution from agricultural chemicals.
Careful choice of cover crop crucial
Useful as they are, cover crops have to be chosen in accordance to what you want them to do and local climate taken into account – or growers can become disillusioned. For example, winter wheat develops in cooler fall temperatures where legumes might suffer, whereas legumes are better at converting nitrogen (N) gas in the atmosphere into soil nitrogen that plants can use, enabling an easy calculation of N fertiliser savings against the cost of the cover crop.
Cover crops also facilitate faster infiltration of excess surface water relieving compaction and generally improving the soil structure. They can also reduce the need for herbicides, points out Mr Pitt. This because in many cases they can out-compete weeds for water and nutrients; block sunlight and reduce soil temperatures to hinder weed development; provide a natural source of root compounds that provide natural herbicide effects.
Additionally, certain cover crops wage war on pests by hosting beneficial microbial life – beneficial insect predators and parasitoids; enable beneficial nematodes (round worms) and inhibit others. Their breakdown in turn adds beneficial organic matter to the fray and general enhanced nutrient recycling.
Depending on the farmer’s strategy, cover crops are usually either sprayed-off with a herbicide or left for the frost to kill-off. Their residues then often increase water infiltration and reduce evaporation, resulting in less moisture stress during drier crop-growing seasons.
“Timely spring killing of cover crops will avoid the negative impact of too much moisture in wet planting conditions, or living plants consuming too much moisture in drier years.”
There have been numerous studies of the quantitative impact of cover crops on their ability to increase yields and/or also reduce input costs. “The trade-offs in each crop area will be different but are certainly worthy of consideration,” says Mr Pitt.
Previous crops affect soil structure
He moves on to talk about crop rotation, pointing out the three major objectives; changing the soil composition and reducing disease, pest and weed pressure. “Fortunately, several disease pathogens have a narrow host range.
This means when a non-host crop is planted, the pathogen is unable to reproduce and inoculum levels are reduced. “Perhaps the most dramatic example is in soybeans, where the soybean cyst nematode population can be cut in half by rotating with corn or wheat.”
Rotation combats insects in a similar fashion, particularly insects that are fairly immobile and feed on a narrow range of crops, and overwinter as eggs or larvae.
However, certain cunning insects have adapted to rotation – either by developing a 2-year life cycle, or by laying eggs in one crop and feeding off the next one, he warns.
The application of differing chemistry herbicides for each crop can lead to better overall weed control. In addition, as they allow growers to use herbicides with different modes of action, they can play an important role in reducing the threat of resistance.
Certain summer crops, such as alfalfa, are harvested early and can also reduce weeds by being cut before germination as part of the harvest process. Further benefits from long rotations include improvements of the soil’s physical properties. For example, he adds, growing a hay crop will improve tilth and soil density by minimising erosion, the growth of roots and the formulation of humus from decomposition of those roots.
No-till systems can benefit from the incorporation of taproot and fibrous root crops in the rotation. Over time improved soil structure leads to an increase in the soil water-retaining capacity, while the development of macro pores promotes new root growth in successive crops. “The classic Iowa/Illinois rotation alternates high carbon to nitrogen ratio corn with low carbon to nitrogen ratio soybeans, which helps establish a diverse community of soil micro-organisms.
Soybeans release nitrogen through their roots and root exudates the following season (into the corn and if occurring at the right time can reduce the amount of nitrogen fertiliser required. Other legumes, such as alfalfa, can perform the same role.”
A second string to the bow
A secondary benefit of crop rotation is the management of risk and investment, emphasises Mr Pitt.
“Having a variety of rotating crops across the farming operation enables the farmer to spread out the preparation, planting and harvesting of each crop.
“This enables a more efficient use of labour and machinery. “Furthermore, the impact of severe weather – drought or flood – can be mitigated somewhat by having the crops at differing levels of maturation.
“This benefit also defines the rotation downside; all crops differ in their relative profitability in any given year, and so rotation inherently has a short-term opportunity cost in that the most profitable crop may not be in the biggest field.”
Crop residue options
Mr Pitt also discusses crop residue options, pointing out that they can be burnt, left on the surface, or covered by soil. No-till, mulch-till and ridge-till techniques deliberately leave the residue on the surface to protect the soil particles from wind and rain, he affirms.
The resultant ground cover prevents soil erosion and protects water quality, while its gradual decomposition improves soil tilth and adds organic matter.
“The reduced tillage reduces soil compaction, and saves time and energy. Moreover, in Saskatchewan (Canada), surface residue is used to trap snow to enhance soil temperatures for overwintering wheat.”
Burning stubble has effectively been prohibited in many jurisdictions. Its advantage, particularly in wheat and rice, is that it is a quick and cheap way to clear the field and kill weeds and pests. The downside is the loss of nutrients, smoke pollution and the risk of fires spreading out of control. It can have a viable use in times of bumper crops – too much straw to cope with; water-soaked fields too wet to till; late harvests/early freeze-ups leaving too little time to till; high clay soils giving rise to drainage/compaction issues also preventing tillage.
“Manitoba (Canada) typically has a lot of these factors, and consequently has a relatively high proportion of residue burning,” adds Mr Pitt. “The primary goal of conventional tillage is the producing of an environment most conducive to seed germination, growth and harvest. Burying surface residue removes its capacity to insulate the soil, and hence solar radiation will heat the soil more quickly.
“Tilling crop residue into the soil increases their decomposition rate and decreases organic matter content on the soil surface and in the shallow soil layers.
“The buried residue becomes more accessible to soil organisms and facilitates the incorporation of oxygen into the soil.
“Conventional tillage is also very effective at killing weeds, cover crops and perennials, while the burying of residue and weeds restricts the impact of harboured disease pathogens. Tillage can also incorporate the application of growth enhancers – and of course good seed-to-soil contact is ensured by breaking up the soil, while planting and harvesting performance is improved by having level fields and uniform seeding patterns and stands. “
When it comes to ‘burying trash’ tool choice for the job will depend on the type of tillage strategy used on the farm he points out. “Tillage is traditionally split into ‘before cultivation’ and ‘after cultivation’ and activities before cultivation are in turn classified into three: primary tillage, secondary tillage and seedbed preparation,” he explains.
Primary tillage is the cutting and inverting of the soil. It is done after the prior crop is harvested, or on fallow or virgin land that is being brought into production. The aim is to open the hard soil; separate the top soil from lower layers; invert the soil to bury residues if necessary, and uproot the weeds and stubbles. This in turn will aerate the soil and allow the infiltration of water.
Secondary tillage is much shallower and aims at the surface treatment of the soil. Its main objectives are the breaking up of clods; surface levelling; residue management; weed control, and the incorporation of fertilisers and other chemicals.
Activities after cultivation can include the side dressing of fertilisers; “earthing up” – the provision of extra support at the base of the crop as in potatoes, or to provide irrigation channels, and inter-cultivation – working
the soil between the rows to control weeds or to conserve moisture.
Burying residue has become a more significant challenge as a result of the use of higher yielding corn varieties with thicker, tougher stalks, particularly in a corn-on-corn planting scheme. The basic idea is to get the residue laced into the soil but to leave pieces of it sticking up so that oxygen can be drawn down to facilitate its decomposition