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Fertilisers

Nitrogen (N)

Nitrogen Cycle - The original source of nitrogen used by plants is an inert gas that constitutes about 78% of the earth's atmosphere. Unfortunately, elemental nitrogen is useless to higher plants. Regardless of how it enters the soil system, nitrogen must be converted to one of several common forms before it can be utilised by plants. Most of these are ionic forms, meaning they carry a particular electrical charge. However, some nitrogen can be taken up in a gaseous form.

There are several sources or inputs of nitrogen that plants can utilise for growth:

1. Rail or lightning that returns nitrogen compounds in the atmosphere to the earth.
2. Microbial decomposition of soil organic matter in the spring and autumn that releases native nitrogen to the plant.
3. Clippings returned to the turf and broken down by micro-organisms that recycle some available nitrogen.
4. Fertilisers may apply nitrogen in several plant available forms.

Regardless of how nitrogen enters the soil, it will be converted to ammonia (a gas symbolised as NH3) or ammonium (an ionic form symbolised as NH4+) and taken in by one of several pathways to the plant. Ammonia can be immediately taken up by plants in small amounts to provide quick responses, or it can be held biologically, converted to ammonium (NH4+), or lost to the atmosphere as a gas through volatilisation.

Some of the nitrogen is quickly absorbed by plants and utilised for growth in the ammonium form. In clay soils, portions of this plant - available form become fixed or tightly trapped between the clay particles, making it unavailable to the growing plant. This entrapment is called ammonium fixation and can cause a temporary deficiency of plant nitrogen. For this reason, heavy clay soils usually require additional nitrogen fertiliser inputs to meet plant needs.

Most of the ammonium form is converted by soil micro-organisms called nitrosomanas to a plant-unavailable form, nitrite (symbolised as NO2-). The nitrite form is then converted again, this time by different soil micro-organisms known as nitrobacter, to another plant-available form, nitrate (symbolised as NO3-). This process makes the nitrogen slowly available to the plant and is called nitrification. Most of the plant-available nitrogen is found in the nitrate form because of the intense nitrification that occurs in most soils. This fact illustrates the importance of good soil aeration to support the soil micro-organisms population responsible for converting unavailable nitrogen to more plant-available forms. Soil pH, moisture and temperature are other factors that affect the activity of the soil micro- organisms.

Once in the nitrate or NO3- form, the nitrogen is subject to loss from the soil system before it can be absorbed by the plant. For example, it may be washed through the soil profile by leaching because of its weak attraction for negatively charged soil particles. *Much of the soil nitrogen is lost in this manner. Small amounts of nitrate may be immobilised to unavailable forms by other soil micro-organisms, and still more nitrogen may be lost as a gas through volatilisation. The nitrate nitrogen that remains is then absorbed by the plant roots and translocated to the shoots to be utilised in the photosynthetic and respiration processes for growth, and the cycle repeats itself.

Nitrates, like soil particles, have a net negative charge and tend to repel similarly charged soil particles. Ammonium, because of its net positive charge, is attracted to soil particles and thus is not lost through leaching.

Nitrogen in fertilisation

Most of the nitrogen inputs responsible for improved turfgrass growth are a direct result of fertilisers. Fertilisers apply the nitrogen in several different ionic forms, but only two, the ammonium and nitrate forms, can be taken up by plants in any appreciable quantities. Except for the differences in response, the turn can not differentiate between a natural organic, an inorganic or synthetic organic fertiliser.

An inorganic fertiliser such as common ammonium nitrate applies nitrogen in two plant available forms - ammonium (NH4+) and nitrate (NO3-). Since both of these forms are immediately available and do not have to undergo microbial decomposition for release, the plant will utilise more nitrogen than is really necessary for the photosynthetic process. The result is an unhealthy surge of growth, and unfortunately, very little residual nitrogen remains for later uptake due to more extensive losses through volatilisation and leaching.

Natural organic fertilisers work in just the opposite fashion. Most of the nitrogen must be changed to an inorganic form such as NH4+ or NO3- through microbial decomposition before it can be released to the plant. This phenomenon is similar to organic matter decomposition in the soil, and the result is a slow, steady release of nitrogen with very little loss to volatilisation or leaching. Their disadvantage lies in the fact that the plant shows very little initial response and may require supplemental applications of more immediately available forms to get the desired response.

The synthetic organic fertilisers such as methylene urea apply nitrogen in forms that are immediately less available to the plant along with forms that require microbial degradation for release. This means the initial response is rather quick, but there is still residual nitrogen that becomes slowly available for continuous, healthy growth.

Phosphorus (P)

The Phosphorus native to most soils originated from the weathering and decomposition of rocks containing the mineral apatite, usually in an inorganic but unavailable form. Today's soils derive a large portion of their available Phosphorus from the decomposition of organic matter that has accumulated in the soil, but the greatest quantities are supplied by Phosphorus fertilisers derived from rock phosphate or apatite.

The Phosphorus ions are present in small concentrations in the soil solution as either the primary orthophosphate form (symbolised as H2PO4-), or the secondary orthophosphate form (symbolised as HPO42-), but there are about ten times as many absorption sites on plant roots for H2PO4- as there are for HPO42-. Small concentrations of these ions must be maintained for proper turfgrass growth since higher concentrations may tie up the phosphates in plant unavailable forms. The concentration and thus the availability of the phosphate ions is controlled by the soil pH. A pH range of 6.7 - 7.5 gives the greatest availability of phosphate ions to turfgrasses.

