Soil Amendment for Lawns and Landscapes

By John Fech, UNL Extension Horticulture Educator

It Ain't Just Dirt

Image of garden soil and tillerThere are many types of soil, each varying in the proportion of three basic components--clay, silt, and sand. A soil is commonly described by its predominant ingredients, such as "silty clay". In its ideal combination, soil is known as loam, a mixture of roughly 20 percent clay, 40 percent silt, and 40 percent sand. Loam encourages nutrient retention and proper drainage. You may have to add certain amendments to your soil to bring it near this ratio.

Sandy soils
Sandy soils are very well drained, and sometimes excessively so. The angular shape of the particles creates a good deal of air space so that water passes through the soil quickly. In general, roots thrive in sandy soils, because they get the oxygen they need to grow well. Another advantage is that sandy soils hold their shape well, and resist compaction. This is very important in high-traffic areas such as the path to the trash cans, the mail carrier's everyday route through the front yard, and the lawn adjacent to the patio or deck. Golf course greens and professional sports fields are heavily sand based; they require compaction resistance and rapid drainage to maintain a firm, safe playing surface.

The down side to a predominantly sandy soil is that it will not hold water and nutrients well. Moisture tends to rush right through these soils, making it necessary to water frequently. And because sand particles are slick and hard, much like marbles, they have few sites for holding on to nutrients and fertilizing must be done frequently. Sandy soils are managed best by applying light but frequent doses of water and nutrients, and they can be improved by adding components that hold water and nutrients well, such as compost, aged manure and leaf mold.

Clay soils
Clay soils are made of extremely small particles that are packed close together. Clay particles are flattened in shape, something like tiny pancakes. When lawns growing on clay soils receive even moderate foot traffic, the particles smash together in what is known as compaction. There is little room for water and air, because the percentage of voids is reduced. The limited spaces tend to fill up with water after a rainfall, leaving little or no oxygen for the root system. Water does not pass quickly through clay soils, and often will move downward only after the soil voids are completely saturated. The roots remain wet, and are vulnerable to rot.

Clay soils do have their advantages. They hold water well, reducing the loss of nutrients through runoff and percolation. Lawns growing in soils predominant in clay can be fertilized and watered less often, due to their holding capacity.

Silty soils
Silt particles are smaller than those of sand and not much larger than clay particles. Silty soils perform much like clay soils. Their particles tend to stick together, and they are prone to compaction. In some parts of the country, they are referred to as "muck" soils. They hold nutrients well, and most are naturally plentiful in soil nutrients, reducing the need for the homeowner to fertilize the lawn.

Organic matter
Organic matter is a very important component of the soil, even though it makes up only 3 to 5 percent of a good soil's solid portion. This nutrient-rich material serves to moderate the hardness of the mineral component.  When the soil is predominantly clay or sand, the addition of organic matter (aged manure, Canadian peat moss, ground bark, sawdust, compost) makes clay soil looser, and easier to work.  In clay soils, organic matter additions improve drainage, and allow air to move into the soil more readily.  In sandy soil, it helps to hold moisture and nutrients in the root zone of the lawn.  Organic matter has the magical quality of being able to help both soil problems.  If you want to know what it looks like, buy a bag of compost at a garden center. High-quality composts are predominantly organic matter.

Fixing Soil Problems

In home construction, the topsoil is often carelessly mixed with the other layers, reducing its ability to support a quality lawn. Ideally, the topsoil should be scraped off and placed in a pile, to be redistributed when the home is finished. This is rarely done, especially if the contractor is in a hurry to complete the job and the homeowner is more concerned with picking out the curtains than making sure the soil is properly managed.

In another common scenario, the topsoil and subsoil are trucked away by the contractor for resale or use somewhere else, leaving the new home with the poor soil left over after digging the basement. The homeowner is faced with trying to establish a landscape on a yard that has reduced infiltration, porosity, and fertility, and that is easily compacted. The end result is a very nice-looking home in the middle of a stunted landscape. If you still have the chance, talk with the contractor about reserving the topsoil and then spreading it over the graded yard. You may even want to call in a landscape architect or designer at this early stage to direct the contractor so that the job is done right.

