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Preparing the soil for Turf Planting

From Turfgrass Producers International (TPI),

Lush green turfgrass is at least partially due to a carefully prepared soil bed with balanced pH, which will allow the grass access to the key element iron and to other nutirents.
Photos courtesy of Turfgrass Producers International
Public Skatepark Development Guide

Enhance your lawn's ultimate beauty and success by improving the soil before any planting takes place.

The Benefits of Soil Preparation

  • Improved Uniformity
  • Increased Density
  • Faster recovery from wear
  • Reduced Use of water, fertilizer, & chemicals
  • Reduced Maintenance

Planning and preparation are the key to a successful turf installation. Take the time to measure and chart the selected area in order to better estimate the amount of turf, soil amendements and fertilizers to use.

Good Soil Is Important

For optimum growth, turfgrass needs just four things (in the proper balance) to grow...sunlight, air, water and nutrients. Reduce any of these, or provide too much of any one, and the grass may die or simply suffer. In the right proportions, the grass will flourish, providing not only beauty to the landscape, but also a clean and safe place to play and many benefits to the environment.

Grass obtains three of these four essential factors (air, water and nutrients) from the soil, but many soils are less than ideal for growing grass. Some soils contain too much clay and may be very compacted... great for roads, bad for grass, because air and water aren't available to the roots and the roots can't grow. Other soils may have too much sand... beautiful on a beach, but difficult to grow grass because water and nutrients won't stay in the root zone long enough for the plant to use. Another frequently observed problem with many soils is that its pH (the degree of acidity or alkalinity) is too high or two low for optimum grass growth.

Initial tilling should be completed before the addition of topsoil or soil amendments in order to control weed growth, alleviate subsoil compaction and permit a fusing of the topsoil to the subsoil to improve root penetration and water movement.

The Best Type of Soil For Planting Turf

Loams, sandy loams and loamy sands, with a pH of 6.0 to 7.0 are the very best soils for producing a beautiful, high-use, low-maintenance lawn. Unfortunately, this idea soil mixture is seldom found on any property after construction.

Fertilizer and chemical applications will depend on climate, sod type, soil, insects, weed and disease conditions. However it is usually a good idea to apply a "starter fertilizer" that is high in phosphate (P, or the middle number on a bag of fertilizer), at a rate recommended for the particular product. Then water the prepared area to settle the soil and provide a moist base for turf.

Ideal Depth Of The Soil For Planting Turf

The absolute minimum quality soil depth for a carefree lawn is 10 cm (4 inches); however, for deeper root penetration and the benefits that brings, the accepted standard is 15 cm (6 inches).

Before planting, rototill or spade the area to a depth of 10 to 15 cm. (4 to 6 in.). Then, using a wide rake to produce a fine surface. Be sure to clear the site of all building materials (wood, cement, bricks, etc.) as well as any debris larger than 4-5 cm, or 2-3 inches in diameter.

Improving Soils

Practically without exception, not only can most soils be improved, they usually need to be improved to get the maximum results with only a minimum of other on-going effort.

The knowledge of what's necessary, the amount and availability of materials and the immediate costs of time and money are the factors that typically deter people from taking the steps necessary to improving the soil. While some people do not fully understand the importance of good soils for grass, many also believe they can save time and money by ignoring the need to improve their lawn's soil.

The fact is that failing to improve the soil before planting is only inviting a much greater and continual investment of both time and money, that will never return its value as fully as preparing the soil properly before planting any grass.

Be prepared to install all the sod delivered each day. Any left over turf must be watered before you leave and should be installed prior to any newly delivered sod the next day.

Steps To Preparing The Site

"The beauty is in the blades, but the 'action' is in the roots," is a good adage to remember when growing grass. Thus, the value of proper site preparation and soil improvement, before any planting takes place, is that it will be easier for the grass roots to penetrate deeply and evenly. Deep roots will make the lawn more drought resistant, a more efficient water and nutrient user and more dense as new grass plant shoots emerge.

Rolling the area with a lawn roller one third full of water will firm and settle the surface and reveal any low spots that should be filled to match the surrounding grade surface.

