The Role of Nutrients in Turfgrass and the Soil
By George Attwood-Harris BSc MBPR
We can only produce good quality turf playing surfaces when we have the correct balance of chemical, biological and physical properties in the soil. Of course we also need the right weather, the right machinery, and the right staff, but that is another story.
For now I want to consider the balance of chemical nutrients in the soil, how they interact, and how they are used in the turf plant. An understanding of this helps us to avoid nutrient deficiencies, and is designed to promote the growth of strong, healthy, disease-free turf.
Cation exchange capacity
Before we consider the individual nutrients, we ought to mention the soil's cation exchange capacity. The CEC is also known as the buffering capacity, it is a measure of the soil's ability to store nutrients. This measurement is the cornerstone of understanding soil analysis as it allows us to define target nutrient values for each particular soil.
Sands which have a low organic matter content have a low CEC, typically 1 to 4, while clay soils have a much higher CEC, normally in the range of 12 to 20.
There is no perfect CEC, but often a high value is useful as it enables you to get better value for money from the fertiliser that you use. If the CEC values are low, the soil cannot hold onto applied nutrients, so useful management techniques would include the use of liquid feeds, controlled release fertilisers, or the use of regular light applications of fertiliser.
The pH level of the soil is also of great importance as it affects the availability of the nutrients to the grass plant. Some agronomists would argue that the pH is best maintained at 7.0, as nutrients are most available at this level. However, fine turf grown at this high pH is more prone to worm disease and Poa problems. Consequently, a lower pH of around 6.0 is thought to give the turf good access to nutrients whilst helping to control worm problems.
Many golf or bowling greens have a slightly lower pH, perhaps averaging 5.0, which means that they can be more prone to moss. Raising the pH of acidic greens can cause some difficulties, but does help to encourage more biological activity in the soil which then promotes thatch breakdown. The tendency towards a low soil pH can be alleviated by reducing the use of high sulphur / high iron fertilisers. Also, using a sand or topdressing of slightly higher pH (normally 6.5) will gradually raise the pH. The irrigation water should not be forgotten, as if it is of high pH and will gradually raise the soil pH over time.
Nutrient interactions in the soil
It is important to understand that nutrients do not stand alone, but interact in the soil solution. These interactions affect the availability of each nutrient to the turf, and so we strive to keep them in optimum balance for healthy growth.
This interaction is simplified and summarised in the diagram above.
|A high level of the nutrient below||Reduces the availability of the nutrient in this column|
|Calcium||Boron, Iron and Magnesium|
So what role does each nutrient fulfill?
Phosphorus is critical for the promotion of root development and the establishment of turf, so we often recommend higher levels for young turf. It is also required for the breakdown of carbohydrates and the transfer of energy. Turf deficient in phosphorus has spindly shoots, and can be purplish or reddish in colour. Excessive levels should be avoided on fine turf as it encourages weed grasses such as annual meadow grass, and makes the turf more prone to disease.
Phosphate is very persistent in the soil and so should only be used with care. Ryegrass dominated winter sports turf can cope with higher levels of phosphate than fine turf. It is normal to recommend adopting a zero phosphate fertiliser programme on golf or bowling greens.
Potassium serves to promote the formation of strong cell membranes and improve tolerance to stress such as from cold, heat, wear, and disease. Turf which is short of potash often has yellowish soft drooping leaves, and a poorly developed root system.
However, potash is very mobile in the soil and can be quickly leached out, so it would be worthwhile to try to maintain high potash levels.
Potassium and magnesium work in combination and it is important to try to maintain a ratio of about 1.5:1 potassium to magnesium.
Calcium aids in the uptake and movement of other nutrients around the plant, and is a major constituent of cell walls. It also encourages good respiration especially in periods of high heat and humidity. Turf, which is deficient in calcium, has reduced root growth and pale leaves and does not respond well to nitrogen or iron fertilisers.
Calcium needs to be used with care, as it is also alkaline and can affect the pH of the soil. Fine turf does best in an acidic soil so calcium if needed should only be applied in small amounts, ideally as a liquid.
Magnesium promotes winter hardiness and early growth. It is essential for nitrogen metabolism and chlorophyll synthesis, and helps the turf to utilise iron and phosphate. Turf which is short of magnesium has yellowing drooping leaves.
Magnesium interacts with both calcium and potassium so should be kept in balance with those. Ideally the calcium to magnesium ratio should be about 10:1.
Sulphur is necessary for the utilisation of nitrogen, and forms proteins in the plant. Turf requires one unit of sulphur for every 14 units of nitrogen that it uses. Since the introduction of the clean air act sulphur is becoming deficient, symptoms include slow growth and yellow leaves. However we often encounter greens with high levels of sulphur which can lead to the formation of black layer when iron is present. This is also indicative of poor drainage and anaerobic conditions. These lead to a reduction in microbial activity and an accumulation of thatch, which forms an ideal environment for disease. In high sulphur situations it is often recommended to choose a fertiliser with a low sulphur content.
Zinc is important for cell elongation, sugar consumption and uniform growth. Turf which is deficient in zinc is withered and has mottled leaves. High levels of phosphate in the soil can interfere with zinc uptake.
Golf or bowling greens which are high in Zinc; can have problems with the lock up of various nutrients especially iron. Application of seaweed and humic acid products can help to reduce the level by increasing the cation exchange capacity.
Manganese is important for nitrogen uptake, activation of enzymes, and photosynthesis. It may also play a part in protecting the turf from disease. Deficiency symptoms include spotted or mottled leaves. Manganese is often deficient in sandy soils, and is most available at pH 6, but high pH reduces its availability greatly.
If manganese is in short supply it is often worthwhile to apply a manganese supplement to help with vigour and disease control.
Copper is essential for chlorophyll production and the correct operation of photosynthesis. It also acts as an enzyme activator. Deficient turf has stunted withered leaves with often a dark blue/green colour, and dead or brown spots.
Iron is necessary for chlorophyll formation, good colour, and resistance to disease. Since iron is very widely use for cosmetic greening, it is unusual to find any deficient turf, but symptoms can include yellowing, spindly leaves. Most golf courses and bowling greens have a high level of iron in the soil because of historic usage. This can lead to lock up of potassium and phosphorus and has the tendency to promote black layer when high levels of sulphur are present. The high iron levels are likely to be resistant to change; however it is possible to use a specialist wetting agent to release the locked in iron and flush it out of the soil profile. The reduction in iron levels can also be helped by reducing the amount of iron applied to the turf.
Boron aids in shoot and root growth by assisting in the control of hormones and the translocation of sugars around the plant. Turf, which is deficient in boron, is stunted with discoloured leaves and cracked roots.
I hope that this brief look at the properties of nutrients has shown that as a turf professional you cannot afford to rely on guesswork or random trial and error.
Regular assessment of the turf by a reputable Agronomist is a great help in improving the quality of your turf, and will normally include soil sampling followed by analysis by a specialist laboratory. The agronomist should then provide an individual interpretation of the results with recommendations for an appropriate fertiliser, nutrient and management programme.
Regular soil analysis and careful monitoring are the key to making sound management decisions and producing sustainable, durable high quality playing surfaces.
Sports Turf Consultancy Associates Ltd