16.2: Tillage Systems

Tillage systems can be classified by the amount of surface residue left on the soil surface. Conservation tillage systems leave more than 30% of the soil surface covered with crop residue. This amount of surface residue cover is considered to be at a level where erosion is reduced by more than half (see Figure 16.2). Of course, this residue cover partially depends on the amount and quality of residue left after harvest, which may vary greatly among crops and harvest method (corn harvested for grain versus silage is one example). Although residue cover greatly influences erosion potential, it also is affected by factors such as surface roughness and soil loosening.

Each pass of a tillage tool incorporates some residue and thereby reduces the amount of residue on the surface that helps reduce runoff and erosion. Table 16.2 shows estimates of the percent residue that remains on the soil surface after different tillage passes. In cases where one pass is followed by another, the remaining residue cover can be estimated by multiplication. For example, starting with 80% residue cover after a grain corn crop harvest and over-wintering, the sequence of 1) a chisel with straight points, 2) a tandem disk, 3) a field cultivator and 4) a row crop planter is expected to leave 0.8 (80%) x 0.7 (70%) x 0.45 (45%) x .75 (75%) = 0.19, or an estimated 19% of residue remaining on the surface, thereby not qualifying as conservation tillage. By eliminating the tandem disk and keeping the soil slightly rougher, the residue level will be 42%.

Table 16.2 Estimated Crop Residue Levels Remaining After Field Operations

Field operation	After corn or cereals	After soybeans
After harvest	90–95%	60–80%
Over-winter decomposition	80–95%	70–80%
Moldboard plow	0–10%	0–5%
Chisel (twisted points)	50–70%	30–40%
Chisel (straight points)	60–80%	40–60%
Disk plow	40–70%	25–40%
Disk, tandem-finishing	30–60%	20–40%
Field cultivator	60–90%	35–75%
Row-crop planter	85–95%	60–70%
Note: Speed, depth and soil moisture can affect the amounts. Source: USDA-NRCS

Another distinction of tillage systems is whether they are full-width systems or restricted-width systems. The former disturbs the soil across the entire field, while restricted tillage limits various degrees of soil loosening to narrow zones in the crop row. The benefits and limitations of various tillage systems are compared in Table 16.1.

Figure 6.3. Conservation tillage system (full width) that leaves at least 30 percent or more residue on the surface. Residue adequately controls erosion by both wind and water on this soil type. Photo courtesy of USDA-NRCS.

Conventional Tillage

A full-width system manages the soil uniformly across the entire field surface. Such tillage systems typically involve a primary pass with a heavy tillage tool to loosen the soil and incorporate materials at the surface (fertilizers, amendments, weeds, etc.), followed by one or more secondary passes to create a suitable seedbed. Primary tillage tools are generally moldboard plows (Figure 16.4, left), chisels (Figure 16.4, right) and heavy disks (Figure 16.5, left), while secondary tillage is accomplished with finishing disks (Figure 16.5, right), tine or tooth harrows, field cultivators, roller-packers, etc. These tillage systems create a uniform and often finely aggregated seedbed over the entire surface of the field and thereby good conditions for seed germination and crop establishment. Before farming was mechanized, farmers would use broadcast seed applications by throwing seeds out by hand followed by harrowing, but this task is now accomplished with mechanical planters. If a good seedbed is prepared the planter does not require special attachments to deal with surface residues or firm soil.

Figure 16.4.Left: Moldboard plowing inverts the soil and leaves no surface protection. Right: Chisel plow shanks loosen soil and leave some residue cover.

But moldboard plowing is also energy intensive, leaves very little residue on the surface, tends to result in high organic matter (carbon) losses and requires secondary tillage passes (Table 16.1). It also tends to create dense pans below the depth of plowing (typically 6–8 inches deep). However, moldboard plowing has traditionally been a reliable practice and almost always results in reasonable crop growth. Chisel implements provide similar results but require less energy, allow for faster speeds and leave more residue on the surface. Chisels also allow for more flexibility in the depth of tillage, generally from 5 to 12 inches, with some tools specifically designed to go deeper, which may be useful for breaking up compacted layers.

