What is Topsoil?
Some people have the notion that topsoil is a marvelous, naturally deposited material that contains all the essential chemical, physical and biological components necessary for growing beautiful plants. Unfortunately, this is rarely the case. Topsoil often needs amending with lime, fertilizer or organic matter.
Use of topsoil is increasing in rapidly developing areas where the native soil is predominantly clay. Concerns have been raised about using clay-based soils for landscaping or lawns. Questions frequently asked include
- When should topsoil be used?
- What factors are important in selecting topsoil?
- How can the quality of a topsoil be evaluated?
- How does addition of topsoil affect drainage
What is topsoil used for?
To use topsoil effectively, you must know two things: what kind of soil you are amending and what kind of result you hope to achieve. If your goal is to improve drainage, your strategy will be different than if your goal is to increase water-holding capacity. Applying appropriate amendments can help alleviate either problem.
In North Carolina, topsoil is often applied to “improve” existing clay-based soils. The topsoil chosen for this purpose is likely to be sandy. This scenario sets the stage for increased nutrient deficiencies, acidity problems and moisture stress. The very qualities of sandy soils that improve drainage can trigger these other problems. In such cases, the soil is better amended with a combination of topsoil and organic matter. Amendments that include compost, rotten sawdust, manure, peat moss or aged tree bark can improve drainage, soil porosity, nutrient retention and soil pH. They can also increase biological activity and promote root growth.
Your decision to apply topsoil will be based on the qualities of the topsoil and the qualities of the soil you want to amend. The following sections give background information on topsoil that you will need to consider.
How do you place topsoil?
Topsoil can be placed as the top 100mm of soil. This usually has the essential nutrients that generally all plants require. This shows the importance of buying soil as opposed to subsoil. Make sure of the type of soil you need before ordering.
You may require a more specific soil for exotic plants. This makes it very important for you to do some research on what your plants actually require.
Topsoil colours can differ due pure geography so not all topsoil is the same colour but the quality will still be there.
We can determine the to constituents of the soil from the analysis sheet. This sheet will give you an indication as to what the soil contains and what you may need to add or subtract to meet your planting requirements.
Soil has four main components: 1. minerals 2. organic matter 3. Air 4. Water
NPK stands for (n) nitrogen (p)phosphorous (k) potassium.
Good quality topsoil must have all of the following attributes:
- It is rich in nutrients necessary for basic plant nutrition, these will be nitrogen, phosphorus and potassium.
- Good quality topsoil must contain these minerals in sufficient quantities for plant nutrition, boron chlorine, cobalt, copper, manganese, magnesium, molybdenum,sulfur , and lastly zinc.
- Topsoil can contain organic matter also known as greenwaste that improves soil structure and helps to retain moisture.
- The ph of topsoil is in the range 7 to 9 for most plants but some plants will prefer acid or alkaline differences.
Topsoil is commercially available either bagged or in bulk. Bagged topsoil usually is sold in 40- to 50-lb quantities and has been amended with lime, fertilizer and organic matter. Bulk topsoil generally is a native soil taken from the surface and sold in truckload lots. Unfortunately, bulk lots of North Carolina topsoil rarely meet the standards of a good topsoil.
The texture and fertility of bulk topsoils vary across geographic regions. In North Carolina, coastal plain soils are sandy whereas piedmont and mountain soils are mostly clayey. Although the topsoils from these two regions are vastly different, both can be made productive when properly amended with lime and fertilizers.
Sandy coastal plain soils have physical properties that make them easy to distribute, but they have low nutrient- and water-holding capacity. As a result, plants growing in these soils are more subject to nutrient deficiency and drought stress. Sandy soils generally require more frequent applications of lime and fertilizer, particularly nitrogen, potassium and sulfur. Addition of organic matter increases the water and nutrient retention of these soils.
In contrast, clay soils aggregate and are hard to distribute. However, they hold water and nutrients quite well. As a result, lime and fertilizer applications are needed less frequently than on sandy soils.
