Tillage systems are often 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 significantly reduced (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 or silage is one example). Although residue cover greatly influences erosion potential, it also is affected by factors such as surface roughness and soil loosening.
Another distinction of tillage systems is whether they are full-field systems or restricted systems. The benefits and limitations of various tillage systems are compared in table 16.1.
|Table 16.1: Tillage System Benefits and Limitations|
|Moldboard plow||Allows easy incorporation of fertilizers and amendments.|
Buries surface weed seeds.
Allows soil to dry out fast. Temporarily reduces compaction.
|Leaves soil bare.|
Destroys natural aggregation and enhances organic matter loss.
Commonly leads to surface crusting and accelerated erosion.
Causes plow pans.
Requires high energy use.
|Chisel Plow||Same as above, but leaves some surface residues||Same as above, but less aggressively destroys soil structure; leads to less erosion, less crusting, no|
plow pans; requires less energy us
|Disk harrow||Same as above.||above|
|No-till||Leaves little soil disturbance.|
Requires few trips over field.
Requires low energy use.
Provides the most surface residue cover and erosion protection.
|Makes it more difficult to incorporate fertilizers and amendments.|
Makes wet soils dry and warm up slowly in spring.
Can’t alleviate compaction.
|Zone-till||Same as above.||Same as above but compaction is alleviated.|
|Ridge-till||Allows easy incorporation of fertilizers and amendments.|
Provides some weed control as ridges are built.
Allows seed zone on ridge to dry and warm more quickly.
|Is hard to use with sod-type or narrow-row crop in rotation|
Requires wheel spacing to be adjusted to travel between ridges.
A full-field system manages the soil uniformly across the entire field surface. Such conventional 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 (see figure 16.3, left), chisels (figure 16.3, right), and heavy disks (figure 16.4, left), while secondary tillage is accomplished with finishing disks (figure 16.4, right), tine or tooth harrows, rollers, packers, drags, etc. These tillage systems create a uniform and often finely aggregated seedbed over the entire surface of the field. Such systems appear to perform well because they create near ideal conditions for seed germination and crop establishment.
But moldboard plowing is also energy intensive, leaves very little residue on the surface, and often requires multiple secondary tillage passes. It tends to create dense pans below the depth of plowing (typically 6 to 8 inches deep). However, moldboard plowing has traditionally been a reliable practice and almost always results in reasonable crop growth. Chisel implements generally provide results similar to those of the moldboard plow but require less energy and leave significantly 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.
Disk plows come in a heavy version, as a primary tillage tool that usually goes 6 to 8 inches deep, or a lighter one that performs shallower tillage and leaves residue on the surface. 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 is not sustainable in the long run.
Although full-field tillage systems have their disadvantages, they can help overcome certain problems, such as compaction and high weed pressures. Organic farmers often use moldboard plowing as a necessity to provide adequate weed control and 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, full-field tillage systems with intensive secondary tillage crush the natural soil aggregates. The pulverized soil 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.
Full-field tillage systems can be improved by using tools, such as chisels (figure 16.3, right), that leave some residue on the surface. Reducing secondary tillage also helps decrease negative aspects of full-field 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 through the use of modern conservation planters, which create a finely aggregated zone around the seed without requiring the entire soil width to be pulverized. A good planter is perhaps the most important secondary tillage tool, because it helps overcome poor soil-seed contact without destroying surface aggregates over the entire field. A fringe benefit of reduced secondary tillage is that rougher soil often has much higher water infiltration rates and reduces problems with settling and hard-setting after rains. Weed seed germination is also generally reduced, but pre-emergence herbicides tend to be less effective than with smooth seedbeds. Reducing secondary tillage may, therefore, require greater emphasis on post-emergence weed control.
In more intensive horticultural systems, powered tillage tools, which are actively rotated by the tractor power takeoff system, are often used (figure 16.5). Rotary tillers (rotovators, rototillers) do intensive soil mixing that is damaging to soil in the long term. They should be considered only 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.
Restricted Tillage Systems
These systems are based on the idea that tillage can be limited to the area around the plant and does not have to disturb the entire field. Several tillage systems—no-till, zone or strip-till, and ridge-till—fit this concept.
BEFORE CONVERTING TO NO-TILL
An Ohio farmer asked one of the authors of this book what could be done about a compacted, low-organic-matter, and low-fertility field 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. 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 later on.