Some lawns have a perched water table. This means abrupt differences between soil layers prevent water from freely percolating into the ground. The most common example is a predominantly clay layer over a predominantly sandy soil. The clays have much smaller and more numerous voids that must be filled with water before it will percolate down into the clay. This limits the depth of the root zone to the upper layer. The upper layer is also prone to compaction, and there may be standing water on the lawn for several hours after a rainfall.

It's very difficult to correct a perched water table. The only real solution is to rebuild the soil, from "the ground up," so to speak, so that the duff, topsoil, subsoil, and bedrock are in the proper location and proportion. You can go through the work of stripping off the sod for reuse, then changing the soil's composition. Consider this if your lawn continually has standing water, or if the roots are commonly rotten upon inspection. The process is expensive and quite laborious. If you choose not to go ahead with it, you can prevent root rot and problems with standing water by watering more often and putting on less water with each application.

Other soils are excessively well drained, if layers of larger particles occur over layers of smaller ones, or if the top layers are quite deep. A sandy soil is particularly apt to drain too quickly. These soils are unable to retain enough moisture and nutrients to sustain the lawn, and the grass may look anemic, rather thin, and yellow. On the other hand, these soils are excellent for drainage and discouraging root rot diseases.

Fast or slow draining soils can be corrected through rebuilding and mixing several inches of compost with the existing soil as deep as is practical.  Soil rebuilt to a depth of of 24 inches is ideal, but you may only be able to enhance the upper 12 to 18 iches or so. Use a rented front-end loader for this operation or hire a contractor to do the job. An easier but somewhat less effective process is to aerate the soil over time with a core cultivation machine, available at rental stores.  In these circumstances, aerate the lawn twice in fall and twice in spring. After each cultivation pass, apply dried compost with a fertilizer spreader. The compost should be of a uniform particle size, or the spreader will clog and distribute unevenly. You can drag the lawn with a piece of chain link fence or a stiff-tined rake to help move the compost off the grass blades and into the holes left by the aerator.  Whether rebuilt through deep incorporation or gradually via aeration and topdressing, the goal is to improve root development and tolerance to drought or excessively wet soils.

Soil testing

Much as an IQ test measures a person's ability to learn, a soil test computes how capable this medium is to support plant growth. You'll need tests in several places to get an accurate reading of a yard of any size, so take a sample from each part of the lawn that you suspect to be responding to different soil conditions.

To collect samples, arm yourself with a clean, dry plastic bucket (metal could interfere with the test) and something to dig with (a bulb planter, a garden trowel, or small shovel). Take a few tablespoons or small handful of soil from a depth of 2 or 3 in.. Remove any thatch, roots, or grass blades from the soil sample, as they will cause inaccurate readings.

A real slick way to take a soil sample is with a sod spade. Its cutting head is straight or vertical, rather than curved or bent. This allows you to simply slide the spade under the sod, lift a flap of grass, remove the sample, then fill the hole with soil from the garden. Lower the sod back into place, tamp it slightly, and water to encourage re-rooting.

Take about 10 samples from each lawn area you've identified and mix them together in the bucket. The laboratory will need about a pint of the mixture for each soil test. The lab can run all of its testing procedures from this combined sample.  Make sure that no fertilizers, bird food or garden soil amendments contaminate the samples, or your test may be thrown off.

Generally, you'll get a test report analysis returned to you in via surface or electronic mail. Most labs report their information in an easy-to-read chart format, along with recommendations on how much of a particular nutrient or amendment to add to make the soil more fertile or to make the existing nutrients more available. If the pH is out of the best range, modifications may be made according to the chart; the lab may caution you about adding too much in one growing season, particularly if your pH is extremely high or low.

Sulfur and Limestone Amendments

Adding sulfur and ground limestone (also referred to as agricultural lime) is much more effective if you can do it before seeding the lawn, or when regrading the site. But you also can incorporate them through aeration as you would compost. Wear a drywaller's mask to avoid breathing these dusts, and long pants and sleeves to avoid a skin rash. Allergic reactions are not common when using these materials, but it's better to be safe than sorry.