Follow these steps to ensure a healthy and easy to maintain lawn:

  1. Clear the site of all buried stumps, rocks, stones or other debris that is larger than 4-5 cm (2-3 inches) in diameter.
  2. Rough grade the entire area to eliminate any drainage problems on the property. This would include sloping the grade away from building foundations, eliminating or reducing severe slopes and filling low-lying areas. A tractor-mounted blade and/or box are most often used for rough grading, but if the area is smaller, it can be done with hand tools. The rough grading will probably uncover more debris that should be removed and not buried.
  3. The tilling should be completed to a depth of at least 5 cm (2 inches), before the topsoil or soil amendments are added.
  4. Add topsoil to achieve a total topsoil depth of 10-15 cm (4-6 inches), after firming. The topsoil should be a loamy sand, sandy loam, clay loam, loam, silt loam, sandy clay loam or other soil suitable for the area. To the extent possible, practical, affordable and available, incorporate humus (fully decomposed organic matter) into the topsoil.
  5. Testing the soil with a chemical soil test will reveal any acidic soils (pH of 6 and below), which can be improved with the addition of lime. The type (or source) and total amount of applied lime will be determined by the level of acidity and should be based on the recommendations of a reliable garden center or turf professional. As with acidic soil correction materials, the type and total amount of materials will be determined by the level of alkalinity and should be based on professional recommendations.
  6. To prevent root injury to newly installed turfgrass sod, the "starter fertilizer" should be worked into the top 7 to 10 cm (3-4 inches).
  7. Finish grade the entire site, maintaining the rough grading contours and slopes, with a tractor-mounted box blade on larger areas or heavy-duty rake on smaller sites.
  8. If time permits, after rolling the area with a lawn roller one third full of water, allow the area to settle further with rainfall or by applying irrigation water.

Give a new lawn at least 2 to 3 cm. (1 in.) of water within one half hour of installation. Sandy or granular soil (shown here) may need shorter, more frequent periods of irrigation. You (or your client) must keep the turf wet until it is firmly rooted, which usually takes about two weeks. Generally remove no more than 1/3 of the grass height during mowing and keep your mower blade sharp to limit moisture loss.

This site is now ready for turfgrass sod. With this degree of careful and thoughtful soil preparation, the resultant lawn will be absolutely beautiful and require less maintenance, smaller quantities of applied water, fertilizer and pesticides, as it maintains a high degree of density and uniformity and recovers much more rapidly from wear. For years to come, your investment in soil preparations will yield a high return.

Villagers in India make lime the old-fashioned way, by heating sea shells (which are composed of calcium carbonate). Pure lime is among the most alkaline of substances, with rating numbers around 12 on the pH scale. PHOTO COURTESY OF Salvation Army International

What is Soil pH?

Soil pH refers to the acidity or alkalinity of soil and is measured in pH units. The pH scale goes from 0 to 14 with pH 7 as the neutral point. As the amount of hydrogen ions in the soil increases, the soil pH decreases thus becoming more acidic. From pH 7 to 0 the soil is increasingly more acidic and from pH 7 to 14 the soil is increasingly more alkaline or basic.

Most beers rate low (or acidic) on the pH scale, with numbers in the 4.5-5 range. Lemon juice is even more acidic (lower on the scale), with rating numbers around 2.5. PHOTO COURTESY OF Steven A. Edwards

Examples from everyday life can help one understand the nature of various pH unit ratings.

  • Extremely acid: < than 4.5; lemon=2.5; vinegar=3.0; stomach acid=2.0; soda=2-4
  • Very strongly acid: 4.5-5.0; beer=4.5-5.0; tomatoes=4.5
  • Strongly acid: 5.1-5.5; carrots=5.0; asparagus=5.5; boric acid=5.2; cabbage=5.3
  • Moderately acid: 5.6-6.0; potatoes=5.6
  • Slightly acid: 6.1-6.5; salmon=6.2; cow's milk=6.5
  • Neutral: 6.6-7.3; saliva=6.6-7.3; blood=7.3; shrimp=7.0
  • Slightly alkaline: 7.4-7.8; eggs=7.6-7.8
  • Moderately alkaline: 7.9-8.4; sea water=8.2; sodium bicarbonate=8.4
  • Strongly alkaline: 8.5-9.0; borax=9.0
  • Very strongly alkaline: > than 9.1; milk of magnesia=10.5, ammonia=11.1; lime=12

The testing of the soil pH with a chemical soil test will determine if any pH correction materials are required. A desirable pH for soil is neutral, which means that the soil is neither too basic nor acidic for maximum turf growth. Photo courtesy of SUNY Oneonta

Acidic soils are those with a pH of 6.5 or lower, and are often revealed by the chemical soil test. They can be improved with the addition of lime (calcium carbonate). Sulfer-based additives are used to correct alkaline soils. The additives should be raked in as needed to a depth of 7 to 10 cm. (3 to 4 in.).