The no-till system loosens the soil only in a very narrow and shallow area immediately around the seed zone. This localized disturbance is typically accomplished with a conservation planter (for row crops) or seed drill (for narrow-seeded crops; figure 16.6). This system represents the most extreme change from conventional tillage and is most effective in preventing soil erosion and building organic matter.
|Table 16.2 The Effect of 32 Years of Plow and Tillage under Corn Production on Selected Soil Health Indicators|
|Soil Health Indicator||Plow-Tillage||No-Tillage|
|Aggregate stability (%)||22||50|
|*Bulk density (g/cm3)||1.39||1.32|
|*Penetration resistance (psi)||140||156|
|Plant-available water capacity (%)||29.1||35.7|
|Infiltration capacity (mm/hr)||1.58||1.63|
|Early-season nitrate-N (lbs/ac)||13||20|
|Organic matter (%)||4.0||5.4|
|Cellulose decomposition rate (%/week)||3.0||8.9|
|Potentially mineralizable nitrogen (mg/g/week)||1.5||1.7|
|Easily extractable glomalin (mg/g/soil)||1.2||1.7|
|Total glomalin (mg/g soil)||4.3||6.6|
No-till systems have been used successfully on many soils in different climates. The surface residue protects against 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—reduces the susceptibility to drought. This tillage system is especially well adapted to coarse-textured soils (sands and gravels) and well-drained soils, as these tend to be softer and less susceptible to compaction. No-till systems sometimes have initial lower yields than conventional tillage systems. One of the reasons for this is the lower availability of N in the early years of no-till. Knowing this allows you to compensate by adding increased N (legumes, manures, fertilizers) during the transition years. It takes a few years for no-tilled soils to improve, after which they typically out-yield conventionally tilled soils. The transition can be challenging because a radical move from conventional to no tillage can create failures if the soil was previously degraded and compacted. It is best to first build degraded soils with organic matter management and use intermediate tillage methods, as described in the next sections.
Organic No Till?
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, front mounted, heavy roller-crimper, resulting in a weed-suppressing mat through which it is possible to plant or drill seeds (figure 16.7) or set transplants. For this system to work best sufficient time must be allowed for the cover crop to grow large before rollingcrimping, 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 not be fully matured to avoid viable seeds that could become weeds in the following crop. Since timing of any farm operation is critical, careful attention to the details of these biologically based systems is needed for them to be successful.
With the absence of tillage, seed placement, compaction prevention, and weed control become more critical. No-till planters and drills (figure 16.6) are advanced pieces of engineering that need to be rugged and adaptable to different soil conditions yet be able to place a seed precisely at a specified depth. The technology has come a long way since Jethro Tull’s early seeders, especially in the past decades when no-till seeders have been continually improved.
The quality of no-tilled soil improves over time, as seen in table 16.2, which compares physical, chemical, and biological soil health indicators after thirty-two years of plow and no tillage in a New York experiment. The beneficial effects of no tillage are quite consistent for physical indicators, especially with aggregate stability. Biological indicators are similarly more favorable for no tillage, 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.
Zone, strip, and ridge tillage.
Zone-, strip-, and ridge-tillage systems are adapted to wide-row crops with 30-inch spacing or more. Their approach is to disturb the soil in a narrow strip along the plant row and leave on the front (figure 16.9). The planter creates a fine seedbed approximately 6 inches wide by 4 inches deep and uses trash wheels to move residue away from the row. Zone tillage provides soil quality improvements similar to those of no tillage, but it is more energy intensive. It is generally preferred over strict no-tillage systems on soils that have compaction problems (for example, fields that receive liquid manure or where crops are harvested when the soil is susceptible to compaction) and in humid and cold climates, where removal of residue from the row is desirable for soil drying and warm-up.
Strip tillage (figure 16.8, right) uses a similar approach, but the tillage shanks are shallower (typically to 8 inches), thereby reducing energy consumption. In temperate climates, zone building and strip tillage 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.
The zone planter (figure 16.9) can also be used as a single-pass system when deeper disturbance is not needed.
Ridge tillage (figure 16.10) 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 ridging operation can be combined with mechanical weed control and allows for band application of herbicides. Ridge tillage often 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.