Module 6:

6.3 Soil science

6.3.1 Plant nutrition

1   What are the main nutrients given to turfgrasses in a fertiliser?

There are many hundreds of different fertiliser products to choose from. So, you can be pretty certain that there are at least several that will meet your specific pitch needs.

Some fertilisers might only contain one nutrient, whilst others might contain all the essential elements needed for plant growth.

For most turfgrass situations, however, the only nutrients that might be needed to be applied as a fertiliser on more of a routine basis are N, P, and K. Fe and sometimes Mg are also included for specific situations.

2   Nitrogen

This has the most influence on the grass plant and the most noticeable effect is that of encouraging leaf growth and influencing colour in particular. It is also the nutrient that is most needed and applied to turfgrasses as part of a fertiliser programme. A nitrogen deficiency will typically show up as a lighter green / yellowing of the turf, thinning of the sward, reduced leaf extension and growth, with these deficiencies being noticed on the older leaves first.

Nitrogen can be supplied as many different sources and formulations. A typical fertiliser programme might consist of a blend of these over the year. For quick response the use of a granular or liquid quicker release nitrogen source might be required, whilst for a slower release consideration might be given to an organic fertiliser or a synthetic controlled release fertiliser which can last for many months.

There are benefits and limitations to all available sources and formulations, so investigating what is available and suitable from fertiliser suppliers should be an important part of forward planning for a maintenance programme.

3   Phosphorus

This contributes to seed germination, which is essential during renovation and overseeding, and also encouraging good root development. Phosphorus deficiency will typically be noticed in the older leaves first, with them having a darker green appearance along with reddish tinge to the leaf margins.

For this nutrient it is important to get it well mixed into the rootzone, or down into the rootzone via hollow-tine holes. Phosphorus has poor movement within a soil and where it is applied to the surface it will accumulate there and encourage undesirable grasses, especially annual meadow grass and weeds. So, incorporation into the soil profile of this nutrient is key to getting the most out of it.

4   Potassium

This contributes to heat and cold resilience, as well as disease resilience, thereby producing a hardier plant. A key role it plays is in managing water use as it helps control what are called the guard cells which enclose the pore spaces (stomata) in the leaves where water vapour is lost to the environment. This is an especially important function during drying and prolonged dry periods.

A deficiency of potassium can be indicated by a more stressed plant which is slower to spring back to a more upright position after being walked on, a slight curling of the leaf and yellowing occurring in-between the leaf veins, and also affects the older leaves first.

5   Existing soil supply of P and K

There will often be adequate quantities of available phosphorus and potassium within a soil. However, there will also often be an excess of what the plant actually needs. This is one of the main reasons why it is important to base fertiliser requirements on soil tests for these two macro-nutrients. This should not be to the exclusion of what is also happening to the sward, primarily what it looks like and is composed of following a sward and soil profile analysis, because a soil test might indicate there should be adequate supplies, but the sward and root quality are not what might be expected.

In many cases (on soil-based pitches) no further supplies will be needed as a routine practice, especially where clippings are let fly and returned to the soil, as there will already an excess present and available. Clearly, phosphorus and potassium will be needed to be applied over time, especially where grass clippings are boxed off and removed from the pitch, but often not to the routine frequency sometimes suggested.

Where soil supplies are classed as just adequate, neither excessive nor deficient and where clippings are boxed off then the ratio of nutrients to be supplied will typically be equivalent to 4:1:3 of N:P₂O₅:K₂O over a year. Remember, this is what was indicated earlier as the ratio that is removed within grass leaves from mowing where clippings are removed and not let fly and returned to the soil. The challenge arises as to how to calculate, or estimate, the quantity of clippings removed and determine the estimated dry matter equivalent from that. In general terms freshly cut grass is about 80% water and 20% dry matter.

Example

We will estimate that 4,000kg of dry matter of leaf was boxed off and removed, over a year, from a 7,000m² pitch (maintained to say a grade 2/3 level). We will just focus on potassium for this example.

If the potassium element of dry matter is 1.5%, the equivalent potash value would be 1.2% (we’ll look at how this ratio is worked work out soon); so, 1.2% of 4,000kg would mean that 48kg of potash has been removed in the clippings from this pitch area.

To replace the same amount of potash something along the lines of the following might be considered:

• A fertiliser which contains 10% potash (K₂O) applied at a typical rate of 35 g/m², would provide 350kg of fertiliser per hectare, of which 35kg (i.e. 10%) would be potash. As the pitch is just 70% the size of a hectare then the actual quantity of potash supplied to the pitch area is 24.5kg. This is half the amount that has been removed from the clippings.
• So, a similar application would provide the full amount - this approach is what might be applied in a spring and also late summer/ early autumn fertiliser programme.
• Alternatively, a single application with 10% and then two applications at half the content (5% each) might also be considered.
• Whatever is decided, there can be many ways to replenish any nutrient reserves that are depleted through boxing off and removing clippings.

Regular monitoring of soil status and pitch condition will help to better inform a groundskeeper as to what is actually needed rather than a general guess as to what a plant might typically need. This latter approach of just guessing is just going through the motions of routine fertiliser applications, ignoring cost-effectiveness and waste implications, as well as the contaminating impact excess nutrients can have if leached or washed into nearby watercourses.