The Practical Farmer on Safe, Sustainable & Natural Farming Practices

Friday, June 13, 2014

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no. 7.235

Choosing a Soil Amendment

by J.G. Davis and D. Whiting^*(2/13)

Quick Facts...

On clayey soils, soil amendments improve the soil aggregation, increase porosity and permeability, and improve aeration, drainage, and rooting depth.
On sandy soils, soil amendments increase the water and nutrient holding capacity.
A variety of products are available bagged or bulk for soil amendments. However, soil amendments are not regulated. Many are extremely high in salts.
With Colorado’s large livestock industry, manure and manure-based compost are readily available. These are often high in salts, limiting application rates. Use with caution.
Plant-based composts are low in salt. These may be applied at higher application rates, more effectively improving the soil. Plant-based composts are typically higher in price.

A soil amendment is any material added to a soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration and structure. The goal is to provide a better environment for roots.
To do its work, an amendment must be thoroughly mixed into the soil. If it is merely buried, its effectiveness is reduced, and it will interfere with water and air movement and root growth.
Amending a soil is not the same thing as mulching, although many mulches also are used as amendments. A mulch is left on the soil surface. Its purpose is to reduce evaporation and runoff, inhibit weed growth, and create an attractive appearance. Mulches also moderate soil temperature. Organic mulches may be incorporated into the soil as amendments after they have decomposed to the point that they no longer serve their purpose.

Organic vs. Inorganic Amendments

There are two broad categories of soil amendments: organic and inorganic. Organic amendments come from something that was alive. Inorganic amendments, on the other hand, are either mined or man-made. Organic amendments include sphagnum peat, wood chips, grass clippings, straw, compost, manure, biosolids, sawdust and wood ash. Inorganic amendments include vermiculite, perlite, tire chunks, pea gravel and sand.
Not all of the above are recommended by Colorado State University. These are merely examples. Wood ash, an organic amendment, is high in both pH and salt. It can magnify common Colorado soil problems and should not be used as a soil amendment. Don’t add sand to clay soil — this creates a soil structure similar to concrete.
Organic amendments increase soil organic matter content and offer many benefits. Over time, organic matter improves soil aeration, water infiltration, and both water- and nutrient-holding capacity. Many organic amendments contain plant nutrients and act as organic fertilizers. Organic matter also is an important energy source for bacteria, fungi and earthworms that live in the soil.

Application Rates

Ideally, the landscape and garden soils are improved to 4-5% organic matter. At this level, the mineralization (release) of nitrogen from the organic matter will be adequate for most plants without additional fertilizers. Many cities now require that the landscape soils be brought up to this level in new developments as a water conservation technique. With the improved aeration and deeper rooting, plants are more efficient in capturing rain events.
Table 1 gives the routine application rates. Where the soil amendments may be high in salts, the rate is limited due to the salt problem. Salt burn of roots and death of landscape and garden plants is common from over application of salty soil amendments.

Wood Products

Wood products can tie up nitrogen in the soil and cause nitrogen deficiency in plants. Microorganisms in the soil use nitrogen to break down the wood. Over several months to years, as microorganisms complete the rapid decomposition process, the nitrogen is released and again becomes available to plants. This hazard is greatest with sawdust, because it has a greater surface area than wood chips.
Compost wood products, before using them as soil amendments. For these products to decompose rapidly, add a nitrogen source to the compost pile. This could be plant residues high in nitrogen (such as grass clippings or manure), or a nitrogen fertilizer. Do not use uncomposted wood products or sawdust as a soil amendment. It is slow to break down, ties up nitrogen, interferes with seedbed preparation, and interferes with soil and water movement through the soil profile.

Sphagnum Peat vs. Mountain Peat

Sphagnum peat is an excellent soil amendment, especially for sandy soils, which will retain more water after sphagnum peat application. Sphagnum peat is generally acid (i.e., low pH) and can help gardeners grow plants that require a more acidic soil.
Sphagnum peat is harvested from bogs in Canada and the northern United States. The bogs can be revegetated after harvest in this moist environment. However, the harvest rate greatly exceeds the vegetation rate of the peat bogs, so it is considered a semi-renewal resource.
Colorado mountain peat is not a good soil amendment. It often is too fine in texture and generally has a higher pH.
Mountain peat is mined from high- altitude wetlands that will take hundreds of years to rejuvenate, if ever. This mining is extremely disruptive to hydrologic cycles and mountain ecosystems.

Are Biosolids Safe?

Biosolids are byproducts of sewage treatment. They may be found alone or composted with leaves or other organic materials.
The primary concerns about biosolids are heavy metal content, pathogen levels and salts. Use only Class A biosolids, it has been treated to reduce the bacterial content. Class A biosolids are approved for use in production agriculture. However, it is advisable to avoid application to vegetable gardens due to the potential for heavy metals (such as cadmium and lead).
Some cities sell or give away biosolids or compost made with biosolids. It is often extremely high in salts. Ask about the salt content. Use with caution.

