Building Soils for Better Crops, 2nd Edition

	Preventing and Lessening Compaction A lasting injury is done by ploughing land too wet. S. L. Dana, 1842 We've discussed the benefits of cover crops, rotations, reduced tillage, and organic matter additions for improving soil structure. However, these practices still may not prevent compacted soils unless specific steps are taken to reduce the impact of heavy loads from field equipment and inappropriately timed field operations. The causes of compaction were discussed in chapter 6, and in this chapter we'll discuss strategies to prevent and lessen soil compaction. The first step is to decide whether compaction is a problem and which type is affecting your soils. The symptoms, as well as remedies and preventive measures, are summarized in table 14.1. Crusting and Surface Sealing Crusting and surface sealing are easy to see at the soil surface after heavy rains in the early growing season, especially with clean-tilled soil. Keep in mind that it may not happen every year, if heavy rains do not occur before the plant canopy protects the soil from direct raindrop impact. Certain soil types, such as sandy loams, are particularly susceptible to crusting. Their aggregates usually aren't very stable and, once broken down, the small particles fill in gaps between the larger particles. The impact of surface crusting is most damaging when heavy rains occur between planting and seedling emergence. The hard surface that forms may delay seedling emergence and growth until the crust mellows with the next rains. If such showers do not occur, the crop may be set back considerably. Crusting and sealing of the soil surface also reduce water infiltration capacity. This increases runoff and erosion, and lessens the amount of available water for crops. Reducing Surface Crusting Crusting is a symptom of poor soil structure that develops especially with intensively and clean-tilled soils. As a short-term solution, farmers sometimes use tools, such as rotary hoes, to break up the crust. The best long-term approach is to reduce tillage intensity, use tillage systems that leave residue or mulch on the surface, and improve aggregate stability with organic matter additions. Even residue covers as low as 30 percent will greatly reduce crusting and provide important pathways for water entry. A good heavy-duty conservation planter with rugged coulter blades for in-row soil loosening, tine wheels to remove surface residue from the row, and accurate seed placement is a key implement because it can successfully establish crops without intensive tillage (see chapter 15). Reducing tillage and maintaining significant amounts of surface residues not only prevent crusting, but also rebuild the soil by reducing decomposition of organic matter. Practices that improve soil structure, such as cover cropping, rotations with perennial crops, and adding organic materials, also help reduce crusting problems. Soils with very low aggregate stability may sometimes benefit from surface applications of gypsum (calcium sulfate). The added calcium and the effect of the greater salt concentration in the soil water both promote aggregation. Plow Layer and Subsoil Compaction Deep wheel tracks, extended periods of saturation, or even standing water following a rain or irrigation may indicate plow layer compaction. Compacted plow layers also tend to be extremely cloddy when tilled. A shovel can be used to visually evaluate soil structure and rooting. This is best done when the crop is in an early stage of development, but after the rooting system had a chance to get established. Digging can be very educational and provide good clues to the quality of the soil. If you find a dense rooting system with many fine roots that protrude well into the subsoil, you probably do not have a compaction problem. Compacted soil shows little aggregation, is more difficult to dig, and will dig up in large clumps rather than granules. Compare the difference between soil and roots in wheel tracks and nearby areas. Roots in a compacted plow layer are usually stubby and have few root hairs. They also often follow crooked paths as they try to find zones of weakness. Rooting density below the plow layer is an indicator for subsoil compaction. Roots are almost completely absent from the subsoil below severe plow pans and often move horizontally above the pan (figure 6.6). Keep in mind, however, that shallow rooted crops, such as spinach and some grasses, may not necessarily experience subsoil compaction problems under those conditions. Compaction also may be recognized by observing crop growth. A poorly structured plow layer will settle into a dense mass after heavy rains, leaving few large pores for air exchange. If soil wetness persists, anaerobic conditions may occur, causing reduced growth and denitrification (exhibited by leaf yellowing), especially in areas that are imperfectly drained. In addition, these soils may "hardset" if heavy rains are followed by a drying period. Crops in their early growth are very susceptible to these problems (because roots are still shallow) and commonly go through a noticeable period of stunted growth on compacted soils. Reduced growth because of compaction affects the crop's ability to fight or compete with pathogens, insects or weeds. These pest problems may, therefore, become more apparent simply because the crop is weakened. For example, compacted soils that are put into a no-till system