Critical values are still quite useful and are frequently referred to when interpreting a plant analysis result. A brief discussion of the known critical values for the elements included in a plant analysis is given below:
Critical value or range indicates the soil or tissue content below which the plant most likely is deficient in that specific nutrient and production could be enhanced by addition of the nutrient. Below that critical value, the nutrient levels are below optimum.
|
Nitrogen (N) |
The critical level of N in many plants is around 3 percent. For several crops, when the N level in leaves drops below 2.75 percent, N deficiency symptoms appear and yield and quality decline. The primary exceptions are for the very young plants when the critical level may be 4 percent or more, and for leguminous plants, such as soybeans, peanuts, alfalfa, etc., where the critical N percentage is 3 to 4.25 percent. For some tree fruits and ornamentals, N levels may be as low as 2 percent before deficiency occurs. Deficiencies as well as excesses can be a problem.
Nitrogen leaf levels in some varieties of pecans exceeding 3.50 percent may
result in early defoliation. Nitrogen leaf levels greater than 4.50 to 5
percent retard fruit set in greenhouse tomato. High N levels (>3.50 percent)
in forage crops such as fescue is thought to be related to the incidence of
grass tetany. Small changes in N content for some crops can result in
large effects on yield, plant growth, and the quality of forage and fruit.
Therefore, it is important that the N level be maintained within the
prescribed limits of the sufficiency range by the proper use of N fertilizer.
|
|
Phosphorus (P) |
The P requirement of plants varies considerably. Tree
crops have relatively low P requirements with the critical values ranging
from 0.12 to 0.15 percent. Grasses have higher P requirements with critical
values ranging from 0.20 to 0.25 percent. Legumes and some vegetable crops
have relatively higher P requirements with critical values being 0.25 to 0.30
percent or slightly higher. Most plants grow to the extent to maintain a near
constant P level within the plant. When a P deficiency occurs, it is usually
due to a severe inadequacy of P in the soil solution, or in some cases it may
be due to a restricted root system as a result of cool-moist growing
conditions. Phosphorus deficiencies normally occur early in the growth cycle
of the plant when the P requirement is high. The P content of plants is
initially high and declines with age. Since P is a fairly mobile element in plants,
deficiencies generally occur on older tissue. The excess range of P is not clearly known. The P level
in young plants can be very high such as 0.50 to 1.00 percent, but these high
levels may reflect actual need. In some instances, high P plant levels may
cause imbalances and deficiencies of other elements, such as Zn, Cu, Fe, etc.
Plant P can be maintained within the sufficiency range by proper P
fertilization and the maintenance of the soil P level within the medium to
high soil test range. |
|
Potassium (K) |
The K requirement of plants varies widely depending on
plant species. The tree crops such as pecans, peaches, apples, etc., have
relatively low K requirements. The critical value for K in tree leaves ranges
from 0.75 to 1.25 percent. For grasses, the K requirement is higher with the
critical value in leaves ranging from 1.20 to 2.00 percent. For legumes, the
critical value for K generally ranges from 1.75 to 2.00 percent. The K level
in a plant can change quickly as K is quite mobile and moves readily within
the plant. Potassium can be easily leached from growing plants by rain to be
reabsorbed through the roots. Because of K mobility, both in the plant and
soil, deficiency symptoms can develop quickly. Deficiencies frequently occur
during both the early and latter stages of growth, particularly during
fruiting. Young plants may contain 3.00 to 5.00 percent K, although the
actual requirement may not be that high. Because it is mobile in the plant, K
deficiency symptoms appear in the older plant tissue first. The K
concentration in the plant decreases with age. Potassium balance in plants is
important. The K/(Ca+Mg) and K/N balances must be maintained at a proper
level to avoid deficiencies of Mg in the first instance and K in the second.
High K can induce Mg deficiency in most plant and tree crops. Plants which
are Mg deficient may have high K and Ca contents as the plant tends to
maintain a constant cation concentration. As a result of these balance
phenomena, heavy applications of K or N fertilizer, respectively, can induce
a Mg or K deficiency. Under Georgia soil conditions, K deficiency is
difficult to induce unless the K soil test level is low and the soil is
heavily limed or fertilized with large quantities of N. The K to N balance is
becoming increasingly important in pecans. As the N level in tree leaves
increase, the K level must also be increased to maintain the proper balance
and prevent K deficiency from occurring. Plant K can be maintained within the
sufficiency range by proper K fertilization and the maintenance of the soil K
level within the medium to high soil test range. |
|
Magnesium (Mg) |
Magnesium deficiency occurs in many plants when the
leaf level is less than 0.10 to 0.15 percent. Small grains may exhibit
deficiency symptoms when the Mg level is less than 0.10 percent. When corn is
less than 12 inches in height, magnesium deficiency may occur when the Mg
level is below 0.15 percent. However, as corn matures, deficiencies may not
be evident until the Mg level is less than 0.13 percent. For legumes such as
peanuts and soybeans, the critical level is 0.25 to 0.30 percent. The
critical level for cotton and pecans is 0.30 percent. Several vegetable crops
such as tomato, turnips, and collards have a high Mg requirement with the
critical level near 0.40 percent Mg. Magnesium is a fairly mobile element in
the plant, therefore, deficiency symptoms occur in the older plant tissue.
