Amending soils

Amending soils

Landscape managers commonly manage soils to improve their physical structure. Doing so entails cultivation and, often, the addition of some organic or inorganic amendment.

One of the main reasons we amend and cultivate soil is to alleviate compaction. Thus, it's appropriate that this discussion should address preventing compaction as the first step in improving soil structure.

Trees commonly suffer from construction activity, which compacts soil to an extent that often can kill the plant. On construction sites, create a zone around trees in which equipment is prohibited. In areas with high foot traffic, take steps to route people along paths that will not affect the root zones of existing ornamentals. The same thing applies to vehicular traffic. Other practices also help reduce or prevent compaction:

Do not cultivate when the soil is wet. This can be a very frustrating situation during wet periods because it seemingly takes forever for soil—especially clay—to dry. However, cultivating soil when it’s wet will only destroy soil structure and cause the formation of blocky, hard clods impossible to break up.

Keep traffic, including foot traffic, off wet soil. Soil compacts more easily when it’s wet.

Improve drainage to speed soil drying and reduce saturation during wet periods.

Apply mulch around trees, as far as the drip line if possible. This will lessen compaction effects on the root zone and improve the soil environment for root growth.

l Physical cultivation. Cultivation can take place in a variety of situations and by several means. The easiest and best time to perform cultivation is before the installation of the landscape or turf.

If pan layers exist in your soil, now is the time to break them up, because it is nearly impossible to do so after the landscape is established. This may require some heavy-duty equipment but is well worth the trouble because pans can cause you no end of problems. Breaking a pan layer may require the use of a deep-ripper implement. If you cannot do this over the entire landscape, at least use augers or some other method of punching through the pan layer in your tree- and shrub-planting holes. Otherwise, the plants will sit in a “bathtub.” If you must dig the planting hole deeper than you normally would to accomplish this, do so. Just be sure to compact the backfill below the root ball to prevent too much settling.

In established landscapes, cultivating soil is a more complex matter. To treat compaction problems around trees, several options exist. Air injection and vertical mulching are techniques finding some use, but they have their drawbacks. A treatment gaining in popularity that provides excellent results for trees growing in compacted soil is soil replacement with radial trenching. This involves digging a trench starting near the trunk and extending it outward to near the drip line. A recent study of this method used trenches starting 10 feet from the trunk of white oaks and radiating outward. The trenches were 10 feet long, 2 feet deep, 14 inches wide, and held about 1 cubic yard. The trenches were refilled with amended soil rich in organic matter. These trenches reversed the decline of trees suffering from highly compacted urban soils by providing a favorable soil environment for the tree roots. Such trenches are easy to dig with a variety of equipment (or even by hand) and so represent a viable method of alleviating compaction around existing trees. Any digging around trees should avoid damaging major roots.

Surface mulching around trees is also an effective method of improving soil conditions if the mulch covers a large enough area. Mulch should extend to the drip line if possible. This produces results more slowly but is perhaps the best long-term strategy for alleviating compaction around trees.

Turf-soil amendment is a different matter. The most common method of cultivating turf soil is through core aeration. This method uses hollow tines that pull soil cores from the turf and deposit them on the surface. The resulting holes, though they soon fill in with material, increase air and water penetration to the root zone. In many instances (low- to medium-traffic sites), doing this once or twice a year provides adequate relief from compaction. In high-traffic situations, such as golf courses and athletic fields, turf managers may core-aerate several times a year.

Repeated coring at the same depth gradually can create a compacted soil layer. Deep-tine aeration, using much longer tines, reduces this problem. Drills or water jets also are aeration options that avoid the problem of a compacted layers. Many golf-course superintendents use a combination of these aeration techniques.

l Amending soil. Cultivation techniques such as aeration help alleviate compaction created through traffic. Often, however, soil has innate properties that make it difficult to manage. You can improve these soils with amendments that impart more desirable qualities to the soil.

v Organic amendments benefit soils in several ways. They increase nutrient- and water-holding capacities and improve drainage and aeration. In different ways, organic amendments benefit both coarse and fine soils. Because OM increases nutrient and water-holding capacity, it helps counter the drawbacks of sand-based soils. In clay soils, water and nutrient-holding capacities are not usually a concern. However, tilth (the quality that allows you easily to work a soil into a loose state), infiltration and drainage often are poor in clay soils. These, too, benefit from organic matter, as already discussed.