Evaluating Topsoil Quality
A soil test is the most reliable way to determine the quality of topsoil. Typical test results from a bulk soil and a bagged soil are given in Table 1. Most native bulk soils tested have low pH and nutrient content, both of which can be corrected with appropriate amendments. The addition of amendments, such as lime and fertilizer, also improves granulation of soil particles. Keep in mind, however, that clays remain clayey and sands remain sandy even after amendments are applied. You can modify the chemistry of soils, but their basic structure remains the same.
Table 2 presents some guidelines for evaluating topsoil quality. The parameters shown are within ranges that should produce good plant growth. They apply to both bulk and bagged topsoils. Soil can be tested by sending samples to the NCDA&CS Agronomic Division Soil Testing Section at the address given on the front of this note.
|pH||5.2 to 6.2|
|phosphorus (P-I)||index of 50|
|potassium (K-I)||index of 50|
|calcium (Ca%)||40–60 of CEC*|
|magnesium (Mg%)||8–-10% of CEC*|
|base saturation (BS%)||60–80% of CEC*|
|manganese (Mn-I)||index > 25|
|zinc (Zn-I)||index > 25|
|copper (Cu-I)||index > 25|
| * CEC = Cation Exchange Capacity = (Ca + Mg + K + Acidity);
CEC is a measure of the quantity of nutrients a soil will hold.
Ca% = [(Ca/CEC) x 100]
Mg% = [(Mg/CEC) x 100]
Optimizing Water Movement
The mechanics of water movement through soil is often misunderstood. If a layer of sand is placed on top of a clay-based soil, water moves readily through the sand and is absorbed by the clay. In contrast, if sand is placed below the clay, water moves into the sandy layer only when the clay soil is totally saturated. Therefore, placing a layer of sand on a poorly drained clay-based soil has no effect on drainage of the clay.
Drainage of soils can be improved with any treatment that enhances granulation or aggregation of soil particles. Such treatments include lime, gypsum (calcium sulfate), organic matter, sand or a combination of all of the above. When adding topsoil or other treatment to an existing soil, mix the two thoroughly for best results.
Main categories of topsoil
Natural topsoil is highly variable in quality and availability. It can range from an acid, nutrient deficient sand, to a strongly alkaline silt loam to an organic rich peaty clay. If the characteristics of the topsoil are not suitable for the end use then widespread expensive plant failures are possible.
Natural topsoil will virtually always have a seed bank with annual and perennial weed seeds. It can often contain rhizomes of persistent weeds and grasses such as creeping buttercup, couch grass or even worse Japanese Knotweed which is regarded as one of the most invasive plants in the UK. Natural topsoil which is derived from former agricultural land may contain elevated levels of heavy metals as a result of the repeated application of sewage sludge. This is of particular importance if the topsoil is to be used in sensitive end uses, such as domestic gardens and allotments.
Skip Waste Soils
Skip waste soils are more common in urban areas and are a result of screening materials derived from building and demolition operations. Skip waste soils usually consist of a mixture of topsoil, subsoil, clay and numerous fragments of building waste materials – brick, concrete, mortar, ash, clinker and to a lesser extent asbestos, glass, metal, wood and plastic.
In terms of its physical and chemical properties, the material is usually extremely alkaline with a pH range of 8 – 10, saline, deficient in organic matter and plant nutrients and can often have elevated levels of zootoxic and phytotoxic contaminants. These soils can look like good topsoil however they often result in either plant failure or, at the very least stunted growth not to mention the consequences of having potentially contaminated soils present in a garden.
Blended soil, as the name implies are formed when two or more components are intimately mixed to form a growing medium.
The Ideal Landscaping Topsoil
The ideal landscaping topsoil can be described as an organic rich, slightly acid to slightly alkaline, fertile sandy loam, free from contamination with no waste materials or sharps. It should be available all year round in bags or bulk.