Nutrients other than nitrogen, phosphorus and potassium (NPK) for cereals

Summary

The elements which are essential for plant growth are classified into two groups, according to the relative amounts (kg/ha or g/ha respectively) required by crops:

Major nutrients: nitrogen (N), phosphorus (P), potassium (K), sulphur (S), magnesium (Mg), calcium (Ca), and chlorine (Cl)
Trace elements or micronutrients: boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn).
Balanced crop nutrition, with an adequate supply of all the essential plant nutrients, is crucial for optimum yield and quality of cereals. Inorganic NPK and, increasingly S fertiliser inputs, represent a significant proportion of the variable costs for cereal production and, to help ensure their efficient use, the availability of other nutrients must not be a limiting factor. Previous HGCA-funded reviews have covered both Cu deficiency and S nutrition in cereal crops, but little attention has been given to other non-NPK nutrients in recent years. This review draws together and updates existing information on the incidence, diagnosis and treatment of deficiencies of nutrients other than NPK in cereals, including the effects of deficiency on grain yield and quality. The soil chemistry and role in plants of these nutrients are also summarised.

Research studies have shown that deficiencies of Mg, S, Cu, Mn and, very rarely Zn, can occur in cereal crops, and are usually associated with specific soil types. Each deficiency produces characteristic symptoms in cereal crops and different cereal types can vary in their susceptibility to a particular deficiency. However, any inputs of these nutrients must be targeted according to deficiency risk, and not applied simply as insurance dressings, to ensure their cost effective use. Cereals are not susceptible to deficiencies of the other non-NPK nutrients (B, Cl, Fe, Mo) under UK growing conditions.

Cereals may show visual, and often transient symptoms of magnesium deficiency but seldom give a yield response to Mg applications, unless soil reserves of Mg are very low. The latter situation is most likely to occur in sandy soils where sugar beet or potatoes are not grown in the rotation. This deficiency can, however, be induced on a wide range of soils under conditions of crop stress caused by poor soil structure, restricted rooting and/or drought. Treatment is very rarely necessary, unless symptoms persist, in which case a foliar Mg spray should be applied. The soil Mg status should be maintained above Index 0 in arable rotations to avoid any risk of Mg deficiency limiting cereal yields. Magnesium application is very unlikely to improve grain quality on non-deficient soils.

Sulphur deficiency in cereals has increased over the last decade because of the continuing decline in the atmospheric deposition of S, due to restrictions on sulphur dioxide emissions from industrial sources. The occurrence of S deficiency is variable and depends on the interaction between crop N and S supply during the growing season. Grain yield responses have ranged from 4 to 40% across deficient sites in the UK and have sometimes occurred in the absence of any visual response to S application. Conversely, yield responses to applied S are not always obtained where deficiency symptoms appear in the untreated crop. Leaf analysis between flag leaf emergence and mid flowering (GS39-65), also grain analysis, can be used to assess crop S status and whether S is required for subsequent cereal crops. Soil analysis does not, however, reliably predict S deficiency in cereals. Currently, cereal crops grown on well drained sandy or shallow soils in areas where atmospheric deposition is less than 20 kg S/ha/year, are likely to need S. Sulphur deficiency will gradually become more widespread and may occur on a wider range of soil types in the future. Modelling predictions suggest that 30-40% of the UK land area may currently be at some risk of S deficiency for cereals and that this proportion will increase to about 50% by 2003. Sulphur deficiency is best prevented by a spring application of 10-20 kg/ha S as a water soluble sulphate fertiliser. The development of plant diagnostic techniques for earlier identification of S deficiency during the growing season will enable more effective corrective treatment of deficiency in the growing crop.

Loaf volume, as a measure of breadmaking quality in wheat, is reduced by S deficiency and may sometimes be increased, without any response in grain yield, by S application. There is little information on whether malting quality in barley is affected by low S (relative to N) in the grain. Further work is also needed on S availability from organic manures and on effective options for late foliar S applications.

