ARCHIVE: Fertiliser Manual (RB209)

Crop nutrient requirements

Some 13 elements, in addition to carbon (C), hydrogen (H) and oxygen (O), are known to be essential for plant growth and they can be divided into two groups:

  • Macronutrients: these are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S) and are required in relatively large amounts.
  • Micronutrients (trace elements): these include iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), boron (B), molybdenum (Mo) and chlorine (Cl), and are required in smaller amounts than the macronutrients.

The names macro- and micro- nutrients do not refer to relative importance in plant nutrition; a deficiency of any one of these elements can limit growth and result in decreased yield. It is therefore important to ensure that there is an optimum supply of all nutrients – if a plant is seriously deficient in, for example, potassium it will not be able to utilise fully any added nitrogen and reach its full potential yield and any unutilised nitrogen may be lost from the field.

In the UK, two conventions are used as follows:

  • For fertiliser contents and for recommendations, phosphorus is expressed in the oxide form phosphate (P2O5) and potassium as potash (K2O). Sulphur, magnesium and sodium also are expressed in oxide forms (SO3, MgO and Na2O).
  • Soil and crop analysis reports usually show elemental forms for example mg P/kg or mg K/l. 

Oxide or elemental forms are used in this Manual according to context.

Achieving the right timing of nutrient application is as important as applying the correct amount. Crop demand varies throughout the season and is greatest when a crop is growing quickly. Rapid development of leaves and roots during the early stages of plant growth is crucial to reach the optimum yield at harvest, and an adequate supply of all nutrients must be available during this time.

Excess application of nutrients, or application at the wrong time, can reduce crop quality and cause problems such as lodging of cereals or increases in foliar pathogens. Excessively large amounts of one nutrient in readily plant-available forms in the soil solution may also decrease the availability or uptake by the root of another nutrient.

Other elements found in plants, which may not be essential for their growth include, cobalt (Co), nickel (Ni), selenium (Se), silicon (Si) and sodium (Na). Sodium has a positive effect on the growth of a few crops. Some elements, such as cobalt, iodine (I), nickel and selenium are important in animal nutrition. These are normally supplied to the animal via plants, and must consequently be available in the soil for uptake by plant roots.

All these elements are taken up by plant roots from the supply in the soil solution (the water in the soil). They are absorbed in different forms, have different functions and mobility within the plant and hence also cause different deficiency, or very occasionally toxicity, effects and symptoms.

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