Disk plows come in a heavy version, as a primary tillage tool that usually goes 6–8 inches deep, or in a lighter version that performs shallower tillage and leaves residue on the surface (Figure 16.5). Disks also create concerns with developing tillage pans at their bottoms. They are sometimes used as both primary and secondary tillage tools through repeated passes that increasingly pulverize the soil. This limits the upfront investment in tillage tools, but it is not sustainable in the long run because it does a lot of soil disturbance.

Figure 16.5.Left: A heavy disk (disk plow) can be used for primary and secondary tillage.Photo by Mark Brooks. Right: A finishing disk.

Full-width tillage systems clearly have disadvantages, but they can help overcome certain problems such as surface compaction (temporarily at least, but they create more compaction over time), high weed pressures and the challenges of terminating a previous crop or cover crop. Although no-till options exist for some organic crop sequences, organic farmers often use moldboard plowing as a necessity to provide adequate weed control (a big challenge without herbicides) and to facilitate nitrogen release from incorporated legumes. Livestock-based farms often use a plow to incorporate manure and to help make rotation transitions from sod crops to row crops.

Besides incorporating surface residue, plowing with intensive secondary tillage crushes the natural soil aggregates and promotes decomposition of organic matter that had been protected inside but is now accessible to soil organisms. Some conservationists say that inverting the soil by moldboard plowing is very unnatural. Soil in its natural state is never turned over, inverting and burying surface plant residues. (Earthworms and other critters do that without inverting the entire soil.) The pulverized soil after plowing also does not take heavy rainfall well. The lack of surface residue causes sealing at the surface, which generates runoff and erosion and creates hard crusts after drying. Intensively tilled soil will also settle after moderate to heavy rainfall and may “hardset” upon drying, thereby restricting root growth.

Reducing secondary tillage also helps decrease negative aspects of full-width tillage. Compacted soils tend to till up cloddy, and intensive harrowing and packing are then seen as necessary to create a good seedbed. This additional tillage creates a vicious cycle of further soil degradation and intensive tillage. Secondary tillage often can be reduced with the use of modern conservation planters, which create a finely aggregated zone around the seed without requiring the entire soil to be pulverized. A good planter is perhaps the most important tillage tool because it helps overcome rough seedbeds without destroying surface aggregates over the entire field. A fringe benefit of reduced secondary tillage is that rougher soil often has higher water infiltration rates and reduces problems with settling and hardsetting after rains.

Vertical tillage is a concept that incorporates a range of tillage tools that do not move the soil from side to side but mostly move it vertically with limited compaction. This generally includes tools with large rippled or wavy coulters, and blades that are aligned with the direction of travel and cut into crop residue or push it into the soil. Sometimes they are combined with a field cultivator, light chisel-type tools, finishing tines or rolling baskets to level the ground. They may also be used with fertilizer applicators.

Figure 16.6.Powered tillage tools used with horticultural crops: Left: rotary tiller; Right: spader.

In more intensive horticultural systems, powered tillage tools are often used, which are actively rotated by the tractor power takeoff system (Figure 16.6). Rotary tillers (rotovators, rototillers) do very intensive soil mixing and create fine uniform tilth that is advantageous when establishing horticultural crops that are small seeded or sensitive to compaction. But it is quite damaging to soil in the long term, which can only be sustainable if the soil also regularly receives organic materials like cover crop residue, compost or manure. A spader is also an actively rotated tillage tool, but the small spades, similar to the garden tools, handle soil more gently and leave more residue or organic additions at the surface than a rototiller.

Figure 16.7. A modern row-crop planter for conservation tillage systems. Coulters in front and closing wheels in the back allow for seed placement without soil preparation; equipment positioning is controlled by a GPS system; seed depth is controlled by hydraulics; seed is delivered by vacuum; and seed placement is digitally monitored. Photo by Larissa Smith.

Restricted Tillage Systems

These systems are based on the idea that tillage can be limited to the zone immediately adjacent to the crop and does not have to disturb the entire area between crop rows. Several tillage systems—no-till, strip-till (similar to zone-till) and ridge-till—fit this concept. No-till system. The no-till system was developed on the concept that soil disturbance is not needed as long as good seed placement and weed control can be achieved. The planter only loosens the soil in a very narrow and shallow zone immediately around the seed. This highly localized disturbance is typically accomplished with a no-till planter (for row crops; Figure 16.7) or seed drill (for crops seeded in narrow rows; Figure 16.8). This system represents the most extreme change from conventional tillage and is most effective in preventing soil erosion and building both organic matter and overall soil health.