may initially show greater weed pressure because the crop is unable to effectively compete. Also, dense soils that are poorly aerated are more susceptible to infestations of certain soil-borne pathogens, such as Phytophthora during wet periods. Preventing or Lessening Soil Compaction Preventing or lessening soil compaction generally requires a comprehensive, long-term approach to addressing soil health issues and rarely gives immediate results. Compaction on any particular field may have multiple causes and the solutions are often dependent on the soil type, climate and cropping system. Let's go over some general principles of how to solve these problems. Tillage is a problem, but sometimes can be a solution. Tillage can either cause or lessen problems with soil compaction. Repeated intensive tillage reduces soil aggregation and compacts the soil over the long term, causes erosion and loss of topsoil, and may bring about the formation of plow pans. On the other hand, tillage can relieve compaction by loosening the soil and creating pathways for air and water movement and root growth. This relief, as effective as it may be, is temporary. Tillage may need to be repeated in the next growing seasons if soil management and traffic patterns stay the same. Lessening and preventing soil compaction is important to improving soil health. The specific approaches: should be selected based on where the compaction problem occurs (subsoil, plow layer, or surface); must fit the soil and cropping system and their physical and economic realities; and are influenced by other management choices, such as tillage system and use of organic matter amendments. Over time, farmers frequently use more intense tillage to offset the problems of cloddiness associated with compaction of the plow layer. The solution to this problem is not necessarily to stop tillage altogether. Compacted soils frequently become "addicted" to tillage and going "cold turkey" to a no-till system with a seriously degraded soil may result in failure. Practices that perform some soil loosening with minor disturbance at the soil surface help in the transition from a tilled to an untilled system. This may include a zone-building tool (figure 14.1a) with narrow shanks that disturb soil only where future plant rows will go. Also, paraplows (figure 14.1b) that loosen the soil by lifting it from underneath can relieve some compaction. Another approach may be to gradually reduce tillage intensity through the use of tillage tools that leave residue on the surface (for example, chisels with straight points, or specifically designed for high-residue conditions) and a good planter that ensures good seed placement even with minimal secondary tillage. Such a system reduces organic matter losses and erosion over the long term and through better germination rates may produce more crop residues. a) zone-builder b) paraplow Figure 14.1 Tillage implements that loosen soil with minimum surface disturbance. Deep tillage (subsoiling) is a method to alleviate compaction below the depth of normal tillage, although it is often erroneously seen as a cure for all types of soil compaction. It is a rather costly and energy-consuming practice that should not be done regularly. (Practices such as "zone building" and paraplowing also may loosen the soil below the plow layer, but are less rigorous and leave residue on the surface.) Deep tillage may be beneficial on soils that have developed a plow pan. Simply shattering this pan allows for deeper root exploration. To be effective, deep tillage needs to be performed when the entire depth of tillage is sufficiently dry and in the friable state. The practice tends to be more effective on coarse-textured soils (sands, gravels), as crops on those soils respond better to deeper rooting. The entire subsoil of fine-textured soils is often hard, so the effects of deep tillage are then less beneficial and in some cases even harmful. After performing deep tillage, it is important to prevent future recompaction of the soil by heavy loads and plows. Better attention to working and traveling on the soil. Compaction of the plow layer or subsoil is often the result of working or traveling on a field when it is too wet. The first step when addressing compaction is to evaluate all traffic and practices that occur on a field during the year and determine which field operations are likely to be most damaging. The main criteria should be: The soil moisture condition under which the traffic occurs; and The relative compaction effects of various types of field traffic (mainly defined by equipment weight and load distribution). For example, with a late-planted crop, soil conditions during tillage and planting may be generally dry, and minimal compaction damage occurs. Likewise, mid-season cultivations usually do little damage, because conditions are usually dry and the equipment tends to be light. However, if the crop is harvested under wet conditions, heavy harvesting equipment and uncontrolled traffic by trucks that transport the crop off the field will do considerable compaction damage. In this scenario, emphasis should be placed on improving the harvesting operation. In another scenario, a high-plasticity clay loam soil is often spring-plowed when still too wet. Much of the compaction damage may occur at that time and alternative approaches to tillage should be a priority. Better load distribution. Improving the design of field equipment may help