Raising pH with limestone (lbs. ground limestone per 1,000 sq.ft.)

Desired change in pH Sandy soil Silty soil Clay soil
4.5 to 6.4 50 160 200

5.0 to 6.5

40 130 150
5.5 to 6.5 30 90 100
6.0 to 6.5 15 50 55

Lowering pH with sulfur (lbs. elemental sulfer per 1,000 sq.ft.)

Desired change in pH Sandy soil Silty soil Clay soil
8.5 to 6.5 45 60 70
8.0 to 6.5 30 35 45
7.5 to 6.5 10 20 25
7.0 to 6.5 3 5 7

Organic matter

One of the most common deficiencies is too little organic matter, either because the soil was initially poor or because it was devalued in the construction process. Organic matter content will gradually improve with the age of a lawn, but it is a very slow process--even a 30-year-old home may have a yard deficient in this component.

A soil test may reveal your yard has no more than 1 to 1.5 percent organic matter, when the ideal range is 3 to 5. To improve a serious shortfall before putting in a lawn, add liberal amounts of finished compost to your soil and till it in by with a rototiller or small plow.  Work it in to the upper 6 inches of soil, and it will trickle downwards over the next few years. You'll need 3 to 4 inches for a soil with a reading of less than 1 percent. If the soil needs only a slight improvement, spread about an inch of compost over the entire area and then incorporate it.

Infiltration testing
The soil beneath the lawn must be able to handle the water that falls on the yard as precipitation and when watering, or the resulting pools of water may cause the roots to rot. On slopes, the water may run off, resulting in dry soils where the grass suffers from drought stress. The infiltration rate (the speed at which water moves downward) is affected by several factors, especially soil particle size and the relative position of soil layers.

If you suspect your yard may absorb water too slowly, you can perform a simple test. Cut the bottom and top out of a coffee can, and place it upright on the lawn. Use a pocket knife to trace the outside edge of the can, remove the can, and deepen this circular cut to 3 in. or so. Then carefully work the can down into the cut without displacing the sod. Pour a gallon of water into the can, or as much as it will hold, and time how long it takes for the water to move downward.  Take note of the time required for the water to drop an inch, 2 inches and then 3 inches. These observations help determine the infiltration rate, which is expressed in volume of water in a unit of time; such as a half inch per hour.   If the water in the can doesn't disappear in a few hours, then you need to do something to help your soil.    For the best determination of infiltration capacity, perform this test when the soil is moist, not overly dry or soggy wet--the cracks that often develop in dry soil and the filled up voids of wet soils will distort the test results. 

Knowing your lawn's capacity to accept rainfall will suggest how often to water, and how long to water each time. For example, if the water is absorbed quickly, you know that your soil should be watered more frequently and with less volume.

Don't Try This at Home--Adding Sand to Improve Drainage

While on a beach, you've probably noticed that the water moves through sand quite rapidly. You might be tempted you to add sand to improve the drainage of a heavy soil, but as logical as this sounds, it's not a good idea. The opposite effect would occur. Drainage problems are usually due to a predominance of silt and/or clay, and when combined with sand these soils become harder, not more permeable. This reminds me of reading with my daughter from her social studies textbook about Adobe Native Americans making bricks out of sand and clay.

The silts and clays which cause the lawn to drain slowly can only be improved through liberal amendment with compost, or by amending the exising soil to bring the sand percentage to 85%.  Doing so would mean adding a whole lot of material to the lawn; it's usually not practical to do this, or to replace the existing soil with a sandy mix.

If the lawn remains too wet after remediation attempts fail, consult a contractor about installing a lawn drainage system. The most common type uses perforated pipes, made of various thicknesses of hard plastic--usually PVC. The system collects water and directs it away from the lawn. It is usually desirable to place gravel around the drain tiles to prevent clogging by fine soil particles. At several locations in the wet area (usually 6-8 feet apart), columns of the gravel extend up to the surface of the lawn and function as catch basins for collecting surface runoff water. A perforated cover is placed on top of the columns to discourage soil particles from clogging the system.