Manure

Fresh manure can harm plants due to elevated ammonia levels. To avoid this problem, use only aged or composted manure.
Human pathogens, including E coli, are another potential problem with fresh manure, especially on vegetable gardens. For vegetables with direct contact with the soil, fresh manure must be applied at least four months prior to harvest. For other fruits and vegetables, fresh manure must be applied at least three month prior to harvest. In simple words, fresh manure would be only fall applied for the spring garden. For additional information on E.coli, refer to fact sheet 9.369 Preventing E. coli from Garden to Plate.
Aged manure refers to manure that has been piled for at least six months. Excessive ammonia will have escaped. Salt levels may be higher as the salts concentrate in the decomposing material, or may be leach out with high rainfall. Weed seeds will be viable.
Composted manure technically refers to manure that has been through multiple active heating cycles and turned in between. If heated above 145 degrees F, it will kill pathogens and weed seeds. In composted manure, the organic matter is stabilized (through the rapid decomposition process) making it an ideal soil amendment. Salt level may be concentrated or may be leach out with high rainfall.
As a point of clarification, composts and manures are not regulated. Many commercially available products are labeled as “composted.” However, this does not mean that it has been through the active decomposition process.

Compost

Compost refers to decomposed organic matter. It is not regulated, so there is no standard about the state of decomposition. In commercially available products the term “compost” is often used generically, and does not infer that the product has been through the actively heating, decomposition process.
In Colorado, a wide variety of compost products are available in bagged and bulk products. These may be a combination of plant-based compost, manure-based composts, biosolids, and other agriculture by-products (such as chicken feathers).
With the large livestock industry in Colorado, manure-based composts are most common. These are often high in salts. Use with caution.
Compost made solely from plant-based products (such as wood chips and yard wastes) are low in salts. These are preferred over manure based composts which are often higher in salts. However, they are generally more expensive.
Working with Dr. Jean Reeder, the Colorado Master Gardener Program had soils tests done on samples of locally available, bagged, manures and composts. The majority had high salts. Use with caution!

Factors to Consider When Choosing an Amendment

There are at least four factors to consider in selecting a soil amendment:

how long the amendment will last in the soil,
soil texture,
soil salinity and plant sensitivities to salts, and
salt content and pH of the amendment.

Laboratory tests can determine the salt content, pH and organic matter of organic amendments. The quality of bulk organic amendments for large-scale landscape uses can then be determined.

Longevity of the Amendment

The amendment you choose depends on your goals.

Are you trying to improve soil physical properties quickly? Choose an amendment that decomposes rapidly.
Do you want a long-lasting improvement to your soil? Choose an amendment that decomposes slowly.
Do you want a quick improvement that lasts a long time? Choose a combination of amendments.

Table 1: Routine application rate for soil amendments.
Site		Depth of soil amendment prior to incorporation^A (based on an incorporation depth of 6-8 inches)^B
Site		Plant-based composts and other soil amendments low in salts^C	Manure, manure-based compost, biosolids, biosolid-based compost and other soil amendments that may be high in salts^D
One-time application to new landscapes prior to planting trees, shrubs, perennials, and laws.		2-3 inches	1 inch^E
Annual application to vegetable garden and annual flowerbeds	First three years	2-3 inches	1 inch^E
	Fourth year and beyond	1-2 inches	1 inch^E
^A Three cubic yards (87 bushel) covers 1,000 square feet approximately 1 inch deep. ^B Cultivate the soil amendment into the top 6-8 inches of soil. On compacted/clayey soils, anything less may result in a shallow rooting depth predisposing plants to reduced growth, low vigor, and low stress tolerance. Rate should be adjusted if incorporation depth is different. ^C Plant-based compost are derived solely from plant materials (leaves, grass clippings, wood chips, and other yard wastes). Use this application rate for other soil amendments know to be low in salt. ^D Use this application rate for any soil amendment with maure or biosolids, unless the salt content is actually known, by soil test, to be low. Excessive salts are common in many commercially available bagged and bulk products. Use with caution. ^E For soil amendments with high salts, this routine application rate may be too high. Use with caution.

