All about Topsoil
By Laurence Gale MSc
Like many things in life, we take topsoil for granted; we assume that it is easily available, will always be around and generally do not worry about how valuable this resource is.
It takes nature over 1,000 years to produce a 25mm depth of topsoil. Man can remove metres of soil in a matter of minutes. Even when we move and redistribute soils we often do untold damage to their structure, which often leads to them failing to undertake the necessary function for which they have been chosen.
This is often demonstrated in our sports turf industry, when we see the results of poor sports field constructions where soils have been moved, poorly ameliorated or wrongly placed, resulting in playing pitches that do not drain or sustain healthy grass cover, often turning into anaerobic quagmires.
Topsoils are complex arrangements of mineral particles, air, water, organic matter, living organisms and nutrients. The proportion of these elements is not always critical, however it is important that a soil contains all these elements in one form or another.
Topsoil is a major component of almost all natural sports turf facilities, the exception being pure sand constructed facilities. It's primary role is to provide a medium for grass growth that is both sustainable and stable. This sustainability and strength only comes about when the soil has a good structure.
Soil, naturally, is separated into layers called horizons. These horizons are described as topsoils and subsoils. Topsoil material is generally darker due to the impact of the organic matter that has incorporated itself within this layer. This organic matter is also responsible for enhancing the physical properties of the soil such as tilth, structure, water infiltration and water holding capacities.
The topsoil horizon plays a major part in plant growth and biological diversity as well as some hydrological processes. Topsoil is the interface of air, minerals, water and life (soil organisms), which all interact with one another to support and maintain a soil structure.
The four major components of soil are air, water, mineral matter and organic matter. The relative proportions of these components greatly influence the behaviour and productivity/performance of the soils.
The ideal volume composition of a loam topsoil, for example, would be a pore space of 50% air and water, with 45% mineral matter and 5% organic matter. The proportions of air and water will fluctuate as the soil becomes wetter or drier.
The mineral particles of soil are variable in size and are put into three main classifications - sand, silt and clay which, combined in different ratios, determine the soil type.
Probably the most commonly used description about soil is texture. This is a property of fine earth material <2mm and which depends on the particle size distribution of sand, silt and clays found in the topsoil sample. It is these fine particles smaller than 2mm that determine the behaviour of the soil.
Sands have the largest particle sizes ranging between 2.0mm and 0.05mm, typically free draining material that can become unstable when dry, and inefficient in holding nutrients or moisture.
Silts have smaller particle sizes ranging between 0.05mm and 0.002mmm, poor in retaining nutrients but tend to hold on to water.
The smallest class of mineral particles are the clays having <0.002mm that have a large surface area giving it the ability to retain nutrients, and adhere together to form a sticky mass when wet.
The proportions of mineral particles in these different size ranges are called soil texture. Terms such as loam, sandy loam, silty clay and clay loams are used to identify the soil texture.
Soil texture can also be influenced by the amount of organic matter (OM) present and the form in which it derives. OM is important for good soil structure as it aids aeration, infiltration and increases the soil's capacity to hold on to nutrients and prevents the soil from being eroded.
There are a number of ways of classifying soil texture. One simple method is the feel test - take a soil sample, add some water and roll it between your fingers to ascertain the texture. Compare the feel of the soil - is it gritty, silky or smooth? Check against a guide chart to find out the soil classification. British Sugar have produced a leaflet about top soil which includes a feel test identification chart which can be accessed through the following link. http://www.pitchcare.com/media/pdfs/soil_identifier.pdf
The most accurate test is to take a number of samples from the field and send them for laboratory analysis to measure the particle size of the soil. This analysis involves a series of tests where the soil is dried, sieved and allowed to settle through water (Pipette method), separating the sand, silt and clay particles. The final results give a breakdown of the percentage of clay, silt and sand in the sample.
Texture refers to the proportion (%) of sand, silt and clay-sized particles in the soil. The percentages by weight of sand, silt and clay are used with a texture triangle (see diagram) in assigning soils to a specific texture class (e.g. silt, loam).
For example, clay soils (generally more than 40 percent clay) are often poorly drained. On the other hand, well-drained loam soils are mixtures of sand, silt, and clay in roughly equal proportions, and are well drained. A sandy loam, however, has much more sand and much less clay than does a clay loam.
Soil texture is an inherent soil property that you as a groundsman / gardener cannot change (except through extreme interventions). It is not feasible to change soil texture on a large scale because huge amounts of sand, silt or clay would be required to effect any change.
Soil structure is the aggregation of soil particles (sand, silt and clay) into granules, crumbs or blocks and is the shape that the soil takes based on its physical and chemical properties. It is very important since, along with texture, it affects the drainage and aeration capacity of the soil and also supports microbial life. Without structure a soil collapses and compacts, resulting in a number of problems for landscaping and sports field constructions.
Soils need an open structure in order to function as a growing medium. In particular, it influences the main soil and plant root functions - aeration, drainage and root development. Without structure, soils will suffer from anaerobism, waterlogging and nutrient lock-up and, ultimately, plants will wither and die!
Check topsoil colour. Generally, light coloured materials have lower organic matter contents than dark brown or black soils. Soils that are very light or white may contain excess lime. Excess lime aggravates nutrient deficiencies, particularly iron chlorosis. Frequently, on high lime soils, higher application rates of fertilizer (phosphorus and zinc) are necessary to provide adequate nutrients for plant growth.
The physical properties of most natural soils are vulnerable to major changes when mechanical stress disrupts their aggregated structure. Such disruption is most likely to occur when the soils are in a wet state. Anaerobic conditions and waterlogging are the biggest soil-related causes of plant failure in landscaping and pitch constructions.
Therefore it is important to consider weather and ground conditions when handling and moving topsoil materials on site, working with wet soils can resultisin very wet co
The other elements found in topsoil that influence plant growth are soil pH and nutrients. PH is an indication of the acidity or alkalinity (basic nature) of soil. Soil pH must be measured with a pH electrode to obtain an accurate value. A pH of 7.0 is neutral, while values below 7.0 are acidic and values above 7.0 are alkaline.
Guidelines for soil pH:
Ideal: between 5.5 and 7.5
Acceptable: between 5.0 and 8.2
Unacceptable: below 5.0 or above 8.2
Most plants do well at the range of pH's listed as acceptable, however some acid-loving plants will not grow well at pH's above 7.0. Other sensitive plants are susceptible to iron deficiency at soil pH's of 7.8 or above. Select soil with the appropriate pH for the desired grass/plants/vegetation.
A number of nutrients are required to sustain plant growth, these base nutrients are generally present in good quality topsoils. Ideally the four main nutrients Nitrogen (N), Phosphorus (P), Potassium (K) and Magnesium (Mg) should be present in the top soil medium to promote healthy grass growth.
It is important to select topsoils that are compatible with your existing soil structures. Mixing diverse soil materials will only lead to problems such as layering and capping which usually leads to water problems and anaerobic conditions.
Ideally, take soil samples of the site to confirm the nature and properties of the soils, measuring for particle size distribution/analysis (soil texture), soil pH, nutrient status, organic matter content and levels of potential toxic elements (PTEs) if the land has been subjected to industrial use in the past.
Contaminants, this is one area in which the user of topsoil products needs to be vigilant. Especially the suppliers It is in their own interests to be aware of where these materials are sourced.
Where does it come from?
What is the nature of material in that area generally?
How was the material excavated, stockpiled and transported?
What is the chemical make up of the material?
Any reputable supplier will be able to answer all of these points without hesitation.
As regular suppliers with a good turnover of stock their materials will have been tested prior to sale and kept in good condition usually under covered store to ensure it gets to the customer in prime condition.
So let's consider those points:
Where does it come from and what is the nature of the landscape in that area? Sourcing material from an area of heavy clay with high stone or gravel content may not give the best results for sportsfield application.
How was the material excavated, stockpiled and transported? If material has been sourced from a major industrial development for example the stockpile could have been standing for many months and in a highly anaerobic state. Transportation and packaging can also be detrimental if not carefully considered. Topsoil also benefits from being kept under cover and not saturated by Mother Nature.