No-till systems have been used successfully on many soils in different climates. The surface residue protects against water and wind erosion (Figure 14.3) and increases biological activity by protecting the soil from temperature and heat extremes. Surface residues also reduce water evaporation, which, combined with deeper rooting, lowers the susceptibility to drought. This tillage system is especially well adapted to coarse-textured soils (sands and gravels) and to well-drained soils, as these tend to be softer and less susceptible to compaction. No-till systems sometimes experience lower crop yields than conventional tillage systems in the early transition years but tend to outperform them after the soil ecosystem has adapted. Reasons for this are the lower availability of N in the early years of no-till, cooler soil conditions and the compaction that needs to be overcome through natural biological processes like earthworm activity and cover cropping. Knowing this allows you to compensate by adding increased N (legumes, manures, fertilizers) during the transition years.

Figure 16.8.Left: A no-till seed drill requires no-till or seedbed preparation for narrow-seeded crops or cover crops. Right: The cross-slot opener used in no-till planters. The disk slices soil, the inverted T blade allows seed and fertilizer placement on opposite ends of the disk, and the packer wheels (right side) close and firm the seedbed.

Before Converting To No-Till

An Ohio farmer asked one of the authors of this book what could be done about a compacted field with low organic matter and low fertility that had been converted to no-till a few years before. Clearly, the soil’s organic matter and nutrient levels should have been increased and the compaction alleviated before the change. Once you’re committed to no-till, you’ve lost the opportunity to easily and rapidly change the soil’s fertility or physical properties (aside from growing cover crops that can lessen compaction). The recommendation is the same as for someone establishing a perennial crop like an orchard or vineyard. Build up the soil and remedy compaction problems before converting to no-till. It’s going to be much harder to do so later on.

Table 16.3 The Effect of 32 Years of Plow and No-Till Under Corn Production on Selected Soil Health Indicators in a New York Experiment

	plow	no-till
Physical
Aggregate stability (%)	22	50
*Bulk density (g/cm^3)	1.39	1.32
*Penetration resistance (psi)	140	156
Permeability (mm/hr)	2.1	2.4
Plant-available water capacity (%)	29.1	35.7
Infiltration capacity (mm/hr)	1.58	1.63
Chemical
Phosphorus (lbs/ac)	13	20
Potassium (lbs/ac)	20	21
Magnesium (lbs/ac)	88	95
Early season nitrate-N (lbs/ac)	310	414
Calcium (lbs/ac)	7,172	7,152
*pH	8	7.8
Biological
Organic matter (%)	4	5.4
Cellulose decomposition rate (%/week)	3	8.9
Potentially mineralizable nitrogen (µg/g/week)	1.5	1.7
Total protein (mg/g soil)	4.3	6.6
Note: Higher values indicate better health, except for those listed with an asterisk, for which lower values are better. Source: Moebius et al. (2008)

The transition can be challenging because a radical move from conventional to no-till is a big shock to a soil system that has been routinely loosened. It can especially create challenges if the soil was previously degraded and compacted. It is then best to first build the soil with organic matter, cover crops and strip-till (zone-till) methods as described in the next sections. In the absence of tillage, seed placement, compaction prevention and weed control become more critical. No-till planters and drills (figures 16.7 and 16.8) are advanced pieces of engineering that need to be adaptable to different soil conditions yet also be able to place a seed precisely at a specified depth. This technology has come a long way since Jethro Tull’s early seeders.

Figure 16.9.Left: A strip-till tool with hilling disks and rolling basket to create a zone of loosened soil.Photo by Robert Schindelbeck.Right: Strip-till after corn harvest results in a narrow tilled zone that leaves much of the soil surface undisturbed.Photo by Georgi Mitev.

