Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityOctober, 1996
Soil Amendments for a Superior Landscape
Keywords: soil amendments, alternative substrates, landscape
W hy use soil amendments? A fertile loam soil that is easily tilled and retains moisture may require little if any soil amendment to support excellent plant growth. However, such soils are increasingly rare, and are almost non-existent in the urban environment.
Typically, when sites are under construction, developers begin by removing the topsoil, - which is not just 'dirt' but is actually a complex microenvironment. Topsoil is a loose term that poorly describes the upper layers of a soil that have evolved to support an interdependent community of plants, soil-dwelling animals and microbial organisms. The subsoil below this layer is relatively sterile, infertile, and unsuitable for plant growth, - yet after urban development this often is what is left to 'landscape.' Developers may replace some topsoil after construction, but the
site's ability to support plant growth will be drastically altered by the disturbance and compaction from heavy construction equipment.
Soil amendments can be of great value in these disturbed sites.
What is a soil amendment? Soil amendments can include virtually any substance that improves the growth of plants in soil. Generally, we separate fertilizers from soil amendments by their higher nutrient content, but the distinction is not always clear since soil amendments can improve both chemical and physical properties of a soil. The chemical properties that may be altered by amendments include the soil fertility and pH.
The physical properties that are improved by amendments include soil structure, porosity, and water-holding capacity. Poor soil physical characteristics directly constrain root growth and are probably the biggest problem in landscaping newly constructed sites.
Improving soil physical properties. Organic soil amendments are currently the most common solution for improving physical characteristics of these sites. The beauty of organic soil amendments is that they will, on one hand, increase aeration and porosity in heavy clay soils, while increasing water retention in sandy soils. A great variety of composted materials are available at a reasonable cost throughout the various regions of the United States. Composted sludge, composted animal manure, composted leaves, and composted yardwaste are some of the most common materials that make excellent amendments. One word of caution: the composting process and the suitability and consistency of the resulting products can vary a great deal between producers. First experiment with using a compost product in the landscape before using it on a wide scale.
Other types of organic amendments that may be used with excellent results include sphagnum peat and bark soil conditioners. The bark soil conditioners should consist of mostly bark and should not have a lot of wood mixed in. Uncomposted wood chips and sawdust are generally not recommended as soil amendments because wood is broken down rapidly by soil microbes that deplete nitrogen from the soil during this process. Plants growing in soil incorporated with sawdust will turn yellow from the lack of nitrogen. However, after the wood is entirely broken down, the nitrogen will be released back to the soil.
Inorganic soil amendments. Periodically the recommendation to amend a heavy or compacted soil with builder's sand pops up. Usually the result is a disaster, - a much denser and less porous soil! Adding sand to fine silt or clay soils will cause them to compact and should be avoided. Other types of coarse aggregate materials and inorganic soil amendments, - some of them very expensive, are available for specialized applications such as golf-course green construction. By and large, the organic materials are far cheaper and more suitable for landscape applications.
Soil chemical characteristics. Composted sludge or composts that contain animal manure will increase the fertility of the soil in addition to improving physical properties. These materials may also increase soil pH. Sphagnum peat moss lowers soil pH but does not add nutrients to the soil. Soil amendments are also available specifically to raise or lower soil pH. If a soil is excessively acid, finely ground limestone is used to increase soil pH, and also provides calcium, a plant nutrient. A soil test can be used to determine whether liming is necessary, and will provide the recommended rate. Elemental sulfur, iron sulfate, and aluminum sulfate are three materials commonly used for lowering soil pH. Elemental sulfur is the most effective of the three on a pound-per pound basis, although iron sulfate will react more quickly. Aluminum sulfate is the least desirable material for lowering soil pH since aluminum can be toxic to plants. Most plants will tolerate a wide range in soil pH, however lowering the soil pH may be necessary for certain plants that require acid soils. Key examples of these are rhododendrons and azaleas.
