Irrigating layered soils
Proper irrigation is an important component of sound turfgrass management. Managers base irrigation scheduling decisions on the water use or evapotranspiration (ET) of turfgrass, local weather conditions and soil characteristics. These soil characteristics include three basic factors: initial soil infiltration, depth of water penetration and available water per unit depth of soil.
These three characteristics influence the amount of water you should apply. Traditional irrigation models and methods operate on the assumption that the soil in which the roots are growing is a specific type and is uniform through the entire root-zone depth. However, this is not always the case. Soil textures can change with depth due to a variety of natural or manmade factors that result in soil layering.
Naturally occurring events that cause physical changes in soil profiles include the shifting of soil particles downward in the soil profile, formation of natural chemical deposits and geological layering of different soils from events that occurred millions of years ago (for example, sedimentation and volcanic deposits).
Manmade events include topsoil layering and contouring of landscapes that result in the mixing or inversion of surface soils with naturally occurring subsurface layers (or horizons). In addition, subsurface compaction can occur from repetitive mowing and other equipment traffic.
Any of these conditions can result in soils that are called layered, pan or discontinuous. Such soils exhibit discrete layers, or pans, with different physical characteristics that affect the movement and retention of water and air in the soil profile. Quite often, the effects are undesirable.
Layered-soil effects on turfgrass growth Layered or pan soils present a restricted layer for water movement and air content for reasons I will describe later. The result often is poor root growth and a shallow root system. Saturated layers and perched water tables restrict root growth due to the low oxygen content of saturated soil. The resulting shallow root zone needs more frequent irrigation with lesser amounts of water because a smaller portion of the soil-moisture reservoir is available to the turf roots. This is true regardless of which type of soil layering you have.
Often, "black layer" forms above or below the interface layer. Saturated soil is anaerobic (without air) and organisms that grow and live in such environments use sulfur as an alternate energy-transfer source. The by-product causes this soil to smell like rotten eggs. Usually, turfgrass roots cannot penetrate past this black layer without beneficial soil cultivation.
Light soil over heavy soil Let's look at a case where a light-textured soil (high sand content) lies over a heavy-textured soil (with more silt or clay). Here's what happens when rainfall or irrigation occurs: The infiltration rate of the sand is high, and water moves through the sandy upper layer quickly. However, the water front (the farthest advance of water in the soil) in this upper sand layer essentially stops when it hits the layer below it (see figure, upper left, page 18). This is because the second layer is naturally slower to absorb water because of its smaller pore sizes. The result is a saturated upper sand layer. The sand layer will essentially fill with water before the water starts to enter the second layer. Thus, the intake of water into the heavy-textured subsoil layer (which has a lower intake rate) limits water movement through the entire soil profile.
Where do we find such situations and what is the proper irrigation strategy for them? Light-textured layers occurring over heavier soil layers occur naturally in soils that are alluvial in nature. This means that, at some time in the past, a fast-moving river deposited the sandy soil at the surface. (Rivers also can deposit clay layers, resulting in a different kind of layering, which we'll discuss later.) Similar manmade conditions also exist. This can result when you apply sand topdressing over poorly drained heavy-textured soil.
No matter the cause, the result is a soil interface that causes water-flow problems. It is obviously more desirable to have a deep layer of sand over the heavy-textured soil underneath. How much is ideal? That depends. With 8 to 10 inches of sand, you effectively have a sand-based turf system. This is impractical except for facilities with budgets that can afford the high construction costs and maintenance these systems require.
Turf managers often find that topdressings of sand on heavy-textured topsoil are beneficial at first (when the sand is shallow). Over time, however, the sand layer builds up from repeated topdressings. Roots then become restricted to the upper portion of the sand layer as the lower portion becomes saturated due to poor drainage through the pan layer.
* Irrigation management. Irrigation water is going to fill up the surface sand layer no matter how much water you apply to the turf. Sand (see table, above left) has a relatively high infiltration rate (1 to 8 inches per hour), and it will take about 0.10 inch of water to fill the pore spaces of a shallow sand layer about 2 inches deep (see graph, page 18). Irrigating at 0.10 inch per day might serve the water requirements of cool-season turfgrasses in late fall and early spring, but will not provide adequate water storage for warmer weather conditions. Therefore, it is better to irrigate at some intermediate frequency (every 3 days when possible) with a larger amount of water. Water will temporarily saturate the upper sand layer and then slowly infiltrate across the interface of the heavy-textured subsoil. Use the following guidelines to help you:
* The deeper the sand, the more water you can apply at any one application. You must be careful not to apply so much water that you create runoff, because the sand will flood in these situations.
