Abstract. Introduction

Transcription

1 Preventing and Dealing With Soil Compaction A Toolbox Approach Curtis Cavers, Canada-Manitoba Crop Diversification Centre, Portage la Prairie, MB R1N 3V6 curtis.cavers@agr.gc.ca Abstract Soil compaction is the squeezing together of soil particles usually by an external force, such as tillage or equipment traffic, resulting in a decrease in soil porosity that may impede soil aeration or root penetration. Although soil compaction can occur naturally and can be beneficial in some instances for crop growth, the concern is that humaninduced soil compaction often results in negative implications for crop growth. In Manitoba, and especially in the Red River Valley, impacts from soil compaction are offset by the effects of freeze-thaw cycles outside of the growing season and due to the high shrink-swell potential of the clay minerals that comprise these soils. Compared to other parts of the world with different soil types and warmer climates, soil compaction has traditionally been a minor concern for Manitoba producers. However, under severe conditions of deep, excessive tillage or extensive heavy equipment traffic when soils are moist, it may become necessary to consider ways to prevent or mitigate soil compaction. Preventative methods include reducing equipment traffic, lowering axle loads, reducing tillage and the number of passes over a field, especially when soil moisture is approaching field capacity (saturated soils, by definition, cannot be compacted). Minimizing the area impacted by traffic through the use of tram lines and multiple-width equipment is another preventative measure that is more common in other parts of the world but is gaining interest with the adoption of sub-inch accuracy guidance systems. Mitigation of compacted soils can be handled is two different ways: properly timed, non-inversion deep tillage (subsoiling) or the inclusion of deep rooted crops in crop rotations with minimal tillage. Both strategies have the goal of loosening the compacted soil sufficiently to allow crops with less aggressive rooting patterns to re-colonize the traditional rooting zone that was previously limited by the compacted layer. Introduction Soil compaction has multiple causes, varies in its effects on crop performance and may require an array of solutions to overcome and manage the condition effectively. The purpose of this paper is to provide some details to identifying and managing soil compaction in Manitoba. Soil compaction occurs when a force compresses the soil and pushes air and water out of it so that it becomes more dense (New South Wales Dept of Primary Industries, 2005). The reduction in pore space inhibits water holding capacity and soil aeration, while making it more difficult for plant roots to penetrate into the soil. A limited amount of compaction (packing) of the seedbed may be beneficial to emerging plants and may help conserve moisture on lighter-textured soils, but generally soil compaction produces negative impacts to soils and crops, particularly on heavier-textured soils that have inherently smaller pore spaces and are more prone to lack of soil aeration (wetness).

2 Background To appreciate the situation of Manitoba soils and climate and the interactions with soil compaction, it is important to understand some aspects of soil mineralogy and soil structure. In Manitoba, the main type of clay minerals comprising soils is smectite, or montmorillonite, clay. These clay minerals, unlike many of the clay minerals found in other parts of the world, have a high tendency to allow water to enter in between the microscopic clay layers, causing considerable swelling and shrinking (and subsequent volume change) in the clay material (Brady, 1984). It is believed that this phenomenon, coupled with the expansion and contracting of water during freezing/thawing periods is prevalent enough to offset the negative effects of soil compaction on the heavy clay soils in the Red River Valley of Manitoba. Most of our non-saline, grassland-derived, agricultural soils in Manitoba have inherent soil structure that is desirable for crop production. Forest soils, by contrast, tend to have platy structure at or near the soil surface that impede root penetration of young plants. Sodium-rich soils tend to have columnar or prismatic structures that are also undesirable for small plant root growth. Soils with well-expressed, granular or crumbly structure in the surface horizons and blocky or sub-angular blocky structure in the subsurface horizons are most conducive for optimum plant root growth, water infiltration and resistance to erosion. The critical moisture stage at which soil compaction (and its negative effects) occurs is at the plastic limit. This is the moisture content at which a soil sample changes from a semi-solid to a plastic state. More practically, it is the moisture content at which travel across the soil with equipment becomes difficult, if not impossible (MAFRI, 2006). Some key moisture contents of various soils are highlighted in Table 1. Table 1. Soil Moisture Content (% by Weight) for 0-6 Depths of selected soils. Stockton (fine sand) Newdale (clay loam) Red River (heavy clay) Saturation Field Capacity Plastic Limit N/A Wilting Point Air Dry Oven Dry What is important to notice is that the plastic limit (or the trafficability limit ) occurs for most soils at a lower moisture content than field capacity, and this critical value occurs at a relatively lower moisture content for clays than for lighter-textured soils. It is also of interest to point out that saturated soils, by definition, have their pore spaces completely filled with water. As a result, since liquids cannot be compacted, soils cannot experience soil compaction when saturated.

