The Nitrogen Cycle

13 June 2018
  • Whastsapp

Applying the correct level of nutrients, especially nitrogen, to a fine turf surface or sports pitch is essential to maintaining a healthy sward capable of supporting play and withstanding environmental stresses. Too little nitrogen and the turf will lose colour, thin out, become weak and vulnerable to attack by pests and diseases, and weeds and moss will start to invade.  Too much and you may induce soft growth and deplete carbohydrate reserves, again leaving the plants susceptible to pests and diseases as well as environmental stresses triggered by heat and/or water.

Applying too much nitrogen fertiliser also runs the risk of losing a certain proportion to leaching and denitrification, which is an expensive waste of vital resources.  Understanding the nitrogen-cycle and how nitrogen can be lost is vital to optimising your fertiliser usage and will help you minimise losses and reduce potential contamination of the local environment.

 What is the Nitrogen Cycle?

As turf professionals, we all know that nitrogen is the main macro-nutrient required by turf for growth.  It is all around us in the air we breathe but it cannot be utilised directly by turf plants to produce protein.  Most nitrogen is taken up by plants in its oxidised form, nitrate, but ammonium can also be taken up.  Nitrogen is converted to ammonium and nitrates by processes within the soil as part of the nitrogen-cycle.

To fully understand the processes involved in the nitrogen cycle it is perhaps best to break it down into four different stages:

Uptake: turf needs nitrogen.

  • Plants absorb available nitrates and ammonium from the soil and use them for growth.
  • Nitrogen not taken up by the grass plant may be utilized by soil microorganisms or potentially be lost from the system through leaching or denitrification.


Inputs: if the turf is taking up nitrogen it needs to be replenished:


In an unmanaged system this is cyclical.

  • Organic matter made up of dead plant material (leaves, stems and roots of turf) is broken down and decomposed by soil microbes returning nitrogen to the soil as ammonium.
  • Legumes, such as clovers (Trifolium ), can fix nitrogen gas from the atmosphere.

In a managed system, turf growth and N supply is manipulated.

  • We can encourage the efficient breakdown of organic material in the soil through good management techniques, including aeration and top dressing, and ensuring that turf areas are well-drained.
  • When required, additional nitrogen can also be added to the system in the form of nitrogen fertiliser to maintain turf health.


Conversion: the nitrogen cycle is all about microbial transformation, in simple terms:   

  • In a healthy, well-aerated and balanced soil environment, microbes will act to convert locked-up nitrogen, from dead organic material or applied as fertiliser to plant usable forms.
  • Natural organic matter, or organic fertilisers are converted to ammonium by soil microorganisms; a process called mineralisation.
  • If urea is applied as a fertiliser this is converted to ammonium by a specific enzyme – urease which is present throughout the soil.
  • The ammonium from mineralisation, from urea, or added as a fertiliser directly can be taken up by plant roots, or more commonly utilised by soil bacteria to produce nitrates: the process of nitrification.
  • Nitrates from nitrification, or from fertiliser can be taken up by plant roots directly for use in the plant.


Losses

There are a number of ways which nitrogen can be lost from the system.  Leaching is probably the most prevalent, depending on circumstances, but denitrification, volatilization and losses through the removal of grass clippings will also play some role.

  • Leaching is the downward movement of dissolved nutrients through the soil profile with water. Any nitrate compounds moving to a depth below the roots of plants is lost out of the system. Nitrogen is particularly vulnerable to leaching because we apply the greatest amount of nitrogen to turf to stimulate growth and nitrate and urea form, are very soluble in water so very mobile. Nitrogen in the form of nitrate (NO3) is also negatively charged and cannot be held by the negatively charged exchange sites within the soil brought about by the presence of clay minerals and organic matter. 

  • Nitrogen losses through leaching are generally higher in wet regions compared with dry as rainfall levels need to exceed evapotranspiration levels to promote the downward movement of water through the profile. Light sandy soils with high infiltration/percolation rates and low nutrient retention are also more prone to nutrient losses through leaching compared with clay and organic soils for example.  So, golf greens and sports pitches are very susceptible.

  • Denitrification occurs during anaerobic conditions, for example when the soil has been waterlogged for a prolonged period. In these conditions, anaerobic bacteria convert nitrates into nitrous oxide gas which then makes its way back into the atmosphere.

