Designing a Roof Garden - Nutrients

Designing a Roof Garden - A Designers Guide

Nutrients

Most of the bulk materials which are used in substrate have a low nutrient content and some, like peat, also have a low pH. As a result it is necessary to incorporate nutrients in the substrate as well as those calcium compounds which are necessary to raise the pH. It is possible to correct a nutrient imbalance in a substrate by the use of liquid feed but unfortunately this requires a regular input. As a result controlled release fertilisers are now becoming popular.

Bulk Fertilisers

There are many bulk fertilisers on the market. Probably the main division within these is into organic and inorganic materials. Organic fertilisers are produced from natural animal and vegetable sources, and rely upon bacterial activity to make their nutrient content available to the plant.

As the activity of bacteria is regulated by soil temperature their behaviour can be erratic. Inorganic sources are mined or produced artificially and have a more reliable performance. A nitrogen, potassium and phosphorous mix (NPK) with a formulation ratio of 1:1:1 or 3:1:3 is usually to be recommended.

Slow Release Fertilisers

These are usually organic based materials whose nutrient content is made available to plants as a result of bacterial activity. Consequently the availability of the nutrients is related to the prevailing temperature. Their mode of operation also means that the rate of nutrient release is initially high but declines with time. Despite these limitations they are more efficient than inorganic fertilisers when used in situations which are prone to leaching.

There are several slow release fertilisers on the market, such as Vitax Q4 and Plantosan. Slow release nitrogen fertilisers such as Gold Nord SCU and Nutriform are available.

Controlled Release Fertilisers

These are designed to extend the period over which the nutrients are released and to do so in a controlled manner. There are many formulations available but the most common consist of a nutrient pellet coated with an acrylic or epoxy resin, or more recently a polyethylene skin. The coat is perforated and the osmotic potential across this layer causes moisture to be drawn into the pellet to displace the nutrient core.

The rate of chemical release is controlled by temperature rather than pH or microbial activity. The chemicals then slowly spread into the substrate. This is useful as it means that release is limited when temperature and therefore plant growth rates are low. Probably the best known products are Osmocote and Nutricote which are available in a range of analyses. These employ an accurately perforated polyethylene skin to effect control. For roof gardens it is best to use a 9 month formulation.

Fritted Trace Elements

An important source of trace elements are the impurities in fertilisers and irrigation water. The actual amounts are very variable and so it is difficult to quantify the value of these sources. One of the main difficulties with supplying additional micro nutrients is the narrow limit between deficiency and toxicity. Any application must be accurate and incorporated thoroughly. Any subsequent additions must be undertaken with care.

Micro nutrients are best added as fritted trace elements (FTE) or frits which are formed by heating sodium silicate and the desired salts together at 1000C. The molten material is then poured into water where it cools and shatters before being ground into a fine powder. This fine chemical impregnated glass then slowly dissolves into the substrate and releases the entrapped chemicals.

There are many types of FTE available, but the most widely used is:

FTE 253A - 2% B 2% Cu 12% Fe 5%Mn 0.13% Mo 4% Zn

Another formulation for plants which are sensitive to high boron levels and require more copper and molybdenum is:

WM 255 - 1% B 4.3% Cu 13.8% Fe 5.4% Mn 1% Mo 4.3% Zn

Potassium frits are also available to provide a long term source of free potassium.

Chelated Micro ElementsIf there are problems with micro elements becoming bound in the substrate it is possible to produce the element mixed with another chemical which safeguards its integrity. Several materials are available for this purpose, such as naturally occurring citrate and tartaric acids, and synthetic ones based on polyamino polycarboxylic acid. Their main value is to prevent a deficiency of iron, although they can be used to reduce the toxic effect of copper, zinc and manganese. Two of the best known formulations are Librel and Sequestrine

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