Freeland create soils for many applications and all topsoil’s delivered generally have to meet with the many specifications which exist within the industry. Either these specifications are supplied by customers, or Freeland create specifications which are suited for the application intended and that which is fit for purpose. Freeland’s main focus is creating topsoil’s which adhere with both BS
3882 and the CLEA requirements for soils supplied to back gardens of new housing developments. These soils must comply with nutrients levels, heavy metals, hydrocarbons and Freeland would be recognised as the largest supplier of these types of soils within London and the Home Counties, with over 250,000m3 delivered annually from 6 soil blending operations.
From our experience, soils which are selected for high quality situations normally require optimum drainage characteristics, combined with an adequate nutrient base to satisfy the plants demands. The physical properties of the rooting zone require that the top soil and sub soil match up, in order water percolates down through the profile and away via the sub soil or sub surface drains. The texture of
the soil/sand and the level of organic matter will have the largest influence over its long term drainage ability, particularly after a period of settlement.
Soils ability to perform in varying applications can be presented as follows:
Physical Parameters
There are a number of issues which have to be addressed to ensure that a growing media/ substrate performs, with particularly attention on drainage criteria. Clearly, the main physical elements which influence the soils performance in this area are that of texture, weight (bulk density) and organic matter. The optimum mineral to select to ensure control over the permeability would be sand based, as this achieves long term stability and removes the shrinkage and settlement factors often associated when 100% natural top soils are chosen.
For a soil to perform, particularly in situations which require guaranteed drainage performance levels, the soil must demonstrate that it possesses the required physical performance characteristics
over a long term period.
The key areas which would normally be associated with this type of growing media are:
The main focus is to define the requirements of each property and through laboratory testing, a model can be created to expose the product to the forces which the soil will be exposed to over a number of years i.e. wetting and drying cycles, surface compaction etc. and compare these results with that which are required for optimum performance.
By adopting this route, we then can assess the soils suitability for the project and satisfy ourselves that the product is fit for its intended purpose.
It was this approach that ultimately developed all the soils for The Eden Project, Cornwall,Jubilee Park landscape scheme at Canary Wharf and the 3 acre roof top garden at Paradise Street, Liverpool, the country’s largest roof top garden.
The key to these soils performing at the desired level is the particle size analysis of the sand fraction, combined with the organic matter levels incorporated.
The organic matter levels will have a direct influence over the percolation rate i.e. the higher organic content the slower the speed in which water will flow down and into the sub soil below. Top soils also placed at depth require vastly reduced levels of organic matter due to the negative effects of elevated levels of moisture held within the root ball of a large tree i.e. sub soil should be less than 1.0%.
The bulk density of the soil will also have a bearing on the engineering ability of the structure to support the weight from above. Any product selected in this capacity will have a dry density and a saturated density. Assuming the soil drains at a given rate it is unlikely to exceed these limits. Soils/growing media which retain too much water either from rainfall or irrigation will expose greater force on the structure below and over time this problem will compound itself as the structure within the media continues to lose its own supporting properties.
Clearly, the recommended option is to build the profile of the soil from 600mm, selecting a free draining sub soil i.e. x10־6/7m/s, which in turn will create a net negative suction, thus the water in the layers above will drain freely to that below.
The top soils placed up to 300mm should have organic matter contents set at a level not exceeding 6/7% which maintains the percolation rate under consolidation.
For these reasons the following values are placed on the sandy base materials which would be suitable for projects requiring high performance substrates:
NOTE: The soil possesses a sandy loam texture which makes it suitable for use in a variety of planting and landscaping applications. It also provides a number of technical and physical benefits when selected on landscape projects. Air and water filled porosity values for sandy loam can generally be as much as 45% allowing the product to drain at x10¯6m/s. This ensures the percolation rate meets the needs of trees and shrubs and prevents water logging, the most common reason for plant failure.
Chemical Parameters
The nutrient loadings of tree and shrub soils are dependent on the drainage properties linked with the depth of placement and the demands the plant has for nutrients i.e. a large tree planted at depth does not require elevated levels of organic matter or in some cases a high quality lawn. The higher the organic matter content in this situation only encourages the soils to settle and compact, effectively removing the level/even surface achieved after consolidation of the seedbed.
The nutrient loadings normally associated with BS3882:2007 top soil would be in line with that required for the shrubs and plants. Thus the organic matter levels are set at approximately 4 – 6% for shrub soils, this will elevate the total nitrogen content to approx. 0.2%. A 4 - 6% organic content will also slow water percolation rates down to x10-6m/sec, which is ideal for the environment the soil is being placed.
Tree soils should possess slightly lower levels of organic matter and these are set approx. 3 - 4%, in line with that normally associated for sands selected for golf green construction. This reduced level will increase drainage rates to x10-5/6m/sec and reduce the nutrient levels proportionally to the level of organic matter being incorporated within the mineral component. The data below gives a suitable indication of the concentrations in each product.
NOTE: a suitable humus fraction has been incorporated into the soil and as such there maybe a slight variation to above.
Additional Benefits
Withthe addition of organic matter, Freeland’s soils possess elevated levels of humus which gives structure and stability to the material, whilst acting as a reservoir for water and essential plant nutrients which release over a long period of time.
Soils possessing a sandy loam texture, can, to a degree, be handled during adverse weather conditions without destroying the structure, which is so important during the winter months.
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