  • Where nitrogen is applied in the form of urea it is first of all converted into ammonia (gas) before reacting with water to form ammonium that can be taken up by the turf. The rate at which this process can occur is dependent on factors such as temperature, soil pH and soil moisture, but whilst the nitrogen remains in the ammonia state it is susceptible to volatilization, whereby the ammonia gas is lost into the atmosphere.  Volatilization will also occur where ammonium fertilisers are applied to an alkaline soil.

  • A final loss, and one that you may not always consider, is grass clippings cut off by mowing. Such clippings contain approximately 4 % nitrogen which is removed from the system rather than being recycled in the soil to provide further plant food.  Forcing too much growth with fertiliser results in an increase in mowing frequency and the loss of higher quantities of nitrogen, so a balance is required.


Top tips for minimising nitrogen loss

Increasing your understanding of the nitrogen cycle and the potential losses that may occur is a positive step to minimising nitrogen losses and has the potential to save you unwanted expense at the same time as potentially improving your playing surfaces.  There are a number of steps that you should consider when formulating a fertiliser programme and these are outlined below:

  • Apply only as much nitrogen as the turf requires, taking time to carefully consider the growing conditions at the time. Applying too much can weaken the plant and deplete vital carbohydrate reserves and will result in losses to leaching, depending on soil type and weather conditions, but also in excess grass clippings. Using growth potential models or measuring clippings removed is one way to monitor growth rate and help indicate maintenance requirements.

  • Always apply fertiliser in the correct conditions. Ideally granular products should be applied to dry turf when light rain is forecast, or be followed by a light dose of irrigation.  The moisture is necessary to help solubilise the fertiliser granules and move them down to the turf base but, in the case of some urea fertilisers, will also help to minimise volatilization.  Organic fertilisers should only be used during the growing season when soil temperatures are high enough for microbial activity in the soil to breakdown the nitrogen source that has been applied. 

  • To prevent leaching consider using a slow-release or controlled release fertiliser as a base-feed. A coated product, such as those available from the ICL Speciality Fertiliser range, are a good option where the height of cut of the turf is above 8 mm.  Coated products have granules that absorb water from the environment, causing the nutrients inside to gradually dissolve.  As soil temperatures rise, pressure builds-up within the granule forcing the nutrients out through a semi-permeable coating.  The rate at which the nutrients are released depends on the thickness of the coating but can vary between 2-3 and 8-9 months.  The release pattern is not affected by changes in soil temperature, pH or moisture and there is no initial ’surge’ in growth.  Instead nutrients are released at a controlled rate that the turf can utilise effectively, to produce a healthy, strong plant and uniform growth. 

  • Stay open-minded about your fertiliser selection, choosing either granular or liquid materials depending on the prevailing weather conditions and time of year. Granular products provide the most cost-effective source of nutrients in an easy to apply form.  Where you need more control over growth-rate or maybe weather conditions are not conducive to applying granular fertiliser, consider using a liquid product.  These allow a small amount of nutrient to be applied on a little and often basis as the plant requires.  Liquids can be applied as a foliar feed in a low water volume to be taken in by the leaf of the plant thereby minimising loss of nutrients through leaching in the soil.  The response can be very quick but    nutrients will need to be re-applied more regularly.

  • Employ sensible cultural practices to promote a healthy plant with deep roots:

  • manipulate cutting heights to suit the prevailing conditions – cutting too short for prolonged periods will reduce rooting depth;
  • do not over-water – too much irrigation will result in a shallow-rooted turf. Consider irrigation inputs (quantity), duration and frequency;
  • Ensure surfaces are adequately aerated and drained. Poor drainage will encourage the build-up of organic matter and result in a shallow-rooted plant that will make the surface more vulnerable to leaching.  It will also result in a higher loss of nitrogen due to denitrification.


In summary, nitrogen is essential for plant growth but is particularly vulnerable to losses via leaching and denitrification if misapplied.  Taking time to carefully consider your choice of fertiliser, dosage and mode of application is critical to reducing these losses and should help you to minimise expenditure whilst also improving your playing surface.  Using a coated fertiliser or controlled release granule (CRF) can drastically reduce leaching losses and denitrification to the environment and so provide a healthy, good quality turf with optimum inputs.