The quality of no-tilled soil improves over time, as seen in Table 16.3, which compares physical, chemical and biological soil health indicators after 32 years of plow and no-till in a New York experiment. The beneficial effects of no-till are quite consistent for physical indicators, especially with aggregate stability. Biological indicators are similarly more favorable for no-till, and organic matter content is 35% higher than with plow tillage. The effects are less apparent for chemical properties, except the pH is slightly more favorable for no-till, and the early season nitrate concentration is 50% higher. Other experiments have also demonstrated that long-term reduced tillage increases nitrogen availability from organic matter, which may result in significant fertilizer savings.

Strip (zone) and ridge tillage. These tillage systems are adapted to row crops. Their approach is to disturb the soil in a narrow strip along the plant row and leave most of the soil surface undisturbed. Strip-till involves the use of shanks and coulters (Figure 16.8) that create a loosened band that extends 6–16 inches into the subsoil. Lower depths may be appropriate in the first years after conversion from conventional tillage to promote deeper root growth and water movement. Strips at shallower depths can be used after soil health has been improved, saving energy. Strip-till is often followed by a row crop planter with coulters mounted on the front that can handle a range of soil tilth conditions (Figure 16.7). Strip-till provides soil quality improvements similar to those of no-till, but it is more energy intensive. It is generally preferred over strict no-till systems on soils that have compaction problems (for example, fields that receive liquid manure or where crops are harvested when the soils are wet), have imperfect drainage, or are in humid, cool climates. In those situations the removal of residue, slight raising of strips, and soil loosening in the row are desirable for soil drying, warm-up and rooting. In temperate climates, strip-till and zone building are often performed in the fall before spring row crop planting to allow for soil settling. Some farmers inject fertilizers with the tillage operations, thereby reducing the number of passes on the field.

Zone tillage uses the same approach as strip-till: restricting soil loosening to a narrow zone along the crop row. It uses a narrow shank to slit-loosen the soil (Figure 15.5, right) and relies on fluted coulters on the planter to create a residue-free strip. The end result is similar to strip-till.

Ridge tillage combines limited tillage with a ridging operation and requires controlled traffic. This system is particularly attractive for cold and wet soils because the ridges offer seedlings a warmer and better drained environment. The minimal drainage derived from the slightly elevated ridge (often only a few inches) can be beneficial to get seedlings through a very wet period in the early season. The ridging operation can be combined with mechanical weed control and allows for band application of herbicides. This decreases the cost of chemical weed control, allowing for about a two-thirds reduction in herbicide use.

In vegetable systems, raised beds—basically wide ridges that also provide better drainage and warmer temperatures—are often used. Potatoes, for example, require hilling of the ridges to encourage new tubers and to keep them covered. In parts of Africa, contour ridges are popular as a soil conservation practice.

Tillage and cover crops. Combining reduced tillage and cover cropping provides great benefits for soil health. It also offers opportunities for organic crop production where weed suppression is generally a large challenge and the reason for using a plow. Researchers at the Rodale Institute in Pennsylvania have developed innovative cover crop management equipment that facilitates growing row crops in a no-till system. An annual or winter annual cover crop is rolled down with a specially designed heavy roller-crimper, resulting in a weed-suppressing mulch mat through which it is possible to plant or drill seeds (Figure 16.10, left) or to set transplants. For this system to work best, sufficient time must be allowed for the cover crop to grow large before rolling-crimping so that the mulch can do a good job of suppressing weeds. Cover crops must have gone through the early stages of reproduction in order for the roller-crimper to kill them but must not be fully matured to avoid viable seeds that could become weeds in the following crop. A similar approach can be used with a wider variety of cover crop mixes, or even previous perennial rotation crops in non-organic systems. Planting green is a concept where a row crop is no-till planted into an actively growing cover crop (Figure 16.10, right). This allows the benefits of the cover crop to be maximized by extending its growing period rather than killing it 2–3 weeks ahead of planting, which is especially beneficial in cool climates. Planting green is still a relatively new practice but can provide good benefits with adequate attention to cover crop termination and planter equipment details.

Figure 16.10.Planting green with a roller-crimper creates a weed-suppressing cover-crop mat through which it is possible to plant seeds or transplants. Left: A row crop is being planted immediately behind rye that has been flattened by the roller-crimper.Photo by Jeff Mitchell. Right: Planting green into a crimson clover cover crop.Photo by Heidi Kaye.