The Mg concentration in the plant tends to increase with age. Magnesium deficiencies can be induced by excessive K
and NH4-N fertilization. When the soil pH is less than 5.4, Mg
availability and uptake by plants is greatly reduced. The usual cause for Mg
deficiency in Georgia is generally low soil pH and/or low soil Mg. Depending
on the soil conditions, the effect of K and NH4-N
fertilization can vary depending on the soil pH and level of soil Mg.
Continued liming with only calcitic lime will result in a Mg deficiency.
Adequate soil Mg can generally be maintained by liming with dolomitic
limestone to keep the soil pH between 6.0 and 6.5. Supplemental applications
of fertilizer Mg may be needed in some cases to supply some of the Mg crop
requirement. |
|
Sulfur (S) |
It has been generally thought that the S requirement of
plants was comparable to that of P. This has not proven to be so. The S
requirement for grasses is quite low, the critical value being around 0.10
percent. Sulfur deficiencies in corn do not generally occur until the S level
is less than 0.13 percent in the leaves. Under Georgia conditions, legumes,
cotton, tobacco, and tomatoes have a critical S level of about 0.20 to 0.25
percent. The S critical level for crops such as cabbage, spinach, turnips,
and collards is around 0.30 percent. However, additional research in this
area should aid in pinpointing the critical level for these crops. There is a
critical N to S percentage ratio which should be maintained. As suggested by
Reneau (1983) the N:S ratio may be a better indicator of the S status of corn
than the S concentration. For crops such as corn, this ratio should not
exceed 18:1 if S deficiency is to be avoided. Stewart and Porter (1969)
suggested that a N:S ratio above 16:1 indicates a lack of S may be limiting
protein formation. A ratio of 20:1 or greater indicates that S is severely
deficient. For optimum corn grain yields, the N:S ratio should be maintained
between 10:1 to 15:1 (M. E. Sumner, personal communication). The optimum N:S
ratio for Coastal bermudagrass ranges from approximately 9:1 to 12:1 (Martin
and Matocho, 1973). Maintaining the N:S ratio within the range for optimum
production of Coastal also provides the N:S ratio that is about optimum (10:1
to 15:1) for ruminant nutrition (Allaway and Thompson, 1966). Sulfur
deficiencies occur primarily on the very sandy soils of South Georgia and
when low S containing fertilizers are used over several years. Sulfur
deficiencies tend to occur early in the plant growth cycle. The proper S
level can be maintained in the plant by providing a S source near the
germinating seed or by adding S with sidedress and topdress N applications
particularly in sandy soils. Most Georgia subsoils contain sizeable
quantities of S. Provided the pH is not too low when roots enter the subsoil,
sufficient S will generally be available to satisfy the crop requirement. Since S is not a mobile element in the plant,
deficiency symptoms tend to first appear in the upper or newly emerging leaf
tissue. |
|
Calcium (Ca) |
The Ca requirement for plants varies widely with
grasses having the lowest requirement, legumes intermediate, and fruit crops and
cotton the highest. Calcium levels from 0.20 to 0.25 percent are quite
adequate for pasture grasses and corn. Soybean has a critical Ca
concentration in the mature leaves of 0.50 percent, while the level for
peanuts is 1.25 percent. Apple leaves should contain about 1 percent Ca and
peach leaves 1.25 percent. Greenhouse tomato has a critical concentration for
leaves of about 1 percent. Of the crops grown in Georgia, cotton probably has
the highest critical Ca concentration at 2 percent for leaves. Calcium deficiencies are not unusual, although the
crops where Ca is particularly important are the fruit crops, such as apples,
peaches, and tomato. Calcium deficiency will significantly affect fruit
quality. Brown rots, easy bruising of fruit, and blossom-end rot of tomato
are frequently associated with inadequate Ca. Pod-rot in peanuts is also a Ca
deficiency. These deficiencies are not easily "uncovered" by leaf
analysis. When Ca deficiency is severe, newly emerging tissue is affected.
The margins of the leaves tend to stick together, giving a ragged edge to new
leaves. Older leaves will show a browning of the margins. Since Ca is not a
mobile element, deficiencies occur in the newer tissues. The Ca level in
plants tends to increase with the age of the plant. There is increasing evidence that Ca is more like a
micronutrient, as the critical concentration may be in the parts per million
range. Several plant physiologists have grown plants successfully at low Ca
levels in artificial growth media. In these experiments, the balance of Ca
with the other essential elements such as Mg, Cu, Fe, B, and Mn was critical.
Calcium was found to be sufficient with plant and leaf concentrations between
600 ppm to 1000 ppm. It is known that relatively little Ca is in a soluble
form in many plants. Crystals of calcium oxalate have been observed in the
leaves of most fruit trees as well as some field crops which are thought to
have high Ca requirements. Therefore, the sufficiency of Ca in such plants
may be related to the soluble fraction in the leaves rather than the total.
