ARCHIVE: Fertiliser Manual (RB209)

Fertiliser Types and Quality


 

It is important to select the most appropriate and cost-effective material from the many different types of fertiliser that are available in both solid and fluid forms. The following features of a fertiliser should be considered:

  • The total concentration, and the ratio of nutrients in the fertiliser.
  • The chemical form of each nutrient.
  • The physical quality of a solid fertiliser and its suitability for accurate spreading.
  • The form of a liquid fertiliser, true solution or suspension.
  • The cost of the nutrients.

The total concentration, and the ratio of nutrients in the fertiliser

The total concentration of each nutrient in a fertiliser has to be declared. Some fertilisers (straight fertilisers) contain just one nutrient, whereas many fertilisers contain more than one nutrient. The requirements for nutrient declaration are controlled by legislation contained in EC Regulation No. 2003/2003, The EC Fertilisers (England and Wales) Regulations 2006 (SI 2486) and in The Fertilisers Regulations 1991 as amended (see Section 9). The nutrient content of some common fertilisers is given in Appendix 7.

The concentration of nutrients in fluid fertilisers may be expressed as kg nutrient per tonne of fertiliser product (w/w basis) or as kg nutrient per cubic metre (1000 litres) of fertiliser product (w/v basis). To convert from one basis to the other, it is necessary to know the specific gravity of the fertiliser. The fertiliser supplier can provide this information.

Concentration as w/v (kg/m3) = concentration as w/w (kg/t) x specific gravity

The concentration of the nutrient or nutrients in a fertiliser dictates the application rate. When there is more than one nutrient the ratio should be reasonably close to the required application of each nutrient.

The chemical form of each nutrient and its availability for crop uptake

Some nutrients can be present in different chemical forms which may differ in their immediate availability for uptake by roots. The requirements of the declaration are controlled by The EC Fertilisers (England and Wales) Regulations 2006 (SI 2486), EC Regulation 2003/2003 or, in some cases, by The Fertilisers Regulations 1991 as amended (see Section 9).

The physical nature and quality of the fertiliser and its suitability for accurate application

Fertilisers may be sold in many different physical forms, some of which may be difficult to apply accurately. Fertilisers of the same type but from different sources can vary significantly in their physical characteristics. Good fertiliser practice must include accurate, uniform application as well as correct decisions on rate and timing. Inaccurate application of fertiliser will result in uneven crops with lower than expected yields and the quality may be poor. Over application may result in adverse environmental impact from pollution. It is important for good spreading of solid fertiliser that the particle size is consistent, free of lumps and low in dust, while the components of a fluid fertiliser should remain as a uniform solution or suspension.

The cost of the nutrients

The cost of the nutrients in fertilisers can vary significantly. When comparing fertiliser prices it is necessary to calculate and compare the cost of each kg of nutrient. It is cost per hectare, not cost per tonne, which determines the economics of a fertiliser application. Low cost fertilisers can have a poor chemical or physical quality. The availability of the nutrients for crop uptake must also be considered as well as the ability of the fertiliser spreader or sprayer that is used to apply the fertiliser accurately.

Nitrogen fertilisers

Ammonium nitrate (33.5-34.5% N); ammonium sulphate (21% N, 60% SO3), calcium ammonium nitrate or CAN (26-28% N): The nitrate-N is immediately available for crop uptake, the ammonium-N can be taken up directly but is quickly converted to nitrate by soil microbes.

Urea (46% N): Before uptake by plants, urea-N must first be converted to ammonium-N by the enzyme urease that occurs in all soils. This process usually occurs quickly and does not significantly delay the availability of the nitrogen for crop uptake. Typically, around 20% of the nitrogen content of applied urea may be lost to the atmosphere as ammonia. As a result less nitrogen is available for crop use and emissions may lead to impacts on biodiversity and human health. Losses are more closely related to soil moisture and weather conditions than to soil type, and may be minimised if urea is applied shortly before rain is expected, and/or is shallowly cultivated into the soil. Urea is a low-density material which, in prilled form, can be less easy to spread accurately over wide bout widths when using spinning disc equipment.

Liquid nitrogen (18-30% N): Liquid nitrogen fertilisers are solutions of urea and ammonium nitrate. The nitrogen is in forms that are quickly available for crop uptake. Solutions based on urea alone will contain no more than 18% N because at low ambient temperatures urea crystallises out of solution.

Phosphate fertilisers

Water-soluble phosphate: Ammonium phosphates (diammonium phosphate (DAP) and monoammonium phosphate (MAP)) and superphosphates (mainly triple superphosphate (TSP) but occasionally single superphosphate (SSP)) contain phosphate mainly (93 – 95%) in a water-soluble form.

Water-insoluble phosphate: There are many different types of water-insoluble phosphate with different chemical and physical characteristics. The fertiliser declaration should give details of the amount of phosphate soluble in different acid extractants. This information does not indicate the effectiveness of these sources of phosphate on different soil types. Care should be taken not to compare the solubility of water-insoluble phosphates in different reagents (for example, formic acid and citric acid) that extract different forms of phosphate.

Lack of water-solubility does not mean the phosphate is unavailable to crops. Finely ground, reactive phosphate rocks, for example, with close to zero water-solubility, can be used successfully as a phosphate source on grassland where the surface soil pH is maintained below pH 6.0. Some other water-insoluble phosphates are an effective source of phosphate under appropriate soil and weather conditions, and in these situations seek advice from a FACTS Qualified Advisor about their use.

Potash and magnesium fertilisers

Potash in most common sources of potash fertiliser is quickly available to crops. The most common potash source is muriate of potash (MOP) which is potassium chloride (60% K2O). Kainit and sylvinite are naturally-occurring mixtures mainly of potassium chloride and sodium chloride. Potassium sulphate (SOP, 50% K2O) is used for some high-value crops. Potassium nitrate supplies both potassium and nitrogen and is used as a source of both nutrients when added to irrigation systems. Some magnesium fertilisers are quickly available (e.g. kieserite (typically 25% MgO) and Epsom salts (16% MgO)) though others are only slowly available (e.g. calcined magnesite and magnesian limestone). Where both lime and magnesium are needed, magnesian limestone will often provide a cheap though slow acting source of magnesium. An application of 5 t/ha of magnesian limestone will provide at least 750 kg MgO/ha and the magnesium become slowly available to crops over time.

Sulphur fertilisers

Ammonium sulphate (60% SO3) and kieserite (typically 52-55% SO3) provide sulphur in an immediately available form. Gypsum (calcium sulphate) is somewhat less soluble but is an effective source. Elemental sulphur must be oxidised to sulphate before it becomes available for uptake. The speed of oxidation depends largely on particle size and only very finely divided (‘micronised’) elemental sulphur will be immediately effective. Where particle size is larger, elemental sulphur fertilisers can be used to raise the sulphur supply capacity of the soil over a longer period of time.

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