At a low or acid pH, the H2PO4 becomes bonded to aluminium and iron in the soil, making it unavailable to plants. At a high or alkaline pH, the H2PO4- attaches to calcium in the soil and is also held in a plant unavailable form. Phosphorus can be released to the soil solution and made available to the plants by raising or lowering the soil pH into the desired range. This adjustment will break the chemical bonds that hold Phosphorus to calcium, iron and aluminium. The plant will then absorb either orthophosphate ion through the root tip and translocate it freely from the lower to upper leaves where it is used in the growth process. The cycle then repeats itself.

Phosphorus in Fertilisation

A well maintained, established turf is rather efficient at extracting phosphorus from this reserve and needs only two or three small maintenance applications throughout the year to replace that being used by the plant. Over fertilisation with phosphorus may build up soil levels so high that minor elements such as iron, manganese, zinc and copper become inactivated and express deficiency symptoms.

On new seedings or in areas where soil used for established turf is extremely low in phosphorus, a supplemental application of a high-phosphorus fertiliser will correct such deficiencies through more efficient plant use. Ammoniated phosphate as a phosphorus source is desirable as it releases small amounts of quick-release nitrogen along with a water soluble form of phosphorus. This means the phosphorus will release even in cold soils, thus allowing it to be taken up by the plant, even when it is not actively growing.

Potassium (K)

The potassium native to most mineral soils originated from the weathering and decomposition of mineral rocks such as feldspar, muscovite and biotite. Other sources include:

1. The decomposition of organic matter in the soil and clippings returned to the soil surface.
2. The release of potassium from the surface of clay minerals within the soil by freezing, thawing, wetting and drying.
3. The application of a potassium fertiliser.

The potassium ion (symbolised as K+) that plants utilise exists within the soil in three fractions: 1) relatively unavailable or fixed, 2) slowly available and 3) readily available or exchangeable. Soil tests reveal the exchangeable fraction because it is a good indication of the potassium fertility status of the soil at any point in time.

Once in the soil solution, the K+ ion can readily be taken up by plants because it is water soluble. However, some of the K+ ions can become trapped between clay particles in the soils and converted to a fixed or unavailable plant form. Alternate periods of wetting and drying and freezing and thawing can release these trapped ions and make them available to plants. Additional potassium may become slowly available through the continued weathering of minerals within the soil.

Leaching removes relatively small amounts of exchangeable potassium from the upper soil profile and deposits it in the subsoil where it becomes entrapped between the clay particles. Liming of acid soils to raise the soil pH will reduce losses of potassium to leaching.

Potassium in Fertilisation

Potassium is absorbed by plants in larger amounts than any other element except nitrogen. Most soils provide a healthy reservoir of potassium in either the fixed, slowly available, or exchangeable form that plants can efficiently utilise. However, there is a tendency for turfgrasses to take up larger quantities than are needed for maximum growth when this element is present in an available state. This excessive absorption of potassium beyond the plant's needs is designated as luxury consumption and can be avoided by reducing application rates, or by using controlled-release K.

There are several sources of potassium used in various fertilisers today, but some provide several distinct advantages over others. Potassium sulphate K2SO4 is the most desirable for several reasons.

1. Its low salt index provides a safe source of potassium to reduce the chance of burn.
2. Its low water solubility reduces the leaching of potassium from the soil profile.
3. The sulphur contained in potassium sulphate provides another essential plant nutrient and reduces the incidence of diseases, especially Fusarium nivale.

Controlled-Release Nitrogen Fertiliser

Controlled release of nitrogen in fertilisation has been pursued by researchers and manufacturers over the past half century. The slow and steady delivery of nitrogen is highly desirable for:

• Increased efficiency of nitrogen utilisation.
• Reduced potential for turf injury (lower salt index).
• Delivery of more uniform growth (less surge growth).
• Supply of nitrogen synchronised with plant requirements.
• Reduced pollution from leachate and volatilisation.

Historically, two basic approaches have been taken to control nitrogen release - controlled solubility (encapsulation technologies and IBDU) and controlled nitrification (methylene urea and urea formaldehyde).

MU technology also has a very favourable salt index which means it is very safe to turf. This table shows the comparative salt index of a number of common nitrogen sources.

TRIAFORM Technology

Scotts Triaform technology is characterised by a low salt index, limited leaching potential and steady slow release properties. Triaform also supplies organic carbon to help build sustained microbial populations.

POLY S Technology

Poly-S fertiliser is granular urea that has been sulphur coated first, with a thin layer of sulphur, and a polymer is sprayed onto the hot granule over the sulphur precoat. This process is patented and represents the fastest growing technology for Scotts.

ONCE Technology

Once technology combines two of Scott's premier controlled release technologies,Osmocote and Poly S. The Poly S in Once supplies steady, up-front and controlled-release nitrogen, while the dominant Osmocote component supplies N, P and K via temperature controlled release, mirroring the needs of growing turf. Once is available in standard particles for general turf use - and in fine particle (as with Super K Greens) for greens use.

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Salt Indexes of Commonly Used Fertilisers 

Nutrient Deficiency Chart

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Soil pH Effects on Availability of Nutrients

N.P.K. Ratings

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Average Composition of Fertiliser Materials

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Sand Particle Size Classification