Finally chemical make up - how can I feel confident? Heavy metals and potential contaminants can occur naturally within topsoils. Within certain limits (which your reputable supplier will reassure you about) they will cause no harm to anyone or thing that comes into contact with that soil.
The British Standards Institution (BSI) have publish a British Standard for topsoil - BS3882:1965. The standard is intended to provide a simple description and classification of topsoil for use in landscape work. The standard covers the following:
Description of topsoil.
Soil reaction (pH).
Appendix A, describes methods of testing and determination of the above.
Appendix B, advises on information to supply to a topsoil supplier.
The standard is a guide for the provision of topsoil and can be used (if applicable) when ordering topsoil materials. All this information will tell a lot about the soil and it's current condition. It would also pay to evaluate the quality of the soil being supplied. It will ensure you are getting what you are expecting, so that it is compatible and performs to your performance requirements.
Topsoils can be sourced from many suppliers. It can come straight from a site or it can be sourced from stockpiles of materials. Care should be taken when sourcing from old stored soil piles, as the soil may have become de-structured.
Most companies will screen their topsoils, putting them through sieves to ensure that they are uniform in size on delivery and able to meet your specified requirements. Also, most companies are able to mix different soils to achieve a required specification.
Topsoil is now becoming an expensive commodity, and can cost in the region of anything between £5-£60 per tonne, A number of factors will dictate these costs, what is it being used for?, source of material, soil quality, soil type, amount required (bulk or bagged) and haulage charges.
Question the supplier about where the soil is being sourced. Have they enough consistent material to supply what you want? Has it been tested for contaminations? Does it meet your specified requirements? Does the material contain brick, glass and extraneous materials?
Removing and relaying topsoil is an expensive operation both in time and money. It pays dividends to establish the quality of any materials prior to use. Prevention is better than cure.
Questions to ask yourself before purchasing top soils
What do I need? (soil type)
What is my existing soil profile?
How much material do I need? Calculate the cubic volume by multiplying width x length x depth.
What is the performance criteria for the imported material?
Is there a supplier(s) who can provide the specified materials?
When do I want it?
Can I afford to accept sub standard materials?
Questions you should ask your supplier
What is in the soil?
Where is it from?
Is it free of contaminates?
Is it safe for use? How can you prove this?
What is its texture soil classification?
Are soil analysis certificates available on request?
Does it meet British Standard 3882?
Is it stored under cover? How has it been stored?
What can it be used for?
How much have you got?
Can I get hold of your soil all year round?
What are you delivery charges?
How quickly can you deliver?
What size deliveries can you make?
Do you sell it in bags?
What are your guarantees?
What are the delivery terms and conditions?
Good soil suppliers will be able to provide you with relevant information about the materials your are buying. An example of the PSD breakdown that should be provided by reputable suppliers is the following:-
It is vitally important you know what you are receiving. Topsoils can also be altered in many ways, many soil suppliers have the facilities to screen, blend, amend and mix topsoil materials to produce a wide range of variable rootzone products that are generally used for top dressing and planting purposes.
In general these topdressings are us referred to as 70/30, 60/40, 80/20 sand/soil rootzone mixes. However, it is important to identify the nature and type of materials being used. You could easily be sold a 70/30 mix that may contain course sand and clay soil, which would in affect be totally unsuitable for top dressing sportsturf situations. It would be better to know the particle size distribution (PSD) of these ratios of topdressings to enable a clearer identification of the materials being used.
Ideally a good topdressing or rootzone mix should be made up from a USGA standard sand blended with natural organic soils. It is also important to know the organic matter content of these mixes.
Remember to ask for a breakdown of the materials being supplied and, more importantly, how consistent these materials will be, and have they enough material to complete the job/task?
Always deal with suppliers that give a full back up service based on factual evidence of the efficacy of their product, not the 'Honest Johns' that give a cheap product matched by cheap or non existent back up. After all, the cost of digging out a substandard material and replacing it with the real McCoy can end up being a lot more expensive than that initial 'saving'!
David Goodjohn, GSB loams Ltd
John Farrell, Bailey's of Norfolk Ltd
Andy Spetch ,British Sugar http://www.bstopsoil.co.uk/