Plant
Leaf and Herbage testing
For the monitoring of plant nutrient problems.
The leaf of a plant can reveal through physical symptoms its
nutrient status. These symptoms are sometimes similar and can be confused. The leaf
test takes the guesswork out of nutrient problem diagnosis. It is essential to
choose normal leaf material.
Introduction
The analysis of total nutrient contents of leaves is an important tool for the diagnosis of nutrient problems in plants. These analyses are often successful in determining the reason for crop failure or reduced production. The results are also useful for determining fertiliser composition and the need for any foliar sprays. Plants have the ability to move some nutrients from older leaves to newly formed leaves. This makes the selection of the leaf material crucial. The analysis of leaf material when plants are growing and producing well provides a good bench mark for the future reference if problems arise.
Apart from obvious stunted growth, some leaf disorders known to be associated with nutrient deficiencies are
Yellowing (Fe, S, N, Mg, Mo) , dark green with purple pigmentation (P), graying with scorched edges (K), leaf curling with dieing brown edges (Ca), burned & distorted growth tips (B), white tip and bleaching (Cu), yellowing to dying between the veins. (Mn, Zn).
Sampling
Some plant species have specific sampling procedures to coincide with stages of growth. If no specific instructions are available then the upper most recently developed mature leaf should be selected. Leaf selection should be from several plants rather than a single plant. Leaves selected should represent the majority of the crop and not just the affected leaves. 20 - 50 leaves should be collected depending on leaf size. Half a bread bag is sufficient.
Note: Plants infested with insects, disease or physical damage should be avoided.
Comparative sampling involves taking two sample 1 from normal leaves and the second from affected leaves and comparing the results between the two samples. This procedure is useful if reference values are not available.
Decontamination of leaf material is necessary if the plant has been recently foliar feed, fungal sprayed (copper) or covered with soil or dust particles.
Sample s are dried at 80 Deg C and ground to a fine powder before analysis.
Results
Testing generally takes 8 - 12 days from receipt to reporting. Results are compared to a reference range for the plant type and species and a bar-graph indicates the relative interpretation of the result. ie Low, within the expected range or high.
This gives a visual indication of the cause of plant leaf disorder.
In many cases the affected can be remedied with a foliar spray. This works well for ornamentals where leaf quality is paramount but of little value for fruit crops.
Nitrogen (N)
Role
Nitrogen, is a major element and is found in
both inorganic and organic forms within the plant. N is contained within amino
acids, enzymes nucleic acids, chlorophyll, alkaloids and purine bases. N
predominates as high molecular weight proteins. Absorbed as either the nitrate
or ammonium ion. Nitrite is toxic to plants at levels less than 5ppm.
Content
Generally results range between 1.5 % - 6%
dry wgt. Normal results are between 2.5% - 3.5%. Fruit crops will often have a
lower range of 1.8% - 2.2% while legume species may contain 4.5% - 5.5%. Higher
contents are found in new leaves. N removal from soil ranges from 56 to 560 kg
N/ha.
Interactions
A relationship exists between (N and P 3:1)
and (N and K). The uptake of nitrate stimulates the uptake of the cations while
chloride and hydroxyl anions restrict nitrate uptake.
High ammonium levels can lead to Ca and K
deficiency.
Deficiency symptoms
Slow growing, early maturing, with low yield
and reduced quality. Typically the plant is light green to yellow with the
older leaves affected first.
Excess symptoms
Dark green lanky, succulent foliage, weak
plants Easily fall over, not tolerant of dry conditions. Fruit and seed crops
may fail to yield. Succulent growth often leads to increased disease and insect
invasion.
Phosphorus (P)
Role
A component of certain enzymes and proteins,
ATP, ADP, and DNA and phytin.
ATP is an energy exchange compound
Content
Generally results range between 0.15 % - 1%
dry wgt. Normal results are between 0.2% - 0.4%. Above 1% is excessive and
below 0.2% is deficient.
Interactions
A relation ship between N and P (3:1) and P and the micronutrients Mn,Fe,Cu,Zn .P and Zn (200:1) is considered critical.
Deficiency symptoms
Slow growing weak and stunted plants. Colour may be dark green with older leaves showing a purple pigmentation. P is mobile and can be moved from old to new leaves.
