Chapter 17

Soil Management for Field Crops

Conventional Tillage

Conventional tillage is the traditional method of farming in which soil is prepared for planting by completely inverting it with a tractor-pulled plow, followed by additional tillage to prepare the seedbed for planting. Tillage operations used vary considerably for different crops and in different regions. Conventional tillage entails turning under and thoroughly mixing crop residues or cover crops into the top 6 to 10 inches (15–25cm) of soil. The goal of conventional tillage is to leave a residue and vegetation-free soil surface, with a uniformly mixed soil horizon to the plow depth. Conventional tillage systems typically involve a primary pass with a heavy tillage tool (e.g., moldboard plow) to loosen the soil and incorporate materials at the surface (e.g., fertilizers, amendments, weeds, etc.), followed by one or more secondary passes, often referred to as secondary tillage, to create a suitable seedbed. Secondary tillage operations are typically performed with a variety of tillage implements such as a disk harrow, field cultivator, or spring-tooth harrow. Removing all residue from the soil surface and disturbing the soil surface greatly increase the potential for soil erosion. The potential for water erosion is less in flat fields, but the potential for wind erosion is high.

Primary Tillage

Primary tillage is the first soil tillage after the last harvest, which involve loosening the soil, inverting the soil, and uprooting weeds and crop stubble. Primary tillage techniques are always aggressive and usually carried out at considerable depth, leaving an uneven soil surface. It tends to leave large chunks, or clods, of soil in its wake that require additional tillage to break up the clods and level the soil surface prior to planting.

Secondary Tillage

In secondary tillage, the soil is not worked as aggressively or as deeply as in primary tillage. Secondary tillage is usually shallower and aimed at surface treatment of soils. Its purpose is usually for breaking up clods, leveling the surface, resizing residue, controlling weeds, incorporating fertilizers and preparing the seedbed for planting. Secondary tillage produces a layer of soil with particles that are smaller than that of primary tillage. This gives the most seed-to-soil contact which allows good germination. It will also limit the moisture lost to evaporation.

Conventional Tillage Practices

Conventional tillage entails turning under and thoroughly mixing crop residues or cover crops into the top six to eight inches of soil. Most often, this complete form of tillage is accomplished by inverting the soil with a moldboard plow, followed by successively shallower harrow passes. A harrow is a farm implement used to level and smooth soil, break up clumps, and control weed growth. It is often used in conjunction with primary tillage to prepare fields for planting crops. There are several different types of harrows that can be used to penetrate the soil to perform their tasks. The most common type is the disk harrow, which has a series of concave disks that are mounted on a frame.

Timing of Tillage

The decision to till in the fall or spring will be dictated by many factors that are not easy to control. The two main factors for tillage in the fall or spring are soil moisture conditions and soil temperature. These two factors can have significant impact on soil fracturing, tillage depth, clod size, and level of soil compaction. Therefore, soil moisture and soil temperature can influence tillage practice, and ultimately yield and soil quality performance.

Limitations of Conventional Tillage

While conventional tillage can be a critical component in a successful and profitable farming operations excessive tillage is harmful to soil health in a number of ways. The limited amount of residue left on fields from conventional tillage leaves soils more vulnerable to wind and water erosion. An additional problem related to soil erosion is sedimentation. Sediment and other materials (such as pesticides and nutrients) from eroding fields increase water pollution, reduce storage capacities of lakes and reservoirs, and decrease the effectiveness of surface drainage systems.

Conventional Tillage Implements

Tillage implements typically used as the primary conventional tillage operation are subsoilers, moldboard plows, chisel plows, and disk harrows. Secondary operations often include field cultivators and spike-, spring-, and tine-toothed harrows, rotary harrows, and packers. Combination tools that perform primary and secondary tillage, referred to as one-pass tools, are often used.

Subsoiler

The subsoiler is considered primary tillage and is typically used after harvest to reset the vertical profile of the soil and to alleviate soil compaction (Figure 17.2). It operates as deep as practical in a given soil and geographic area—generally 12 to 20 inches (35–51cm). It can be considered as “true vertical” tillage as it buries very little crop residue, but the amount of soil disturbance depends on the shape of the shank and the working angle of the tool bar.

