Plant Nutrition and The Relation to Soil pH We all know that plants require nutrition, but do you understand the correlation of nutrient uptake and soil pH? Applying fertilisers, without knowing how they relate to each other and the plant itself, is a waste of money and effort. There is also risk of nutrient toxicity or deficiency if not applied in the correct method. How Clay and Humus Form Mineral soils are formed by the breakdown of rocks, known as the parent material. Heating and cooling, freezing and thawing, wind and water erosion, acid rain (all rain is acid; carbon dioxide in the air forms carbonic acid in the rain), and biological activity all break down the parent material into finer and finer particles. Eventually the particles get so small that some of them re-form and become colloidal clay, made up mostly of silica and alumina clay particles aggregated into thin, flat sheets that stack together in layers. Clay particles are really tiny. They are so small that they can't even be seen in most microscopes. They are so small that when mixed in water they may take days, weeks, or months to settle out, or they may never settle out and just remain suspended in the water. A particle that remains suspended in water like this, suspended but not dissolved, is known as a colloid. Organic matter, as it breaks down, also forms smaller and smaller particles, until it breaks down as far as it can go and still be organic matter. At that stage it is called humus, and humus is also a colloid; when mixed into water humus will not readily settle out or float to the top. Colloids, because they are so small, have a very large surface area per unit volume or by weight. Some clays, such as montmorillonite and vermiculite, have a surface area as high as 800 square meters per gram! The surface area of fully developed humus is about the same or even higher. Other clays have a much lower surface area; some clays actually have a very low exchange capacity, while humus always has a high exchange capacity. It's still a little-known fact that the calcium to magnesium ratio determines how tight or loose a soil is. The more calcium a soil has, the looser it is; the more magnesium, the tighter it is, up to a point. Other things being equal, a high calcium soil will have more oxygen, drain more freely, and support more aerobic breakdown of organic matter, while a high magnesium soil will have less oxygen, tend to drain slowly, and organic matter will break down poorly if at all. In a soil with magnesium higher than calcium, organic matter may ferment and produce alcohol and even formaldehyde, both of which are preservatives. For example, if you till up last year’s corn stalks and they are still shiny and green, you may have a soil with an inverted calcium /magnesium ratio. On the other hand, if you get the calcium level too high, the soil may lose its beneficial granulation and structure and the excessive calcium will interfere with the availability of other nutrients. The following tables for each element are specific to citrus, but most of the information given can be applied to most plants.
Element
Nitrogen (N)
Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots
Vital part of proteins & chlorophyll. Proteins are involved in metabolism & chlorophyll is involved in making sugar using carbon dioxide from the atmosphere & sunlight. Poor stunted shoot growth, old leaves pale green-yellow, thin foliage cover & dieback of twigs. Poor fruit set & fruit size. Fruit quality good. Vigorous vegetative growth. Thick-skinned large, puffy fruit, delayed maturity, re-greening likely. Juice % & quality declines. Shorter storage life. Mobile Mobile Nitrate form preferred, but ammonium also taken up. Conversion between these forms dependent on soil temperature, organic matter & microbial activity, which is greatest in pH range of 5.5-8.5. Use readily available form (i.e. nitrate) during cooler months.
pH effects on availability to tree None known roots Higher tree N levels (>2.6% in leaves) associated with lower boron &/or sulphur in leaves.
Interactions with other elements Effect of excessive N worsened by low P supply. Easily leached in sandy soils. Lost to atmosphere in waterlogged soils. Other information Symptoms of nitrogen deficiency include pale green foliage, yellowing of older leaves & twig dieback.
An excess of nitrogen or potassium can result in thick skinned fruit with rough rinds.
Element Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots
Phosphorus (P) Metabolism, cell division & growth. Old leaves dull bronzed green. Fewer flowers. Misshapen fruit, open centres, thick coarse peels. Pulpy fruit, lower juice, more acid. Smaller fruit, higher % juice, thinner peels, chance of re-greening. Immobile Mobile Available & unavailable forms present; relative proportions dependent on soil pH. In alkaline soils P fixed in calcium compounds, & iron & aluminium compounds in acid soils. Unavailable forms act as a reserve for available forms. Most P in upper soil layer. Australian soils naturally low.
pH effects on availability to tree Most available in range 6 - 7 roots Too much P can accentuate the effects of low zinc availability, induce iron deficiency
Interactions with other elements and affect copper uptake. Other information
Element Role in plant Symptoms of deficiency
Losses from soil only in erosion of soil particles or in very sandy soils. Soil compaction reduces uptake.
Potassium (K) Metabolism, water relations, internal balance, stress & disease resistance. 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 & are more prone to albedo break-down. Thicker rough rinds & chance of re-greening. Delayed maturity in oranges. Mobile Mobile Soils contain large amounts but most is unavailable.
Symptoms of excess Movement in soil Movement in plant Availability to tree roots pH effects on availability to tree None known roots Interactions with other elements Excessive K supply reduces magnesium & calcium uptake. Large amounts removed in fruit. Other information
Element Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots pH effects on availability to tree roots Interactions with other elements Other information
Sulphur (S) As for N, but required in much lesser amounts. Younger leaves stunted, pale green-yellow with lighter veins. None known Mobile Mobile Available to plants only in sulphate form. Most soil sulphur present in the organic matter. Less available in low pH soils. None known Sulphate forms easily leached. Long-term use of ammonium sulphate leads to lower soil pH.
Element Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots
Calcium (Ca) Metabolism, internal balance, tissue strength. Stunted roots, higher incidence of albedo breakdown. None known Immobile Immobile Plant available Ca associated with soil cation exchange capacity. High Ca levels in high pH soils due to calcium carbonate (lime).
pH effects on availability to tree Deficient in low pH (acid) soils. roots Heavy applications of K may reduce Ca uptake. Deficiencies, more likely on acid soils.
Interactions with other elements Calcium carbonate can reduce iron uptake ("lime-induced iron chlorosis"). Other information
Element Role in plant Symptoms of deficiency
Other important sources of Ca include gypsum (calcium sulphate) & single strength superphosphate.
Magnesium (Mg) Synthesis and function of chlorophyll & protein, internal balance. Yellowing towards apex of leaves with a triangular area remaining green at base in older leaves, defoliation and shoot dieback. Smaller fruit and lower yield. None known Attaches to clay particles. Mobile Plant available Mg associated with soil cation exchange capacity.
Symptoms of excess Movement in soil Movement in plant Availability to tree roots pH effects on availability to tree Lower in sandy, acid soils. roots Interactions with other elements Heavy applications of potassium reduces Mg uptake. Important in cation exchange. Other information
Magnesium deficiency shows up in older leaves first.
Element Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots
Zinc (Zn) Chlorophyll formation, protein synthesis, phytohormone metabolism; stress tolerance. Leaves generally small & narrow, creamy white to yellow blotches on young leaves. Retarded terminal growth, small twigs die, tree vigour reduced. Lowers yield of small poor quality fruit. None known Mobile in acid soils. Immobile Availability 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 Less available above pH 6.0. roots High phosphorous can induce zinc deficiency by reducing both uptake by roots & use
Interactions with other elements within the tree. Other information
Nitrogen materials favour zinc availability. Symptoms more pronounced on north side (sunny) of tree. Zinc deficiency shows up in young leaves first.
Element Role in plant Symptoms of deficiency
Copper (Cu) Tissue strength, stress tolerance, carbohydrate metabolism. Dark brown gum pockets on young shoots & shoot dieback. Peel may be brown, with gum stained areas. Fruit splitting more likely. Stunted roots & shoots. Mobile in acid soils. Immobile Dependent on pH, organic matter content, presence of aluminium, molybdenum & iron.
Symptoms of excess Movement in soil Movement in plant Availability to tree roots pH effects on availability to tree Progressively less unavailable as pH rises above 7.0. roots Interactions with other elements Excess may induce iron deficiency. Availability can be reduced by increasing soil organic matter. Other information
Element Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots
Manganese (Mn) Chlorophyll & protein function, stress tolerance, cell elongation. Young leaves mottled pale green, interveinal yellowing, reduced growth & slight loss in yield. Bright yellowing on margins of old leaves, dark brown tar spots on leaves. Mobile in acid & waterlogged soils. Immobile More available in waterlogged conditions; excessively available in acid (pH<4.3) soils potentially leading to toxicity.
pH effects on availability to tree Becomes less unavailable as pH rises above 5.5. roots High levels can induce iron deficiency. Manganese deficiency more acute when nitrogen
Interactions with other elements levels low. Symptoms more noticeable on southern side of tree. Other information
Manganese toxicity symptoms include yellowing on margins of old leaves & dark brown tar spots.
Element Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots
Iron (Fe) Chlorophyll synthesis & nitrogen metabolism. Young leaves chlorotic, stunted abnormal growth; tips/margins/veins stay green longest. Lower vigour. Reduced yield. No known symptoms. Mobile in waterlogged soils. Immobile In very acid soils, phosphates can be tied up by soluble iron & aluminium. High water table & water logged conditions aggravate problem.
pH effects on availability to tree Becomes less available as pH rises above 7.0 roots Interactions with other elements High iron levels can induce a manganese deficiency. Fixes phosphorous. Symptoms more noticeable on southern side of tree. Other information Iron deficiency shows up on young leaves first.
Element Role in plant Symptoms of deficiency
Boron (B) Pollen tube growth, cell elongation, tissue strength, phytohormone metabolism. Stumpy roots, yellow veins on young leaves. Lopsided malformed grey to brown fruit with gum pockets. Heavy fruit shedding. Yellow, dead leaf tips, leaf fall & dieback. Reduced yield. Mobile Immobile Available forms found in soil solution. Dry periods & over liming can induce a deficiency.
Symptoms of excess Movement in soil Movement in plant Availability to tree roots pH effects on availability to tree None known roots Interactions with other elements Calcium renders boron insoluble therefore used to overcome an excess. Narrow range between deficiency & toxicity. Easily leached. Other information
Element Role in plant Symptoms of deficiency
Molybdenum (Mo) Nitrogen metabolism Symptoms similar to nitrogen deficiency although tissue tests may indicate sufficient N; yellow spots on leaves in spring. No known symptoms. Immobile Mobile Deficient in acid soils which contain iron or aluminium oxides.
Symptoms of excess Movement in soil Movement in plant Availability to tree roots pH effects on availability to tree Decreases below pH 6. roots Interactions with other elements None known
Element Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots pH effects on availability to tree roots Interactions with other elements
Aluminium (Al) None No known symptoms. Stunted root growth. Lack of root hairs. Immobile None Toxicity highly likely in soils below pH 4.3. Availability decreases above pH 5.5. Fixes phosphorus in acid soils.
Element Role in plant Symptoms of deficiency Symptoms of excess Movement in soil Movement in plant Availability to tree roots
Sodium (Na) Internal balance No known symptoms. Leaf burn, defoliation & dieback. Mobile Mobile Uptake more likely in waterlogged soils, on some rootstocks & with saline irrigation water.
pH effects on availability to tree None known roots Interactions with other elements Sodium ions displaced by calcium. Build-up in soil leads to sodic soils. Other information