Excess symptoms
Symptoms usually shows as either a Fe or Zn
deficiency
Potassium (K)
Role
Plays an essential role in water status within the plant and turgor pressure of its cells and the opening and closing of the stomata. Also K is involved in the accumulation and translocation of newly formed carbohydrates.
Content
Generally results range between 1.0% - 5.0%
dry wgt. Normal results are between 1.5% - 3.0%. Below 1.5% is deficient. Most
fruit crops remove sizeable quantities of K from the soil. Most plant will
absorb more K than they need this excess is referred to as luxury
consumption. Over 1500kg/ha may be removed by a crop such as banana’s.
Interactions
A relationship exists between K and the other
cations Mg and Ca. High K will first cause a Mg deficiency and a higher K may
produce a Ca deficiency. Ammonium can also have an effect on the balance
between the cations.
Deficiency symptoms
This causes the plant to become prone to
disease. Fruit yield and quality will be reduced. Older leaves turn greyish and
will become burned along the edges (scorched). The plants sensitivity to
ammonium toxicity increases. K is very mobile and signs first appear in older
leaves.
Excess symptoms
Deficiency symptoms of magnesium then calcium
are more likely with a high imbalance of K.
Sulphur (S)
Role
Involved in protein synthesis and is part of
the amino acids cystine and thiamine. Sulphur is associated with disease
resistance. Sulphur compound often give plants certain characteristic tastes
and odours.
Content
Sulphur content ranges between 0.15% - 0.5% .
Some plant types will accumulate 3 times the sulphur to P level but more
frequently S and P levels are similar. Cereals, potatoes and grasses remove
approx 10 kg/ha while sugar beets, cabbage and alfalfa will remove 15 - 45
kg/ha.
Interactions
Low sulphur reduces N uptake. High sulphur
causes Fe, Mo and B to be low.
Deficiency symptoms
Occur initially as a general yellow green
with younger leaves showing a more pronounced yellowing.
Fruits are light green and lack succulence.
Root nodulation in legumes is reduced and this reduces the N fixation ability.
Stems become woody. Symptoms are similar to N deficiency. Often S deficiency is
pronounce during drought. 90% of soil S is in the organic form and not freely
available. Available S levels in subsoil are often higher than top soil.
Excess symptoms
Premature aging and dying off of leaves.
Calcium (Ca)
Role
Plays an important part in maintaining cell
integrity and membrane permeability. It activates enzymes for mitosis, division
and elongation.
Content
Wide ranging contents from 0.2% - 3.0%. For
most crops levels between 0.3% - 1% are adequate.
Contents are often higher in the plant than
the fruit.
Interactions
Typical cation interactions apply to Ca. Ca
levels in the soil should be higher than K or Mg. Ca:K ratio and Ca:Mg ratio
are important.
Deficiency symptoms
The growing tips of root and leaves become
brown and die. Leaves curl and margins become brown.
Fruit quality will be reduced with blossom
end root and internal decay developing . Ca is immobile in the plant, hence the
symptoms occur at growing terminals.
Excess symptoms
Excess Ca causes Mg or K deficiency problems.
Magnesium (Mg)
Role
A component of chlorophyll and a cofactor in many enzyme reactions.
Content
Ranges between 0.15% - 1.00%. O.25% is the
sufficiency level for most crops. Content of grain crops is low while legume
and some fruit crops and vegetables. Magnesium contents are higher in the older
leaves. Mg removal from the soil by crops varies between 10 - 195 kg/ha.
Interactions
Magnesium is a cation and the level must be
imbalance with the other cations K and Ca. Mg deficiency can be induced by High
Ca, K or ammonium.
Deficiency symptoms
Magnesium is mobile in the plant and the
symptoms are seen first in old leaves. As the deficiency increases yellowing of
the leaves or interveinal chlorosis of new leaves become obvious. Necrosis or
leaves dying is possible in severe cases.
Excess symptoms
There are no specific symptoms of excess. Mg
can interferer with other cation uptake and may reduce growth rate.
Iron (Fe)
Role
Plays a role in many plant enzyme systems, is
a component of some plant proteins, energy transfers, reduction reactions and
as a catalyst. Believed to be involved in protein synthesis and root tip
meristem growth.