Soil Texture

Soil texture, or the way a soil feels, reflects the size of the soil particles. Sandy soils have large soil particles and feel gritty. Clay soils have small soil particles and feel sticky. Both sandy soils and clay soils are a challenge for gardeners. Loam soils have the mixture of different size soil particles.
When amending sandy soils, the goal is to increase the soil’s ability to hold moisture and store nutrients. To achieve this, use organic amendments that are well decomposed, like composts, peat, or aged manures.
With clay soils, the goal is to improve soil aggregation, increase porosity and permeability, and improve aeration and drainage. Fibrous amendments like peat, wood chips, tree bark or straw are most effective in this situation.
Use Tables 2 and 3 for more specific recommendations. Because sandy soils have low water retention, choose an amendment with high water retention, like peat, compost or vermiculite. Clay soils have low permeability, so choose an amendment with high permeability, like composted wood chips, composted hardwood bark or perlite. Vermiculite is not a good choice for clay soils because of its high water retention.

Soil Texture	Permeability	Water Retention
Table 2: Permeability and water retention of various soil types.
Sand	high	low
Loam	medium	medium
Silt	low	high
Clay	low	high

Amendment	Permeability	Water Retention
Table 3: Permeability and water retention of various soil amendments.
Fibrous Peat Wood chips Hardwood bark	low-medium high high	very high low-medium low-medium
Humus Compost Aged manure	low-medium low-medium	medium-high medium
Inorganic Vermiculite Perlite	high high	high low

Soil Salinity and Plant Sensitivity to Salts

Many forms of compost made with manure, and biosolids are high in salts. Avoid these amendments in soils that are already high in salts (above 3 mmhos/cm) or when growing plants that are sensitive to salts. Raspberry, strawberry, bean, carrot, onion, Kentucky bluegrass, maple, pine, viburnum and many other landscape plants are salt sensitive. In such cases, choose plant-based composts or sphagnum peat.

Salt Content and pH of the Amendment

Always beware of salts in soil amendments. High salt content and high pH are common problems in Colorado soils. Therefore, avoid amendments that are high in salts or that have a high pH. Amendments frequently high in salts and/or pH include wood ash, Colorado mountain peat and manures, and manure-based compost, biosolids, and biosolid-based compost.
An amendment with up to 10 mmhos/cm total salts is acceptable if mixed well into low-salt soils (less than 1 mmhos/cm). Amendments with a salt content greater than 10 mmhos/ cm are questionable. Choose a low-salt amendment for soils testing high in salts.
Sphagnum peat and compost made from purely plant sources are low in salts and are good choices for amending Colorado soils.
Ask for an analysis of the organic amendments that you are considering, and choose your amendments wisely. If no analysis is available, test a small amount of the amendment before purchasing a large quantity.
Use caution as the salt content in compost may vary from batch to batch.
¹J.G. Davis, Colorado State University Extension soil specialist and associate professor, soil and crop sciences; and D. Whiting, Extension specialist. 6/00. Revised 2/13.Go to top of this page.
Updated Wednesday, January 08, 2014

TOP SOIL

University of Connecticut

Purchasing Topsoil

Dawn Pettinelli, Manager, Soil Nutrient Analysis Laboratory and

Dr. Harvey D. Luce, Assistant Professor, Agronomy

A common misconception when buying topsoil is that the soil we receive should be a dark, fertile, friable loam capable of serving all of our landscaping needs. There are, however, no legal marketing standards for topsoil. To further complicate matters, the terms “topsoil” and “loam” are often used interchangeably when, in fact, not all topsoils are loams and not all loams are topsoils.

Definitions

Soil is the foundation of our landscape. It is comprised of sand, silt and clay, mixed with varying amounts of organic matter, water and air. It is a complex and dynamic medium, able to serve many functions. For the horticulturist, it is a plant growth medium; for the engineer, a construction material and load bearing medium; to the geologist, a stage in the recycling of rock materials. Soil often becomes the scapegoat for a multitude of lawn and garden difficulties. Though sometimes it can contribute to a problem, cultural or other environmental factors are usually more direct causes.

Topsoil, as the name infers, is the uppermost layer of soil. The surface soil usually is darker in color than the subsoil below it because of the accumulation of organic matter. Virgin topsoils in Connecticut generally range from 0 to 5 inches in thickness. Most Connecticut soils, however, have been plowed so a more functional definition of topsoil is the depth of soil that has been mixed by normal cultivation practices, in most cases 5 to 12 inches.

Loam, on the other hand, is a textural classification. Texture refers to the relative amount of sand, silt and clay in a soil. A loam is technically a soil with between 7% and 27% clay, 28% to 50% silt and less than 52% sand. One may modify the term, loam, to sandy loam, sandy clay loam, clay loam, silty clay loam or silt loam as the proportions of these individual soil fractions change.

Considerable variability exists among topsoils throughout Connecticut. Eastern Connecticut soils contain slightly more sand than those in central and western Connecticut. Naturally wet soils often maintain higher organic matter levels than well-drained soils. Soils often mistaken for clays actually contain considerable amounts of silt and sand. Characteristics of these soils may resemble those popularly associated with clays. These include blue-gray color, extreme hardness, cracking, slow drainage and difficulty in cultivation. Because most purchased topsoil is local in origin, its texture may not be too different from what one presently has on site, but its quality as topsoil may be.