reduce compaction problems by better distributing vehicle loads. The ultimate example of this is the use of tracks, like those on a bulldozer, which especially reduce the potential for subsoil compaction. (Beware! Tracked vehicles may tempt a farmer to traffic the land when it's still too wet. Tracked vehicles have better flotation and traction, but still cause compaction damage, especially through smearing under the tracks.) Plow layer compaction also can be reduced by lowering the inflation pressure of tires. A rule of thumb: cutting tire inflation pressure in half doubles the size of the tire footprint and cuts the contact pressure on the soil in half. Use of multiple axles reduces the load on tires. Even though the soil receives more tire passes, the resulting compaction is significantly reduced. Using large, wide tires with low inflation pressure also helps decrease the compaction effect of loads on soil. Use of dual wheels similarly reduces compaction by increasing the footprint, although this is less effective for reducing subsoil compaction, because the pressure cones from neighboring tires (figure 6.11) merge at shallower depths. Dual wheels are very effective at increasing traction, but again, pose a danger because of the temptation (and ability) to do field work under relatively wet conditions. Duals are not recommended on tractors performing seeding/planting operations because of the larger footprint. Improved soil drainage. Fields that are imperfectly drained often have more severe compaction problems, because wet conditions persist and it is almost impossible to prevent traffic or tillage under those conditions. Improving drainage may go a long way toward preventing and reducing compaction problems on poorly drained soils. Subsurface (tile) drainage improves timeliness of field operations, helps dry the subsoil and, thereby, reduces compaction in deeper layers. Clay soils often pose the greatest challenge with respect to compaction, because they remain in the plastic state for extended periods in the spring. After the top inch near the soil surface dries out, it becomes a barrier that greatly reduces further evaporation losses (figure 14.2). This keeps the soil below in a plastic state, preventing it from being worked or trafficked without causing excessive smearing and compaction damage. For this reason, farmers often fall-till clay soils. A better approach might be to use cover crops to dry the soil in the spring. When a crop like winter rye grows rapidly in the spring, its roots effectively pump water from layers below the soil surface and allow the soil to transition from the plastic to the friable state (figure 14.2). Because these soils have high moisture-holding capacity, there is normally little concern about cover crops depleting water for the following crop. Figure 14.2 Cover crops enhance the drying of a clay soil. Without cover crops (left), evaporation losses are low after the surface dries. With cover crops (right), water is removed from deeper in the soil, because of root uptake and transpiration from plant leaves, resulting in better tillage and traffic conditions. Cover and rotation crops. Cover and rotation crops can significantly reduce soil compaction. The choice of cover/rotation crop should be defined by the climate, cropping system, nutrient needs, and the type of soil compaction. Perennial crops commonly have active root growth early in the growing season and can reach into the compacted layers when they are still wet and relatively soft. Grasses generally have shallow, dense fibrous root systems that have a very beneficial effect on the surface layer, but don't help much with subsoil compaction. Crops with deep taproots, such as alfalfa, have fewer roots at the surface, but can penetrate into a compacted subsoil. In many cases, a combination of cover crops with shallow and deep rooting systems are preferred. Ideally, such crops are part of the rotational cropping system, which is typically done on ruminant livestock-based farms. The relative benefits of incorporating or mulching a cover/rotation crop are site-specific. Incorporation through tillage loosens the soil, which may be beneficial if the soil has been heavily trafficked. This would be the case with a sod crop that was actively managed for forage production, sometimes with traffic under relatively wet conditions. Incorporation through tillage also encourages rapid nitrogen mineralization. Compared to plowing down a sod crop, cutting and mulching in a no-till or zone-till system reduces nutrient availability and does not loosen the soil. On the other hand, a heavy protective mat at the soil surface provides some weed control and better water infiltration and retention. Some farmers have been successful with cut-and-mulch systems involving aggressive, tall cover/rotation crops, such as rye and sorghum-sudangrass. Addition of other organic materials. Adding animal manure, compost or sewage sludge benefits the surface soil layer in which they are incorporated by providing a source of organic matter. The long-term benefits of applying these materials, relative to soil compaction, may be very favorable, but in many cases, the spreaders themselves are a major cause of compaction. Livestock-based farms in humid regions usually apply manure using heavy spreaders (often with poor load distribution) on wet or marginally dry soils, resulting in severe compaction of both the surface layer and the subsoil. The