Content
Iron is a micronutrient and is found in ppm
concentrations ranging from 10 - 1000 ppm.
Sufficiency is usually in the 50 - 75 ppm
range. The majority of plant iron is in the ferric state (Fe3+)
Interactions
High plant P levels decrease the solubility
of Fe. A P:Fe ratio of 29:1is average for most plants. Potassium increases the
mobility and solubility of Fe. N accentuates the Fe deficiency due to increased
growth rates. Bicarbonate interferes with Fe translocation.
Iron (Fe) cont
Deficiency symptoms
Interveinal chlorosis of younger leaves is
the typical septum of Fe deficiency. As the deficiency increases the older
leaves become affected.
Excess symptoms
Iron may accumulate to several hundred ppm
without toxicity symptoms
Toxicity produces a bronzing of the leaf.
Manganese (Mn)
Role
Involve in the redox reactions of the
photosynthetic electron transport system. In Photolysis it acts as a bridge for
ATP and the enzymes.
Content
Generally levels of 10 - 50 ppm are common.
In some plant types however (soybean, cotton, sweat potato) levels of 200 - 700
may accumulate before toxicity symptoms occur.
Interactions
Not known to interfere with the metabolism or
uptake of other nutrients.
Deficiency symptoms
Reduced or stunted growth with dicots showing
interveinal chlorosis of the younger leaves. (Grey speck on lower leaves and
necrotic marsh spot in legume cotyledons describe the Mn deficiency.
Excess symptoms
Old leaves may show a necrotic brown spots
surround by chlorosis. Fruit may show a black spotting.
Zinc (Zn)
Role
Involved in the same enzymatic functions as
Mg and Mn. Only carbonic anhydrase is known to be specifically activated by
zinc.
Content
Ranges fro 15 - 50 ppm are common Deficiency
symptoms not occurring until levels get as low as 12 ppm. Often 1 - 2 ppm may
distinguish between sufficiency and deficiency. Plants may accumulate several
hundred ppm Zn without harm.
Interactions
A very high Zn may interfere with Fe uptake
and produce Fe deficiency symptoms in Fe sensitive plants. High P levels are
thought to interfere with Zn uptake.
Deficiency symptoms
Zn deficiency appears first in young leaves
as a paleness in the interveinal areas. This produces a banding appearance. In
severe cases, Growth is stunted and leaves dye and fall from the plant.
Excess symptoms
May cause iron deficiency in Fe sensitive
plants
Copper (Cu)
Role
Copper is involved in many of the plant’s
biochemical pathways. It is a constituent of plastocyanin participates in
protein and carbohydrate metabolism and in nitrogen fixation. It is a part of
the enzymes that are involved in the respiration process of plants.
Content
The sufficiency range for copper is 3 - 7 ppm
and toxic levels begin at 20 - 30 ppm. Copper based fungicides may increase
leaf test copper results to several hundred without problems.
Interactions
High copper interferes with iron and
molybdenum. A iron deficiency can results from high copper.
Deficiency symptoms
Causes stunted growth with a distortion of
younger leaves. In tree species the young leaves will develop a white tip or
bleaching with summer dieback.
Excess symptoms
Cause iron deficiency with chlorosis. Root
development is inhibited.
Boron (B)
Role
Involved in one of the bases for RNA (uracil)
and in the cellular activities (division, differentiation, maturation,
respiration and growth). It is associated with pollen germination and growth.
Also associated with calcium uptake. Boron is fairly immobile in plants.
Content
Plant content varies with plant type. The
leaf content of monocots is 1 - 6 ppm, docots 20 - 70 ppm and dicots with latex
systems 80 - 100 ppm B. Boron accumulates in the leaf margins at concentration
5 - 10 times the leaf blade concentration.
Interactions
A high plant calcium creates a high
requirement for boron. A high plant potassium accentuates a low plant Boron
Deficiency symptoms
Plants exhibit abnormal growth with the
growth points affected and showing deformation and dying. Leaves and stems may
become brittle. Plant types differ considerably in their sensitivity to B
deficiency
Excess symptoms
Leaf tips become yellow and die off. Leaves
eventually become scorched in appearance and prematurely fall off.