* Control irrigation run times closely by having matched-precipitation heads and the option of cycle-soaking the irrigation to avoid run-off.
* Apply just enough water to replace the turf's ET.
* If water puddling is unacceptable, change the irrigation frequency to every second day, or if necessary, daily, to prevent surface flooding.
Heavy soil over light soil The opposite condition also can occur: heavy-textured (clay, silt or loam) soil on top of a lighter-textured (sandy) soil. As I noted previously, this situation sometimes occurs in alluvial soils. In this type of soil, the sand interface layer represents the "pan" layer. Irrigation water is, of course, slow to enter the heavy surface soil because of the smaller pore space. The upper soil layer fills with water and then slowly penetrates across the barrier into the sand layer (see figure, lower left, page 18). However, it takes a lot of water pressure (head) for the water to finally break into the sand sublayer. This is due, in part, to the fact that larger pore sizes around the sand particles represent large voids. Water initially is physically reluctant to fill in these large voids. This causes the upper layer of heavy-textured soil to stay wet longer than if the entire soil profile were uniformly of the heavier type. Once the sandy layer starts to absorb the water, it can only move it as fast as the heavy-textured soil above it will release it. You can see that this soil arrangement is also a highly undesirable situation.
* Irrigation management. The turf surface quickly floods if the precipitation rate of the sprinkler exceeds the infiltration rate of the heavy-textured surface soil. (Of course, this holds true regardless of any layering present.) Water moves more easily into the sandy subsoil layer if the upper layer of heavy-textured soil is relatively deep. Plus, the deeper the upper soil layer, the more water it can hold for use by turfgrass or landscape plants. A loam soil 6 inches deep should be adequate for turfgrass root growth and irrigation, because about 0.75 inch of available water will penetrate to about a 6-inch depth.
Knowing this, you should: * Apply water in amounts and rates that accommodate the low infiltration rate of the upper soil and the soil's storage capacity (see tables on opposite page). * If necessary, use cycle-soak irrigation to avoid puddling and run-off. In either layering arrangement, water movement is restricted compared to uniform soils. Layered soils are even more difficult to irrigate effectively when the upper layer of soil (regardless of soil type) is shallow. Thus, it's not necessarily the kind of soil present that causes problems--it's the presence of layering that slows water's movement.
Prevention--your best defense We can't turn the world upside down to alleviate layered- or pan-soil conditions. However, some cultural practices can lessen the problem and give you options beyond just careful irrigation. The following guidelines are helpful if you have the opportunity to implement them:
* Deep-rip, chisel and till layered soils and any amendments together before planting turf. This is especially critical when you have a completely impervious layer (see box, "Hard pan: A tough subject," above).
* Avoid adding topical soil amendments of drastically different texture than existing soil.
* If possible, use sod that was grown on soil similar to the soil of the establishment site. Ideally, sand-grown sods should go on sandy soils, and sod grown on heavier soil should go on a similar soil type. Aerate sods with heavy-textured soils--with holes left open where possible--after the sod has rooted and the turf is in actively growing.
* Aerate turf at least once a year, preferably twice. On large turf areas, a deep-tine aerator is the most cost-effective way to promote good turfgrass growth in layered or pan soils. Many models now are available, and they are capable of punching holes or removing a core from 5 to 9 inches deep. They allow the entry of oxygen, water and surface-applied fertilizer nutrients deeper into the root zone. However, an added benefit is that deep-core aeration enhances the downward drainage of water from the soil between the aeration holes. This is important in layered soils. Shallower hollow cores may prevent compaction at the base of the core hole (something that can happen with solid tines), but an open 8-inch-deep hole from a solid tine is a much needed improvement for turf growing in a layered-soil situation.
Observe core holes and see how long they remain open. Aerator holes (0.625 inch) in cool-season turfgrasses can remain open for as long as 5 months. However, warm-season turfgrasses with high growth rates (such as bermudagrass) and cool-season species during peak growth periods may fill the holes with roots in 3 to 4 weeks. In either case, the time to irrigate is after aerating. This helps you leach out soil salts and allows your turf to take advantage of the soil moisture in a less restricted, deeper root zone. In established turf, it's much less expensive and more effective to poke holes in a layered or pan soil than to replace it or attempt to amend it.
Dr. David M. Kopec is a turfgrass extension specialist at the University of Arizona--Tucson.
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