need to incorporate animal manure for efficient nitrogen use and odor control is also often a barrier to the adoption of no-till or zone-till systems. This problem can be overcome only through an additional investment in manure injection tools. In general, the addition of organic materials should be done with care to obtain the biological and chemical benefits, while not aggravating compaction problems. Controlled traffic and permanent beds. One of the most promising, but rarely adopted, practices for reducing soil compaction is the use of controlled traffic lanes. In this system, traffic associated with all field operations is limited to the same lanes. A controlled traffic system is easiest adopted with row crops in zone, ridge or no-till systems (not requiring full-field tillage, see chapter 15), because crop rows and traffic lanes remain recognizable year after year. Ridge tillage, in fact, dictates controlled traffic, as wheels cannot cross the ridges. Zone and no-till do not necessarily require controlled traffic, but greatly benefit from it, because the soil is not regularly loosened by aggressive tillage. Adoption of controlled traffic lanes typically requires some adjustment of field equipment to insure that all wheel traffic occurs in the same lanes and also requires considerable discipline from equipment operators. Crops Particularly Hard on Soils Potatoes require intensive tillage and return low rates of residue to soil. Silage corn and soybeans return low rates of residue. Many vegetable crops require timely harvest, so field traffic occurs even when the soils are too wet. Special care is needed to counter the negative effects of such crops. These may include selecting soil-improving crops to fill out the rotation, extensive use of cover crops, using controlled traffic, and adding extra organic materials, such as manures and composts. In an 11-year experiment in Vermont with continuous corn silage on a clay soil, we found that applications of dairy manure were critical to maintaining good soil structure. Applications of 0, 10, 20, and 30 tons (wet weight) of dairy manure per acre each year of the experiment resulted in pore space of 44, 45, 47, and 50 percent of the soil volume. The primary benefit of controlled traffic is the lack of compaction for most of the field at the expense of limited areas that receive all the compaction. Because the degree of soil compaction doesn't necessarily worsen with each load (most of the compaction occurs with the heaviest loading and does not greatly increase with other passes), damage in the traffic lanes is not much more severe than that occurring on the whole field in a system with uncontrolled traffic. Controlled traffic lanes may actually have an advantage in that the consolidated soil is able to bear greater loads, thereby better facilitating field traffic. Compaction also can be reduced significantly by maximizing traffic of farm trucks along the field boundaries and using planned access roads, rather than allowing them to randomly travel over the field. Using a Penetrometer for Assessing Compaction A penetrometer is a tool that costs about $200 and measures the resistance to soil penetration. A penetrometer has a rod with a cone-shaped tip that is pushed into the soil. When penetration resistance is greater than about 300 psi, the soil is usually too hard for roots to grow (see chapter 6). Remember that the strength of the soil depends on the water content as well as bulk density (also chapter 6), so penetrometer measurements need to be repeated several times during the growing season to make a good assessment. However, you can sometimes get important information from a single set of penetrometer measurements made when the soil is very moist (for example at the beginning of the growing season in humid regions). If penetrometer readings at that time are near or above 300 psi, they will surely be higher when the soil dries out later in the season. When making penetrometer measurements, try to notice soil strength in both the plow layer and the subsoil corrective action may be different for each case. When using a penetrometer, also keep in mind that soil strength is extremely variable and multiple penetrations throughout the field should be made and averaged before drawing conclusions. Penetrometers do not work well in rocky soils, as the measurement is not valid when the tip hits a rock. The devices are not very good for predicting rooting behavior in clayey soils. Although clays may get very hard upon drying, they may still have enough large pores to allow roots to proliferate. A permanent (raised) bed system is another way of controlling traffic. In this case, controlled traffic is combined with soil shaping to improve the physical conditions in the beds. Beds do not receive traffic after they've been formed. This is especially attractive where traffic on wet soil is unavoidable for economic reasons (for example, with certain fresh-market vegetable crops) and where it is useful to install equipment, such as irrigation lines, for multiple years. Sources Ontario Ministry of Agriculture, Food, and Rural Affairs. 1997. Soil Management. Best Management Practices Series. Available from the Ontario Federation of Agriculture, Toronto, Ontario (Canada). Kok, H., R.K. Taylor, R.E. Lamond, and S. Kessen. 1996. Soil Compaction: Problems and Solutions. Cooperative Extension Service publication AF 115. Kansas State University. Manhattan, KS. Top