Types of Fertiliser
In general terms there are two categories of granular fertilisers which are commonly used on amenity and sports turf
Quick release mineral fertilisers
These are made from various mineral sources commonly including ammonium nitrate, potassium nitrate, urea, ammonium sulphate and potassium sulphate.
Slow release fertilisers
Slow release fertilisers can be further classified as true slow release and controlled release fertilisers.
True slow release products, often synthetic-organic fertilisers (condensates), include:
- IBDU = Isodur
- UF/MU = Urea Form/Methylene Urea
- CDU = Crotodur
Controlled release fertilisers are coated with a semi-porous substance which controls the rate at which nutrients are released from the granule. These include:
- 100% coated fertilisers
- Part-coated fertilisers made up of a coated and an uncoated fast release fraction
- Fertiliser coated with an ammonium stabiliser
When talking about quick or slow release fertilisers, it should be understood that in the majority of cases we are referring to the Nitrogen element of the fertiliser. Nitrogen has the greatest effect on grass growth but is very mobile in soil. This means that form in which it can be utilised by plants, Nitrate, nitrogen can’t bond with soil particles and therefore leaves the soil either via leaching or volatilisation very quickly. For this reason you will not find the Nitrogen content of a soil on most broad spectrum soil analyses, purely because by the time the analysis is being read, the situation may have changed.
The other two major nutrients, Phosphorus (P) and Potassium (K), are far more stable. Nitrogen is therefore the element of an NPK analysis that is it desirable to be able to control in terms of its release period and subsequent availability to plants.
Quick Release Mineral Fertilisers
These fertilisers are cold water-soluble. As soon as they take on moisture they begin to breakdown and release nutrients into the soil. This makes these fertiliser extremely fast acting and typically they have an effective period of only 2-4 weeks. These types of fertilisers are still widely used and effectively accelerate growth when the grass is actively growing. However, it is widely accepted that these fertilisers are relatively inefficient with up to 60% of the Nitrogen being lost through leaching out in the soil water or volatilised up into the atmosphere.
Examples of Quick release mineral fertiliser:
Slow and Controlled Release Fertilisers
Whatever form these take, the important point about these fertilisers is that they provide a steady supply of nutrients over an extended period of time. This is done by delaying the release of Nitrogen – either by creating a form of nitrogen that breaks down more slowly (a condensate) or by physically coating the nitrogen.
Synthetic Slow Release fertilisers
Essentially these fertilisers contain one of the Nitrogen sources already listed. These synthetic forms of Nitrogen need both temperature and moisture to breakdown and therefore the release of the nitrogen is in line with the nutrient demand of the grass.
Example of a synthetic slow release fertilisers:
Partly Coated Fertilisers
These are fertilisers in which the nitrogen source is surrounded by a polymer coating. As the coating is subjected to heat and moisture, the nitrogen source is gradually released through the coating and into the soil for use by plants.
Examples of a partly coated fertiliser:
Graph showing N uptake and N leaching of turf when different slow release ferts and quick release fert applied.
Ammonium (NH4+) is a positively charged cation which sticks to negatively charged soil particles. Ammonium can be taken up directly by plants, but most N is taken up as Nitrate (NO3). Naturally occurring bacteria (nitrobacter) degrade ammonium in the soil and change it to nitrate (NO3-). Nitrate is available to plants but is also highly mobile within the soil and can easily be lost through leaching as previously explained.
Ammonium stabilisers such as Dimethylpyrazolephosphate (DMPP) delay the degradation of ammonium into nitrate and therefore prolong the effective supply of nitrogen to plants. The effect of ammonium stabilisers is accentuated in the spring before soil temperatures really warm up because the nitrobacter are naturally less active due to the cooler soil temperatures. Once soil temperatures increase, nitrobacter work more quickly and the release of nitrogen will be quicker hence the fertiliser lasts for less time.
Examples of fertilisers coated with ammonium stabilisers:
Benefits of slow and controlled release fertilisers to plant health
Slow and controlled release fertilisers promote steady, uniform growth of grass without the flushes of top growth associated with quick release fertilisers. This steady access to nutrients is typified by stronger root growth and all round improved plant health, as the plants are not putting all their energy into leaf growth.
The other benefit is that slow release fertilisers can be applied just prior to the growing period without fear of the nitrogen being lost before it can be taken up. This means that adequate nutrients are sitting available to be used as soon as the plant needs them rather than a fertiliser application being applied as a reaction to the first signs of growth at the beginning of the year. This means that applications can be made at quiet times ahead of the increasing workload when mowing begins.
Liquid fertilisers are usually concentrated liquids that need diluting with water before spraying onto amenity areas with either a knapsack or tractor mounted sprayer. Liquid fertilisers have the advantage of giving a very quick growth response due to the fact that the nutrients are suspended or dissolved in a liquid solution and instantly plant available.
It is not unusual for liquid fertilisers to contain seaweed extracts which supply contains plant growth hormones such as Cytokinin. These hormones have proven benefits to plant health. Historically seaweed was deposited on pasture land to improve the health of grasses.
An example of a complete liquid fertiliser is: