Quick reference guide to mineral nutrients for citrus production
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Nitrogen (N)
| Role in the plant | Vital part of proteins and chlorophyll. Proteins are involved in metabolism, and chlorophyll is involved in making sugar using carbon dioxide from the atmosphere and sunlight. |
|---|---|
| Symptoms of deficiency | Poor stunted shoot growth, old leaves pale green-yellow, thin foliage cover and dieback of twigs. Poor fruit set and fruit size. Fruit quality is good. |
| Symptoms of excess | Vigorous vegetative growth. Thick-skinned, large, puffy fruit, delayed maturity, and regreening is likely. Juice % and quality decline. Short storage life. |
| Movement in soil | Mobile |
| Movement in a plant | Mobile |
| Availability to tree roots | Nitrate form is preferred, but ammonium is also taken up. Conversion between these forms is dependent on soil temperature, organic matter and microbial activity, which is greatest in the pH range of between 5.5 and 8.5. Use readily available form (i.e. nitrate) during cooler months. |
| pH effects on availability to tree roots | None known |
| Interactions with other elements | Higher tree N levels (>2.6% in leaves) are associated with lower boron and/or sulfur in leaves. The effect of excessive N worsens with low P supply. |
| Other information | Easily leached in sandy soil. Lost to the atmosphere in waterlogged soil. |
Phosphorus (P)
| Role in plant | Metabolism, cell division and growth. |
|---|---|
| Symptoms of deficiency | Old leaves are dull, bronzed green. Fewer flowers. Misshapen fruit, open centres, thick coarse peels. Pulpy fruit, lower juice, more acidic. |
| Symptoms of excess | Smaller fruit, higher % juice, thinner peel, chance of regreening. |
| Movement in soil | Immobile |
| Movement in plant | Mobile |
| Availability to tree roots | Available and unavailable forms present; relative proportions dependent on soil pH. In alkaline soil, P is fixed in calcium compounds, and in acidic soil, in iron and aluminium compounds. Unavailable forms act as a reserve for available forms. Most P is in the upper soil layer. Australian soil is naturally low. |
| pH effects on availability to tree roots | Most are available in the range of 6 to 7. |
| Interactions with other elements | Too much P can accentuate the effects of low zinc availability, induce iron deficiency and affect copper uptake. |
| Other information | Losses from soil are only in the erosion of soil particles or in very sandy soil. Soil compaction reduces uptake. |
Potassium (K)
| Role in plant | Metabolism, water relations, internal balance, stress and disease resistance. |
|---|---|
| Symptoms of deficiency | Slower growth, small leaves, and heavy leaf fall. Severe deficiency causes heavy flower/fruit drop. Small, smooth, thin-skinned fruit that colour early, split easily and are more prone to albedo breakdown. |
| Symptoms of excess | Thicker rough rinds and a chance of regreening. Delayed maturity in oranges. |
| Movement in soil | Mobile |
| Movement in plant | Mobile |
| Availability to tree roots | Soil contains large amounts but most is unavailable. |
| pH effects on availability to tree roots | None known |
| Interactions with other elements | Excessive K supply reduces magnesium and calcium uptake. |
| Other information | Large amounts are removed from the fruit. |
Sulfur (S)
| Role in plant | As for N, but required in much lesser amounts. |
|---|---|
| Symptoms of deficiency | Younger leaves stunted, pale green-yellow with lighter veins. |
| Symptoms of excess | None known |
| Movement in soil | Mobile |
| Movement in plant | Mobile |
| Availability to tree roots | Available to plants only in sulfate form. Most soil sulfur is present in the organic matter. |
| pH effects on availability to tree roots | Less available in low pH soil. |
| Interactions with other elements | None known |
| Other information | Sulfate forms easily leached. Long-term use of ammonium sulfate leads to lower soil pH. |
Calcium (Ca)
| Role in plant | Metabolism, internal balance, tissue strength. |
|---|---|
| Symptoms of deficiency | Stunted roots, higher incidence of albedo breakdown. |
| Symptoms of excess | None known |
| Movement in soil | Immobile |
| Movement in plant | Immobile |
| Availability to tree roots | Plant available Ca is associated with soil cation exchange capacity. High Ca levels in high pH soil due to calcium carbonate (lime). |
| pH effects on availability to tree roots | Deficient in low pH (acidic) soil. |
| Interactions with other elements | Heavy applications of K might reduce Ca uptake. Deficiencies, more likely on acidic soil. Calcium carbonate can reduce iron uptake ('lime-induced iron chlorosis'). |
| Other information | Other important sources of Ca include gypsum (calcium sulfate) and single-strength superphosphate. |
Magnesium (Mg)
| Role in plant | Synthesis and function of chlorophyll and protein, internal balance. |
|---|---|
| Symptoms of deficiency | Yellowing towards the apex of leaves with a triangular area remaining green at the base in older leaves, defoliation and shoot dieback. Smaller fruit and lower yield. |
| Symptoms of excess | None known |
| Movement in soil | Attaches to clay particles. |
| Movement in plant | Mobile |
| Availability to tree roots | Plant available Mg associated with soil cation exchange capacity. |
| pH effects on availability to tree roots | Lower in sandy, acidic soil. |
| Interactions with other elements | Heavy applications of potassium reduce Mg uptake. |
| Other information | Important in cation exchange. |
Zinc (Zn)
| Role in plant | Chlorophyll formation, protein synthesis, phytohormone metabolism and stress tolerance. |
|---|---|
| Symptoms of deficiency | Leaves are generally small and narrow, with creamy white to yellow blotches on young leaves. Retarded terminal growth, small twigs die, tree vigour reduced. Lowers the yield of small, poor-quality fruit. |
| Symptoms of excess | None known |
| Movement in soil | Mobile in acidic soil. |
| Movement in plant | Immobile |
| Availability to tree roots | Availability is reduced by high levels of organic matter, over-liming and overuse of poultry manure. More available when soil temperatures are warm. |
| pH effects on availability to tree roots | Less available above pH 6.0. |
| Interactions with other elements | High phosphorus can induce zinc deficiency by reducing both uptake by roots and use within the tree. |
| Other information | Nitrogen materials favour zinc availability. Symptoms are usually more pronounced on the north side (sunny) of the tree. |
Copper (Cu)
| Role in plant | Tissue strength, stress tolerance, carbohydrate metabolism. |
|---|---|
| Symptoms of deficiency | Dark brown gum pockets on young shoots and shoot dieback. Peel might be brown, with gum-stained areas. Fruit splitting more likely. |
| Symptoms of excess | Stunted roots and shoots. |
| Movement in soil | Mobile in acidic soil. |
| Movement in plant | Immobile |
| Availability to tree roots | Dependent on pH, organic matter content, presence of aluminium, molybdenum and iron. |
| pH effects on availability to tree roots | Progressively less unavailable as pH rises above 7.0. |
| Interactions with other elements | Excess might induce iron deficiency. |
| Other information | Availability can be reduced by increasing soil organic matter. |
Manganese (Mn)
| Role in plant | Chlorophyll and protein function, stress tolerance and cell elongation. |
|---|---|
| Symptoms of deficiency | Young leaves are mottled pale green, with interveinal yellowing, reduced growth and slight loss in yield. |
| Symptoms of excess | Bright yellowing on the margins of old leaves, dark brown tar spots on leaves. |
| Movement in soil | Mobile in acidic and waterlogged soil. |
| Movement in plant | Immobile |
| Availability to tree roots | More available in waterlogged conditions; excessively available in acidic (pH <4.3) soil, potentially leading to toxicity. |
| pH effects on availability to tree roots | Becomes less unavailable as pH rises above 5.5. |
| Interactions with other elements | High levels can induce iron deficiency. Manganese deficiency is more acute when nitrogen levels are low. |
| Other information | Symptoms are more noticeable on the southern side of the tree. |
Iron (Fe)
| Role in plant | Chlorophyll synthesis and nitrogen metabolism. |
|---|---|
| Symptoms of deficiency | Young leaves show chlorotic, stunted, abnormal growth; tips/margins/veins stay green longest. Lower vigour. Reduced yield. |
| Symptoms of excess | No known symptoms. |
| Movement in soil | Mobile in waterlogged soil. |
| Movement in plant | Immobile |
| Availability to tree roots | In very acidic soil, phosphates can be tied up by soluble iron and aluminium. High water table and waterlogged conditions aggravate the problem. |
| pH effects on availability to tree roots | Becomes less available as pH rises above 7.0. |
| Interactions with other elements | High iron levels can induce a manganese deficiency. Fixes phosphorus. |
| Other information | Symptoms are more noticeable on the southern side of the tree. |
Boron (B)
| Role in plant | Pollen tube growth, cell elongation, tissue strength, phytohormone metabolism. |
|---|---|
| Symptoms of deficiency | Stumpy roots, yellow veins on young leaves. Lop-sided, malformed, grey to brown fruit with gum pockets. Heavy fruit shedding. |
| Symptoms of excess | Yellow, dead leaf tips, leaf fall and dieback. Reduced yield. |
| Movement in soil | Mobile |
| Movement in plant | Immobile |
| Availability to tree roots | Available forms found in the soil solution. Dry periods and over-liming can induce a deficiency. |
| pH effects on availability to tree roots | None known |
| Interactions with other elements | Calcium renders boron insoluble, therefore used to overcome an excess. |
| Other information | Narrow range between deficiency and toxicity. Easily leached. |
Molybdenum (Mo)
| Role in plant | Nitrogen metabolism |
|---|---|
| Symptoms of deficiency | Symptoms similar to nitrogen deficiency, although tissue tests might indicate sufficient N; yellow spots on leaves in spring. |
| Symptoms of excess | No known symptoms. |
| Movement in soil | Immobile |
| Movement in plant | Mobile |
| Availability to tree roots | Deficient in acidic soil that contains iron or aluminium oxides. |
| pH effects on availability to tree roots | Decreases below pH 6. |
| Interactions with other elements | None known |
Aluminium (Al)
| Role in plant | None |
|---|---|
| Symptoms of deficiency | No known symptoms. |
| Symptoms of excess | Stunted root growth. Lack of root hairs. |
| Movement in soil | Immobile |
| Movement in plant | None |
| Availability to tree roots | Toxicity is highly likely in soil with a pH below 4.3. |
| pH effects on availability to tree roots | Availability decreases in soil with a pH above 5.5. |
| Interactions with other elements | Fixes phosphorus in acidic soil. |
Sodium (Na)
| Role in plant | Internal balance |
|---|---|
| Symptoms of deficiency | No known symptoms. |
| Symptoms of excess | Leaf burn, defoliation and dieback. |
| Movement in soil | Mobile |
| Movement in plant | Mobile |
| Availability to tree roots | Uptake more likely in waterlogged soil, on some rootstocks and with saline irrigation water. |
| pH effects on availability to tree roots | None known |
| Interactions with other elements | Sodium ions displaced by calcium. |
| Other information | Buildup in the soil leads to sodic soil. |