--From SUNY College of Environmental Science and Forestry

Sulfur: Your Key to Lowering Soil pH

On the pH scale, number 7 is neutral, 0 is the most acidic and 14 is the most alkaline.
Graph courtesy of university of Arkansas

Alkalinity is the opposite of acidity, but both can burn you when it comes to turfgrass health. The ideal soil pH is neither strongly acidic nor strongly alkaline but close to neutral.

When pH is out of whack (either way) plants can't make use of certain essential nutrients. The most obvious example is the iron deficiency that occurs in alkaline soils.

A pH test will show where your soil stands. In the Mid-Atlantic region and elsewhere, excessively high soil pH occurs in areas when high bicarbonate water is used for irrigation. In much of California and elsewhere, soils themselves tend to be high pH.

With alkaline water, soil pH will increase over time and stabilize around pH 8.2, if calcium is the predominant cation (pronounced CAT-ion) in the soil. Calcium carbonate (lime) is formed in the soil at this pH. Soil pH can exceed pH 8.2 when sodium, rather than calcium, is the dominant cation.

In these soils lowering pH is necessary to increase the availability of calcium and micronutrients, particularly iron and manganese. Elemental sulfur (S) is often chosen to lower soil pH, but it must be used carefully.

Potential to Burn

Elemental sulfer has a high potential to burn plant tissue and can lower soil pH too much (pH < 4.0 is possible) if used improperly or at too high an application rate.

Sulfur is oxidized by soil bacteria, forming sulfuric acid which is the substance that lowers soil pH. Each 10 pounds of elemental sulfer generates enough acidity to neutralize 30 pounds of lime. Warm temperatures and good moisture and aeration are required for S oxidizing bacteria to function.

Sulfur oxidation is minimal at soil temperatures less than 50 degrees F.

Consequently Sulfur oxidation in the winter can be limited even in our mild climate. Sulfur that lies dormant in the winter, however, will be oxidized when hot temperatures occur. Even at 75 degrees F the oxidation rate of sulfur is about 15% of that at 85 degrees, so peak rates of sulfur oxidation don't occur until late spring. Applications are best made when temperatures are warm enough for the bacteria to oxidize the sulfur (70 - 80oF), but not hot enough to accentuate tissue burn.

Dig It In

he best time to correct soil pH is before you sod or seed a lawn. Sulfuric acid produced on leaf and crown tissue can burn these tissues. Incorporation of sulfur into the soil by application after core aerification is another good method for reducing burn.

In addition, incorporated sulfur is preferred over surface application because acidification is accelerated and a greater volume of soil is treated.

Sulfur Application Guidelines

  1. There is less chance for leaf and crown damage when sulfur is dug into the soil or applied after core aerification. Sulfuric acid generated on leaves and in thatch can damage foliage and destroy crowns. Contact with the soil buffers the decrease in soil pH around the sulfur particle so this damage is limited.
  2. Faster reaction of incorporated sulfur in comparison to surface-applied sulfur occurs because of higher soil moisture levels. Sulfur oxidation requires good moisture which is more prevalent in the soil than in the thatch or on the leaf surface. Plenty of irrigation water should be applied to wash the sulfur from the turfgrass leaves after any method of application.
  3. Incorporated applications acidify a larger portion of the root zone than surface applications. Elemental sulfur is immobile in the soil so surface applications remain on the soil surface. Even after the sulfur is oxidized, the acidity produced is slow to move into the root zone. Consequently severe decreases in pH may occur in the thatch layer and immediate soil surface with little impact on the remainder of the root zone.

Rate of Application

Sulfur application may be warranted on soils with pH in the high 7s or greater. Using sulfur on soils of lower pH is usually not necessary and can be dangerous due to over-acidification. Calcium should be added to soils dominated by sodium at the same time soil pH is lowered with sulfur.

Sulfur rates should be low to avoid damage to the crowns of the turfgrass plant. Each application to Bermudagrass should be less than 5 pounds per 1,000 square feet, with lower rates being safer. It is wise to check the soil pH before re-application of sulfur to avoid over-acidification, especially in sandy soils that have little capacity to buffer changes in soil pH.

Before taking a soil sample and considering re-application of sulfur, ensure that temperatures and time were sufficient for the original application to have been oxidized, > 75 degrees F and four to six weeks. Commercial sulfur sources range in purity from 50 to 99 percent, so remember to adjust the application rate based on the sulfur content of the material.

--From Jim Camberato, Clemson University

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October 13, 2019, 6:57 pm PDT

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