What to Look For

Before one buys any topsoil it is a good idea to visually inspect the stockpiled soil. It should be free of trash and other debris. Screened topsoil should be completely free of rock fragments greater than 3 inches in diameter (cobbles and stones). Rock fragments of less than 3 inches in diameter (gravel) should comprise no more than 5% of the soil volume.

Texture: Soil texture greatly affects the suitability of topsoil. Sands should be avoided because they almost always have a low water- and nutrient-holding capacity. Clays, sandy clays, silty clays and silts should be avoided because they tend to be difficult to cultivate (cloddy) and insufficiently permeable to water and air. In other words, they tend to stay wet too long after a rain, which prevents air from reaching plants roots. Soil textures that may be suitable for topsoil include loamy sands, sandy loams (including fine sandy loams), loams, silt loams, silty clay loams, clay loams and sandy clay loams. Loamy sands have the disadvantage of having a relatively low water-holding capacity but are generally easier to till. Because they tend to resist compaction, loamy sands may be preferred in areas subject to traffic or other conditions that may result in soil compaction.

Silty clay loams, sandy clay loams and clay loams are suitable but tend to be somewhat difficult to till, may compact somewhat and may not supply plant roots with enough air. Perhaps the ideal textures for topsoils are sandy loams and fine sandy loams. These texture classes tend to have the optimum trade off between water-holding capacity and soil aeration (and soil tilth). Loams and silt loams are equally desirable assuming that no more than 4 inches of topsoil will be applied and that the topsoil will not be artificially compacted or subject to heavy traffic.

Organic Matter: Topsoils should contain at least 2% organic matter but no more than about 10%. Organic matter is an important component of topsoils. It contributes to the capacity of soils to hold water and nutrients and tends to improve soil aggregation which makes soils more porous. This results in excess water draining away more readily and facilitates the flow of air to plants roots. Organic matter also tends to make soils easier to till. Too much organic matter, however, may result in a soil that is subject to compaction during the installation phase or afterwards as a result of normal foot traffic.

When buying topsoil, a preconceived notion is the darker, the better. While an increase in darkness is often associated with an increase in organic matter, soils which are very dark and grayish may have been dredged from wetland areas. These soils often are very acidic and poorly structured. They “evolved” under wet conditions and may not respond well when placed in a well-aerated environment. Avoid them.

Amendments

It is usually unrealistic to believe that purchased topsoil will require no amendments. Lime and fertilizer are frequently necessary. The addition of organic matter can greatly improve the soil’s physical condition. Two or three inches of organic matter tilled to a depth of 6 to 8 inches is a typical recommendation for topsoils low in organic matter. Sources of organic matter include screened compost, peat moss, manure, leaf-mold, rotted sawdust and cover crops. The characteristics of each of these must be considered before use. Mushroom composts, for instance, should be checked for high soluble salt levels. Although it is tempting to add sand to a “heavy” soil, adding organic matter will ultimately give the best results.

Topsoil is generally sold by the cubic yard. Prices vary depending on location, but range from $15 to $20 per yard “picked up” and from $17 to $25 per yard “delivered.” The minimum amount commonly delivered is from 3 to 6 yards. Most soils are screened through a 3/4-inch mesh. Unscreened soil is less expensive than screened, but the few dollars saved is rarely worth the time and effort spent removing coarse fragments after delivery. Estimate the amount needed by figuring 12 cubic yards will cover about 1000 square feet to the depth of 4 inches. Rural companies may mix compost or manure with the topsoil. “Manufactured” topsoil is a more recent market development. It is commonly a coarse-grained (sandy) mineral base mixed with some composted organic material. This serves a dual function of providing a potentially useful horticultural product and disposing of bulky organic wastes. Not all are worthy of consideration as topsoil.

Few companies selling topsoil have it tested before it is sold. One may want to have a soil test performed to determine its lime and fertilizer requirements and to assess its texture. If one plans to grow vegetables in the purchased soil, it may be wise to check the levels of heavy metals such as lead. Due to the high cost, testing for contaminants such as gasoline, oil or pesticides is rare.

Further inquiries may be directed to:

Soil Nutrient Analysis Laboratory

6 Sherman Place, Unit 5102

Storrs, CT 06269-5102

(860) 486-4274

The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available. All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulations as of the date of printing. If the information does not agree with current labeling, follow the label instructions. The label is the law. Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations. The user of this information assumes all risks for personal injury or property damage. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kirklyn M. Kerr, Director, Cooperative Extension System, The University of Connecticut, Storrs. The Connecticut Cooperative Extension System offers its programs to persons regardless of race, color, national origin, sex, age or disability and is an equal opportunity employer.