3 Causes of Soil Compaction Soils can be made up of naturally compacted layers, due to their structure, texture or the action of wind, water, pressure, etc. over time. Soils that have these properties usually have a D subclass in their agriculture capability rating ( D stands for dense material). Usually there will be indications of this in the soil series description of soil survey reports that may provide clues that the soil already is, or is at risk of becoming, prone to soil compaction, such as: Low organic matter Low permeability/slow infiltration Platy, prismatic, columnar or massive soil structure Poor tilth or workability Wetness limitation ( W subclass in agriculture capability) The two main causes of human-induced soil compaction are equipment traffic (i.e. heavy loads) and tillage (especially when soil moisture approaches the plastic limit). For equipment traffic, it is estimated that 80% of soil compaction occurs in the first pass across the field. Headlands can receive as many as 4 or 5 passes with equipment during a single tillage operation. Tillage can induce compaction directly at the depth immediately below the depth of tillage, and indirectly through the breakdown of soil organic matter. Diagnosing Soil Compaction Problems In addition to checking for clues in soil survey reports, examining the root zone of growing plants in normal and suspect areas is an effective way to diagnose soil compaction. Compacted soils will restrict root growth so that the root system is shallow, stunted, and turns horizontal abruptly as if it has encountered a physical barrier in the soil. Using a shovel, trowel or knife to expose the root zone will keep the root system intact and will give the examiner a sense of how difficult it is to penetrate the soil. Using a soil penetrometer may allow the examiner to quantify the degree of resistance to root penetration, but this is highly dependent on: the number of samples taken, the precision and accuracy of the instrument and the soil moisture content (i.e. the effort required to push the soil penetrometer into the ground will vary if the soil is moist as compared to the soil being dry). An example of collecting data with a soil penetrometer is found in Table 2. A study with three tillage treatments and four different row crops was conducted on a clay soil at Portage in A soil penetrometer was used on Oct. 25 to measure soil resistance to penetration to a level of 300 psi to a depth of 24 inches within the plant row. These are average values with no statistical analyses conducted (i.e. only observations). The only conclusion that can be drawn from these observations is that different crops appear to respond differently (in terms of their root growth) to the various tillage treatments. We know that certain crops fare better under certain tillage regimes than others because of factors such as soil moisture and soil temperature, but until more data is collected, we cannot verify the reasons for these trends based on this data alone.

