Primary symptoms

Yield can often be reduced 10-30% by deficiencies of major nutrients before any clear symptoms of deficiency are observed in the field. Stunting or color changes are typical symptoms of deficiency. Field observations, calculations and soil analysis can indicate nutrient deficiencies.

Some nutrient deficiencies can be confused with diseases.

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Identifying nutrient problems based on leaf symptoms

Young leaf symptoms	Young leaf flow

Old leaf symptoms	Old leaf flow

Summary

For best growth, wheat needs many nutrients, in particular, the macronutrients oxygen (O; about 48% of dry matter), carbon (C; 42%), hydrogen (H; 6%), nitrogen (N 2%), potassium (K), phosphorus (P), calcium, magnesium and sulphur. The amounts of each steadily accumulate in the crop as it grows but their concentrations decline as the crop accrues old tissues. Old tissues have lower concentrations of nutrients than young ones. Wheat also needs very small amounts of the microelements iron, manganese, boron, zinc, copper, sodium, molybdenum, chlorine, cobalt, and silicon. Apart from the first 3 elements (O, C and H), which come from the air and water, the remaining 16 can be managed to some degree by soil or crop treatments.

Recommendations for applications of N, P and K differ widely with soil type and fertility and expected use of the fertilizer by the crop. A very short duration crop may not have sufficient time to use as much fertilizer as a long duration crop. Fertilizer policy must be based on local practice, your target yield and crop rotation. Bigger crops need more nutrients and fertilizer. Be cautious though, an excess of nitrogen may result in ground water pollution and lodging. If nitrogen is limiting, yield and probably grain protein will be reduced. Applying nitrogen after spikes emerge generally increases grain protein.

As a rough guide, a 7-t/ha crop removes from the field as grain 150-190 kg N, 25-35 kg phosphate and 45-60 kg potassium. These nutrients must be returned to the soil after every big crop to avoid depleting reserves. Work out the approximate amounts removed for your target yield. For a 4-t/ha yield for example, about 150 * 4/7 kg N would be removed (85 kg N).

When plants do not receive enough of a nutrient to satisfy their requirements, or receive too much, they grow poorly and, if the imbalance is large enough, they show symptoms of the problem. Symptoms for most deficiencies or toxicities are generally most obvious on the leaves.

If a soil nutrient is being progressively depleted by the growing crop, and the nutrient can not be moved from older to newer leaves, the symptoms will be more apparent on young leaves. If the nutrient is mobile in the plant, the plant will extract it from the old leaves for use in the young leaves. Then the old leaves will show the symptom. Be careful with boron however, which shows no significant symptom on the leaves. Only at anthesis, when the sterile florets of heads gape, does the deficiency become apparent.

Be careful also that you do not confuse the symptoms with similar symptoms due to disease. If the cause is nutritional, symptoms will occur in large areas of the field. If the cause is disease, the symptoms are likely to be on isolated plants or in patches in the crop.

First check whether the symptoms are on older or younger leaves and then use the appropriate flow chart and photographs (see top of page) to make a first-order identification of the problem. (Grundon 1987).

Is mineral nutrition a problem?

• Walk through the field at different growth stages. Look for large areas of poorly grown or pale coloured plants with poor growth. Look more closely and decide whether the old or young leaves are most affected. If you see any of the symptoms described on old leaves or young leaves, you can be fairly sure that the problem is already reducing yield.

Causes of nutrient problems

• Soil has a long history of heavy cropping without sufficient replacement of nutrients (not enough fertilizer was applied). Check the cropping history and fertilizer applications of this and earlier years. Calculate whether nutrients are likely to be limiting from the difference between nutrients removed in yields and fertilizer applied.
• Soil is low in organic matter (or insufficient fertilizer was applied).
• Fertilizer was applied but lost due to leaching from heavy rain or over irrigation, run off or volatilisation or lost to competing weeds or an intercrop.
• Fertilizer was applied when the crop could not use it optimally. What was the crop stage when the fertilizer was applied, its type and amount and was it broadcast or banded?
• Soil pH is such that certain nutrients were unavailable (see acid and alkaline soils). Test soil pH. If it is less than 5.5, magnesium deficiency is possible and phosphorus may be unavailable. If greater than 8, deficiencies of zinc, iron, copper and boron are possible. Compare with assessments already reached from the photo keys.
• Waterlogging occurred from heavy rainfall, over irrigation and/or poor drainage on heavy soil. Check for symptoms of waterlogging (see also moisture stress), soil type and check rainfall and irrigations.
• Wheat or maize straw was used as a mulch or large amounts of residues were incorporated. Some reports suggest nitrate may be lost with this practice under high temperature (see crop residues).
• Soil is saline.
• Check soil depth as a plow pan or other restriction may be limiting roots to the upper soil profile so nutrients in deeper layers are unavailable.

What to do about nutrient problems

• Soil test: Before planting get help to do soil tests to check for deficiencies of zinc, phosphorus or potassium particularly if your last crop had deficiency symptoms. Nitrogen is best monitored during the season using tissue analysis.
• Phosphorus: Phosphorus is not mobile in the soil so place it with the seed at sowing. If your wheat follows a rice crop, phosphorus is probably required. Be aware that diammonium phosphate may injure the seed.
• Fertilizer rates: Increase rates of fertilizer for the limiting nutrient. Consider foliar application if a rapid response is required though the effects are usually of short duration e.g. for nitrogen use urea, for manganese use manganese sulphate, for iron use inorganic salts or chelates and for copper use copper sulphate.
• Farmyard manure: Consider adding farmyard manure if available as this contains most micro and macro nutrients and incorporate stubble from the previous crop to build up organic matter and improve aeration. The stubble contains lots of potassium that may be limiting in an acid soil. Alternatively, grow and incorporate a green manure before next season to improve soil organic matter.
• Fertilizer management: Change fertilizer application method and/or timing so that less is lost by run off, leaching or volatilisation. About 65% of nitrogen applied at planting may be lost, but losses of only 35% occur if fertilizer is placed at the first node stage (Z3.1) (Sayre and Moreno Ramos 1997). Generally it is best to split your nitrogen applications between planting and first node. Top-dress fertilizer just before irrigations or before rain to aid infiltration.
• Improve drainage: Consider using a raised bed system for next season with its inherently better drainage and more efficient use of water.
• Weeds: Remove weeds to make more nutrients available to the crop.
• Soil pH: Before next season increase soil pH towards 6 (see acid or alkaline soils) in very acid soils by adding lime or limestone and dolomite but care should be taken not to over-lime as deficiencies of potassium, magnesium, iron, manganese, boron, zinc or copper may result.
• Micronutrients: Apply micronutrients if indications from plant symptoms and pH measurements suggest they are limiting. If your field is sulphur-deficient, use a fertilizer that contains sulphate sulphur at planting.