Organic amendments are available in many forms, often as processed wood products. These amendments take some time to decompose to the point where they create actual humus, but they still provide benefits in the areas of infiltration, drainage and tilth in the meantime. Other common amendments include manure and peat.

Wood-based amendments are infamous for their ability to tie up soil nitrogen. Obviously, this can be a problem and may require the addition of supplemental nitrogen to offset this loss. Manure can contain high salt levels, another problem that may be of concern in your situation.

You will do no harm by adding large amounts of organic amendments to soil. Thus, there is little danger of overdoing it. A more common problem is adding too little. Often, amounts greater than 50 percent by volume are necessary to achieve significant modification. If you feel you need a more precise idea of how much to add to achieve the desired changes, have a laboratory test your soil.

v Inorganic amendments can be quite useful for improving soil quality. The main reason to amend soil with inorganic amendments is to improve porosity and thus increase water and air permeability of the soil. Therefore, this discussion pertains mainly to clay soils. The best way to improve porosity with inorganic amendments is with coarse amendments. These consist of particles that range in size from sand to fine gravel. Smaller particles do not increase porosity enough to be useful as amendments. Coarse amendments should be of uniform particle size: amendments with a wide mix of particle sizes tend to pack tightly and reduce porosity rather than increase it.

For amendments to be effective, the amendment particles must bridge. That is, they must touch each other so that they create large pore spaces in between. This can require between 50 and 80 percent amendment by volume. Small amounts of amendment are not very effective because they are too sparse to bridge with one another.

Sand is the most commonly used inorganic amendment due to its low cost and effectiveness. Calcined clay, perhaps most recognized as cat litter, is another effective coarse amendment that also increases CEC. Other amendments that grounds-care professionals occasionally use include diatomite, zeolite, expanded shale, pumice, blast furnace slag and sintered fly ash. The latter two materials are byproducts that are available on a regional basis. Perlite and vermiculite are materials used primarily in greenhouse and container culture, but have disadvantages in landscape use due to their inability to remain intact under traffic.

Gypsum (calcium sulfate) is an amendment professionals often use to increase infiltration in some types of saline soils. Sodium in saline soils destroys good soil structure by causing clay particles to disperse. This dispersion effectively seals soil to water infiltration and percolation. Gypsum (and lime) displaces sodium causing clay particles to aggregate (clump together) and create large pore space through which water can flow. The displaced sodium is then free to leach through the root zone with enough water.

Incorporating amendments—organic or inorganic—is simply a matter of tilling the material into the soil after you’ve spread it on the surface. Don’t confuse the term amendment with mulch. Mulch refers to material that remains on the soil surface. Mulches can improve soil by reducing compaction, conserving moisture and decomposing to increase OM in the surface layer of soil. However, by definition they are not amendments, which you incorporate into the soil.

You can amend soil in existing turf by core aeration followed by topdressing that you drag into coring holes. This type of soil replacement is not difficult but requires some time—perhaps a year or two depending on frequency of aeration—to achieve significant replacement of soil.

v Topsoil. Many times, it is simply more efficient to bring in high-quality soil than to modify the poor soils already present on a site. Though this use of topsoil does not, strictly speaking, make it an amendment, the idea is the same: Provide a good soil environment for plant growth. Topsoil for sale often is actually loam. It may be of excellent quality, but it is a misnomer to call it topsoil. Of course, it is wise to inspect topsoil before purchasing it to ensure it’s of the quality you’re looking for. Ideally, the soil should be reasonably weed-free and should not contain too many large clods. If the difference between the topsoil and the site soil is great—as it usually is—till a shallow layer of the topsoil into the top few inches of site soil. This will create a transition zone that will aid water movement and root growth between the two soils.

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