Moldboard Plow

The tool most often associated with primary tillage is the moldboard plow that cuts, breaks, loosens, and inverts the soil (Figure 17.3). It is typically used after harvest to bury as much as 80 to 90 percent of crop residue. It is extremely aggressive in lifting and inverting 9 to12 inches (23–30cm)of soil and requires one or more passes with a secondary tillage tool to make the soil suitable for planting. Most moldboard plows work best within a specific speed range. At low speeds, the plow may not fracture the soil and will leave more residue on the surface. By increasing the speed, the soil clumps will be broken down into finer sizes and the plow will bury more residue.

Chisel Plow

Chisel plows are used for primary tillage. Chisel plows are used for loosening the soil without inverting it, thus leaving residues on the soil surface (Figure 17.4). Chisel plows generally provide results similar to those of the moldboard plow but require less energy and leave significantly more residue on the surface. The chisel plow produces a rough surface and generally leaves 50 to 70 percent of the existing corn or grain sorghum residue on the surface depending on chisel point selection, shank spacing, operating speed, and depth. Straight, narrow points, about 2" wide, leave the most residue. For proper shattering action, chisel plows must run at speeds between five and seven miles per hour (8 and 11 km/h) and on soils dry at working depth.

Disk Plow

A disk plow is a heavy-duty agricultural implement that is used to prepare soil for planting crops (Figure 17.6). It consists of a set of concave disks mounted on a frame and mounted on a tractor. When the disk plow is dragged forward by the tractor, the disk rotates around its central axis, the soil is cut around the disk, and the plowed soil rises along the concave surface of the rotating disk and turns sideways and rearward. Furrows are left after plowing.

Disk Harrow

This group of tillage tools falls into the category of primary tillage that, in some cases, is also used in secondary tillage operations (Figure 17.7). Disk harrows are primarily used to level the soil surface, redistribute surface residue, pulverize clods and disturb germination of weeds. Disk harrows are not very effective against root-propagating weeds. They are considered conventional (or horizontal) tillage tools, often designed to kill weeds and loosen the soil. Residue is mixed into the soil about three-quarters of the depth of tillage. It will leave 30 to 70 percent of residue after one pass. It breaks up the residue and loosens the soil surface. Use of a disk harrow in wet soil conditions can lead to compaction at the tillage depth.

Tandem versus Offset Harrows. On a true tandem harrow, the front and rear gangs are directly across from each other. On an offset harrow, the left side will be slightly staggered from the right side, so that the front gangs overlap slightly, eliminating the middle ridge.

Field Cultivator

A field cultivator is an implement used to perform secondary tillage operations such as seedbed preparation and weed eradication (Figure 17.8). Field cultivators shatter clumps and sort them by size. Coarser lumps are brought to the surface while finer ones accumulate at lower depths where the seeds are planted. This also brings residue to the soil surface. Field cultivators are often wider than chisel plows because less power is required per unit of width. Field cultivators generally leave the fields with most of the crop residue remaining or mixed into the upper soil surface and the fields are ridged, rough, and open so that soil moisture infiltration is enhanced.

Spike and Spring-Toothed Harrows

Spike and spring-toothed harrows are effective in breaking remaining soil crusts and clods from primary tillage, smoothing and firming the soil surface, aerating the soil, and pulling emerging weeds. Spike-tooth harrows have rigid spikes (or tines) that often are mounted on an angle to flow over residue (Figure 17.9). They are more aggressive than spring-tooth harrows and will bust up crust and clods and smooth the soil. It will stir the soil to a depth of about 12 inches (5cm), if weighted.

Rotary Harrow

Rotary harrows fluff up crop residue, bust clods, pulverize soil, lift up weeds, and dry out soil (Figure 17.10. Rotary harrows redistribute residue and levels the ground. Tines disturb only the top inch or so of soil. In general, the greater a tool is angled off of the tool bar the more that operation will disturb the soil. They are used for secondary tillage.

Roller Harrow

The single rolling basket harrow breaks clods and churns the soil (Figure 17.11). The roller penetrates up to 2 inches deep, mixing and firming the soil. The unique design allows the roller to operate in either aggressive or passive. Typically mounted with field cultivators and other straight frame secondary tillage tools, also available on disks.

Packers

Packers prepare the seedbed for planting (Figure 17.12). Packers consist of rollers made up with wheels with various types of open edges. Packers are used to finish preparing the seedbed by thoroughly pulverizing and firming the loose soil so that there will be no large air spaces or pockets.

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