How to use soil amendments: amend the site or the planting hole? The answer to this question is, whenever possible, use soil amendments to prepare the entire site before planting. Adding soil amendments to the backfill of a planting hole is of much less benefit than incorporating materials into the planting site prior to planting. Further, researchers have demonstrated that roots often have difficulty penetrating unamended soil beyond the planting pit. When preparing a site with organic amendments, use about 25% to 35% of the material by volume. For example, incorporating a two inch layer of compost into the top six inches of soil would result in mixture that consists of 33% compost by volume. Materials that are used to alter soil pH are used in much smaller amounts; the appropriate rate to use depends on soil characteristics.
A Soil Amendment Glossary.
Bark. The finer grades of bark mulches are sometimes called 'soil conditioners' and make good soil amendments. All are low in nutrients. Because of their low pH, conifer barks make good amendments for azalea and rhododendron beds. The pH of hardwood barks is close to neutral (7.0).
Calcined clay. This inorganic material is a fired clay; when added to soil, it increases porosity and prevents compaction. It is used in high-cost turf applications.
Composts. Composting is a process in which organic matter is broken down by microorganisms over weeks or months. The organic matter that remains is much more stable than the starting materials. A great variety of composted materials are available. Composted leaves or composted mixed yardwaste are made from green wastes collected from municipalities. 'Mushroom compost, ' while used for mushroom growing, is also sold for the home landscape; it usually contains composted manure, straw, and peat. Composted municipal sewage sludge is a byproduct of the sewage treatment plant. Heavy metals are a concern with some sludge composts. If using a composted sludge in the home landscape, be certain it is cleared for this use. Most composts have a relatively high pH (7 to 8). Sludge composts are also fairly high in nutrients.
Humus. Humus is the decayed organic matter in soil. The term is sometimes used to describe well-composted organic soil amendments, such as 'leaf humus'.
Manures. Cow and poultry manures are most suitable for use when well composted. This reduces the problems of odor and weed-seed contamination. Many of these products tend to be high in pH and nutrients and probably should be considered organic fertilizers rather than soil amendments.
Green manure. The so-called green manure crops, small grains and legumes, are actually cover crops that are grown to improve the soil. Upon maturity, green-manure crops are turned into the soil. This age-old practice produces a large amount of organic matter and releases nutrients into soil slowly as the crop decomposes.
Limestone. Limestone is added to soil to increase the pH, but also supplies some plant nutrients. Calcitic limestone is calcium carbonate and is an important source for calcium. Dolomitic limestone contains both calcium and magnesium carbonates, thus supplies two important plant nutrients. Use a soil test to determine the limestone requirement for your soil and crop.
Peat. Most sphagnum peat moss is commercially harvested from huge peat bogs in the northern hemisphere. Sphagnum peat has a spongy, fibrous texture, a high water-holding capacity, and a low pH and nutrient content. Peat moss has been a choice material for incorporating into landscape beds for years. Muck peat is very different from sphagnum peat. It looks more like compost, - black in color and non-fibrous, and has a pH near 7.0.
Rice hulls. Rice hulls are a composted product that is increasingly available. Rice hulls are quite stable, lasting up to ten years in the soil. This material opens up the soil and increases porosity; it does not add nutrients or water-holding capacity.
Sand. Sand is used in special sand mixes for golf course greens but is not a satisfactory landscape amendment.
Sulfur, iron sulfate. These materials are used to decrease soil pH. Usually this is not necessary unless you wish to grow plants that prefer acid soil. Heavy clay soils require more sulfur than coarse-textured soils to enact a pH change. To lower the pH of a coarse soil one-half to one full unit, begin by adding one-half pound of sulfur, or 2.5 pounds of iron sulfate per hundred square feet. Heavy clay soils may require five times this amount or more. For best results, consult your local extension office for application rates for soils in your area.
Wood waste. Wood chips and sawdust are sometimes used as organic matter amendments. As these materials break down they will remove nitrogen from the soil. The best way to use these materials is to compost them before adding to the soil, or to add them to soil with nitrogen, and wait for the materials to decompose before planting the site.