4 Table 2. Penetrometer observations at CMCDC-Portage tillage trial. Conventional Tillage Zero Tillage Strip Tillage Edible beans 20 >24 >24 Soybeans Sunflowers Corn Possible Tools for Managing Soil Compaction We will discuss four possible place-based solutions for dealing with soil compaction. It will be up to the individual producer/agronomist to decide which method gives the most apparent value for the cost required and which method is most effective on the most vulnerable areas in a field/farm operation. 1. Do Nothing and Let Mother Nature Work Based on experience with the clay mineralogy and climate in agro-manitoba, it is widely believed that soil compaction does not become severe enough to inhibit crop performance because of the offsetting effects of shrink-swell and freeze-thaw. What is difficult to prove and quantify is whether any of the other practices to mitigate soil compaction have an agronomic or economic advantage over the status quo, other than in extreme cases. 2. Specialized Tillage One of the possible methods for mitigating soil compaction is the use of non-inversion, deep tillage (i.e. subsoiling, deep ripping, etc.). If this method is to be used, it is important to adhere to the following guidelines or the current situation could be made worse: Know the depth of your compacted layer, then till just below that, to avoid creating a tillage hardpan deeper in the subsoil than what previously existed. Avoid bringing salts, carbonates, stones, gravel and subsoil to the surface Till when soil moisture is as dry as possible (i.e. less than the plastic limit) to the intended depth of tillage, to create a shattering effect in the soil structure rather than a smearing effect. Conduct this type of tillage on affected areas of the field (e.g. headlands) and only when it is needed. 3. Deep Rooted Cropping Previous work has shown that establishing crops with large tap roots and high water demand, such as alfalfa and sunflowers, have the ability to penetrate compacted layers in soil more effectively than crops with fibrous and shallow root systems. In a no-till system where these root channels remain open and undisturbed to the soil surface can increase water infiltration rates by X (Cavers, 1996). Root channels from previous

5 crops are more resilient and effective in transmitting water than planar cracks because the cylindrical shape does not change when soils wet up and begin to swell shut. However, the benefits of increased infiltration are only realized when root channels remain intact to the surface in a no-till system; tillage disconnects these channels from the surface so that water will only enter these large pores when moisture conditions become saturated. 4. Minimize Traffic Area (Controlled Traffic Farming) Controlled Traffic Farming (CTF) builds upon the concepts and benefits of no-till cropping (described above) and utilizes high accuracy, GPS-guidance technology to minimize the area compacted by equipment traffic. Tramlines are established using multiple equipment widths so that all field traffic is restricted to these areas. ( In terms of minimizing soil compaction with individual pieces of equipment, the rule of thumb is that soil compaction increases in depth and severity with increasing loads and increasing soil moisture. The question of whether tires or tracks on farm equipment is better for managing soil compaction is often asked. While track-type tractors have an advantage over wheel-type tractors in terms of their floatation and trafficability, their impact on soil compaction is less advantageous. Work conducted by PAMI (1996), Alberta Farm Machinery Research Centre (1999) and Hofman (2001) found that wheel tractors cause more soil compaction than track-type tractors when tire pressure is high (i.e. using bias tires), but wheels cause less soil compaction than tracks when tire pressure is low (i.e. using radial tires). It is also important to ensure a tractor is properly ballasted and operated for optimal performance to minimize soil compaction. Summary To effectively manage soil compaction it is important to: Correctly diagnose the problem Quantify the magnitude of the problem (acres affected, yield reductions, etc.) Pick the right tool to manage and correct the problem In addition, soil moisture, soil organic matter and tillage are factors that influence the degree to which soils may be compacted. These factors play a role in other agronomic issues, so it may be useful to consider how to manage them to achieve desired outcomes in agronomy, economics and soil quality. References Alberta Farm Machinery Research Centre and Prairie Agricultural Machinery Institute, Research Update #742. Brady, N. C The nature and properties of soils (9 th ed.). Macmillan Publishing Company, New York. Cavers, C. G Characteristics of Red River clays pertaining to vertisolic criteria and macropore flow. M. Sc. Thesis, University of Manitoba.

6 DeJong-Hughes, J., J. F. Moncrief, W. B. Voorhees and J. B. Swan Soil Compaction: Causes, Effects and Control. University of Minnesota Extension Service. Hofman, V Soil compaction is a new management concern in region. MAN-DAK News, Fall, Manitoba North Dakota Zero Tilllage Farmers Association. MAFRI, Soil Management Guide. New South Wales Department of Primary Industries, Protect your soil from compaction. Prairie Agricultural Machinery Institute, Research Update #726.