Soil data indicate that up to 5% of the cereal growing area in England and Wales, and 30% in Scotland may be deficient in copper. This deficiency usually occurs on sandy, shallow chalk and peaty soils; sometimes the deficiency is sub-clinical, where yield is reduced without any apparent symptoms. Soil analysis is particularly useful for identifying Cu deficiency, while plant or grain analysis is less reliable. Copper treatment, to prevent deficiency, is normally applied as a foliar spray of inorganic or chelated Cu in the spring. A large soil dressing of copper oxychloride or copper sulphate, prior to sowing, is an alternative treatment option where a deficiency has previously been identified. In practice, soil applications are rarely used as they are less convenient than using annual foliar sprays. A single soil application has an appreciable residual value, which lasts for at least five years, but probably needs an additional foliar Cu spray in the first season following its application, to be fully effective. More information is needed on the extent of sub-clinical Cu deficiency in UK cereal crops.

Manganese deficiency is the most common trace element deficiency in cereals and 15-20% of the total cereal area is usually treated with Mn each year. Soil applications of Mn are generally ineffective, as the applied Mn rapidly changes into less available forms. Deficiencies are best prevented or, where Mn deficiency only occurs infrequently, corrected by foliar spraying with manganese sulphate or a proprietary chelated or inorganic Mn product in the spring. Autumn, as well as spring treatment, may be necessary on very deficient soils. Manganese seed dressings, combined with subsequent foliar sprays, may also be useful for very deficient sites where deficiency can develop while there is still very little plant cover for foliar uptake. Take-all disease in cereals does not appear to be exacerbated by Mn deficiency under UK conditions, which is in contrast to findings in Australia and America, but increased mildew incidence is often associated with deficient plants. Treatment strategies for controlling Mn deficiency require further development.

Zinc deficiency has occasionally been recorded in barley crops grown on sandy soils with high pH and P status in Ireland and Scotland, but has not so far been encountered in England and Wales. The average annual rate of Zn deposition from the atmosphere exceeds crop removal of Zn, even in high yielding crops, but there is little information on the actual availability of this deposited source of Zn to crops. A re-assessment of the Zn status of crops grown on sandy soils in areas with relatively low Zn deposition would, however, identify whether there is currently any potential deficiency risk. Foliar spraying with zinc sulphate, or a proprietary chelated or inorganic Zn product, in the spring is recommended for the treatment of Zn deficiency.

Trace element inputs from atmospheric deposition can be significant, especially near the coast and in areas of industrial activity. Organic manures, where used in arable rotations, will also add small amounts of these nutrients and contribute to the net balance of trace element reserves in soils. High yielding crops do not necessarily need trace element (Cu, Mn, Zn) applications, despite their greater nutrient demand, as such crops are more efficient at obtaining these nutrients from soil reserves. Greater mobilisation of these trace elements occurs in the rooting zone of high yielding crops. There is no clear evidence at present on whether sub-clinical deficiencies may now be occurring on a wider range of soil types than those where treatment would conventionally be recommended. Information on the Cu, Mn and Zn nutrition of current cereal cultivars is, however, very limited and the risk of sub-clinical deficiencies on marginal soil types should be further investigated. There is also very little information on whether applying combinations of trace elements together can have synergistic effects on grain yield and quality.

Deficiency risks at field level should initially be assessed from a knowledge of soil types and past incidence of observed symptoms or confirmed deficiencies in susceptible crops. Soil1 analysis will accurately predict the likelihood of Cu or Zn deficiency on candidate soil types. Leaf analysis can be used to diagnose or confirm whether visual crop symptoms or a suspected latent deficiency are caused by Mg, Mn, S or Zn deficiency. Where a particular deficiency problem is clearly identified, an appropriate amount and form of the specific element should be applied as a preventative or corrective treatment.

A survey of the main commercial analytical laboratories and product supply companies in the UK showed that a wide range of deficiency threshold levels are used for some nutrients to interpret soil and plant analyses. Greater standardisation of crop and soil sampling procedures, analytical methods and associated interpretative guidelines on deficiency thresholds would help to improve the accuracy of diagnosing nutrient deficiencies in cereal crops.

Future technology transfer requirements and research priorities for the major and trace elements covered by this review are identified.