Dealing With Impoverished or Contaminated Soils

In response to this, there has been an upsurge of interest in the ecology of such problems and efforts are now being made towards their effective restoration. Unfortunately, the problems of restoration can be extremely varied and complex with many sites providing highly toxic and hostile environments for natural re-colonisation.

Poor Soil Structure

Perhaps the biggest obstacle to successful establishment of grass on reclaimed sites is poor soil structure – either as a result of being compacted by heavy machinery during the reclamation process or, increasingly, as a result of the use of reclaim material from across the site being blended to create ‘manufactured soils’.

In many cases, these manufactured soils are arguably not a soil at all. Instead, they are simply a combination of composted material and crushed aggregate. These types of substrate lack any kind of structure and therefore have no macropores (air spaces) which are vital for the gaseous exchange required for healthy grass growth and which enable water to percolate to the roots and on through the soil profile.

These substrates are also typified by a lack of humus or any other positively charge cations which hold on to essential nutrients and keep them in the soil where they are available to plants.

Soil structure can be enhanced through an increase in organic matter either by application and incorporation into the soil (often a costly and impractical process) or by a build-up of organic matter over a period of time by sustaining a mixture of grasses and legumes on the site.

Here in lies the problem however as it often a case of which comes first: the plants or the organic matter. However, recent advances in fertiliser technology have led to silicate colloid products such as Agrosil LR Correction of nutrient imbalance  product which can be invaluable in accelerating the establishment of vegetation on these impoverished soils.

Chemical Problems

Many waste materials develop a toxic nature due to chemical changes, occurring on exposure to the air and other weathering processes. A classic example of this is with some colliery spoils. Iron pyrite is derived from the association of organic and mineral sediments during formation of coal. On exposure during the mining operation the pyrite forms sulphuric acid which leads to a dramatic drop in the pH of the soil.

Methane has of course become another major factor. This is linked with the increased use of landfill sites. The careful planning of venting facilities is essential is this is not to become a major problem in the future.

Nutrient Imbalance

Of the essential plant nutrients those most limiting on many waste land soils and some manufactured soils include nitrogen and phosphorous whilst the potentially phytoxic elements such as some of the heavy metals may be in high concentration. A further problem associated with the nutrient status of many wastes is their inability to ‘hold’ essential elements as a result of their low organic content. Nitrogen in particular is readily leached.

It is therefore extremely important to carry out extensive soil analysis if a long-term strategy is to succeed.

Correction of the Nutrient Imbalance

Phosphorus

Phosphorus is essential for plant establishment and has a strong stimulating effect on roots. It is, however, quickly fixed in the soil and unavailable to plants. Phosphate fertiliser typically only penetrates the soil to a depth of 5cm but roots benefit from a sufficient amount of phosphate at a deeper level.

One of the most recent innovations in fertiliser technology is the development of Agrosil LR: an electrolyte system consisting of partially dehydrated sodium silicate and phosphate. Agrosil LR keeps phosphate completely available and carries it down to a depth of 30cm – the mobile silicate particles congregate in the soil to form large molecules of silicate gel. These molecules are full of pores which improve soil structure and promote greater drainage, gas exchange and good health in even the poorest soils, sandy rootzones, alluvial sand, compost and manufactured soils. The silicate gels of Agrosil LR lock up heavy metals such as lead and cadmium so tightly that they can no longer be taken up by the plant. This allows growth to be possible again on contaminated sites. In a similar way, growth can be promoted on road verges where heavy salt contamination is a problem.

Nitrogen

Of the important fertiliser elements, nitrogen will effect establishment and growth of a new grass sward more than any other single element. Because spoils are naturally deficient in organic material it becomes important to introduce a supply of nitrogen – either using inorganic or organic fertilisers link to fertilisers section, or by the use of leguminous species which will fix nitrogen from the atmosphere. For example, white clover is often used for this latter technique. The conservation of a grass/legume association is therefore of particular importance on reclaimed land as an economical means of maintaining nitrogen efficiency.

White Clover

As mentioned in previous pages white clover not only fixes nitrogen but, as studies at IBERS* have demonstrated, also improves soil structure. White clover has also been shown to significantly decrease the bulk density of soils and increase porosity, resulting in improved movement of nutrients, making them more available to plants and subsequently improving plant health.

[*Institute of Biological and Environmental Research]

Seed Mixtures

Many grasses are tolerant of poor soil and environmental conditions. The species selected for use within reclamation mixtures are not only tolerant of a range of pH, but are able to survive on very low nutrient soils.

By experimentation and practical use the following mixtures have been formulated for use in reclamation areas:

A16 Country Park Mixture

A16 provides a natural ‘upland’ sward being composed of fine grasses. It is ideal for low maintenance, poor soil sites and produces attractive seed heads if uncut.

View A16

The inclusion however of palatable ryegrasses within a reclamation mixture can provide the following advantages

• more rapid establishment
• greater wear tolerance
• some grazing tolerance

It can therefore be prudent to have a ryegrass inclusion within a reclamation mixture.

A15 Reclamation Mixture

The A15 reclamation mixture has been used successfully on a wide range of sites both urban and rural.

  View A15