Unfortunately at this time, all of the current literature related to Ca and
its sufficiency concentration are based on total Ca contents of sampled plant
parts. No doubt there is need to change the method of analysis for Ca to
determine the soluble Ca content and relate this to sufficiency range
standards. |
|
Manganese (Mn) |
Manganese deficiency normally occurs when the leaf
tissue concentration is less than about 15 ppm. Depending upon the crop,
ample but not excessive concentrations of Mn may range from 15 to over 1,000
ppm. Although there is limited data to delineate when toxicity occurs, leaf
levels in excess of several hundred ppm are probably toxic to many plants.
Plants which are sensitive to Mn deficiency are equally sensitive to
excessive Mn. Growth of soybeans, which are particularly sensitive to Mn
deficiency, is reduced when leaf Mn levels approach 200 ppm (Ohki, 1976).
Several plant species have higher Mn critical levels. For example, the
critical Mn level for alfalfa is about 25 ppm. Some plants can tolerate extremely high Mn levels
without detrimental effects. Pecan leaves may contain up to 1000 ppm Mn with
seemingly no adverse effect. Similarly, cotton and peanuts will accumulate Mn
up to 500 ppm without apparent toxicity. However, a high Mn level in plants
is a sign of low soil pH, and is frequently associated with Mg deficiency.
When the Mn concentration in peach leaves exceeds 150 ppm, this is generally
a good indication that the soil pH is low according to George Cummings. The Mn level in plants is usually quite high at the
initial period of growth. It decreases rather rapidly and then levels off to
remain fairly constant during most of the season. Since Mn is not a mobile
element, deficiency symptoms will occur in the newer leaves or upper portion
of the plant. |
|
Iron (Fe) |
Iron analyses are probably invalid unless the leaf
tissue has been washed in dilute acid or detergent solutions. Therefore, for
unwashed leaves, iron analyses are of no real value. When soil contamination
is suspected, usually Al is also high. The Fe content in a plant can vary considerably. In
general, when the Fe concentration in leaves is 50 ppm or less, deficiency is
likely to occur. The grasses and corn have a lower Fe requirement, the
critical level being 20 ppm. Iron toxicity has not been reported for any
field crops growing under natural conditions in Georgia. The only Fe
sensitive field crops would be pecans and soybeans, with possible deficiency
occurring only on soils with pH's at 7.0 or above. Iron deficiency is common
in Centipede grass and azaleas, particularly when grown in soils with pH's
above 6.0 Iron deficiency is very difficult to correct in some
crops. The application of some forms of Fe to the soil is not practical.
Foliar applications of Fe have been found to be effective in correcting Fe
deficiencies in plants such as turf grasses. However, on crops such as
pecans, foliar applications for correction of low Fe levels have been
erratic. Since Fe is an immobile element in plants, Fe
deficiencies appear in the new tissue or upper portion of the plant. Iron
deficiency symptoms may appear early in the growth of the plant only to
disappear in several days or weeks. The Fe level in the plant usually remains
fairly constant during the growing season. |
|
Boron (B) |
Boron requirements vary considerably among crops. The
optimum range in leaf tissue of most crops is from 20 to 100 ppm. Some crops
are particularly sensitive to B and can be injured when the leaf B level is
too high. For example, B levels in excess of 50 ppm have been associated with
B toxicity in peaches. The B critical level for corn is about 4 ppm, while
alfalfa, cotton, peanut,and soybeans have critical levels of 20 ppm. Corn,
having a fairly low B requirement, is also sensitive to excess B. Toxicities
may occur when the B level in young corn leaf tissue exceeds 25 ppm. Members
of the Papilionaceae and Cruciferae have fairly high B requirements with
critical levels being about 25 to 30 ppm B in the leaf tissue. Those plants
which have fairly high B requirements are also ones with fairly good
tolerance to excessive B. Boron is not a very mobile element and deficiency
symptoms occur in the newly emerging tissue. The B concentration in leaves
remains constant during the growth cycle. Boron deficiencies result in
various physiological diseases in plants, such as "hollow heart" in
peanuts, a fairly common disorder occurring in Georgia peanut fields. |
|
Copper (Cu) |
The normal range of Cu in many plants is fairly narrow,
ranging from 5 to 20 ppm. When the Cu concentration in plants is less than 3
ppm in the dry matter, deficiencies are likely to occur. When Cu levels
exceed 20 ppm in mature leaves, toxicities may occur. There is some variation
in the critical values for various plant species; however, most critical
values have been determined to be somewhere between 3 to 10 ppm for most
crops. The Cu level in leaves tends to remain constant during the growing
season. Copper deficiency symptoms often depend on plant species
or variety and the stage of deficiency. In the early stages of deficiency,
symptoms are generally reduced growth. In the moderate to acute stages of
deficiency on crops such as wheat, terminal or new leaves are pale green,
lack turgor, and become rolled and yellowed; older leaves become limp and
bent at the ligule. The leaves die and dry to a bleached gray (Reuther and
Labanauskas, 1966). |
|
Zinc (Zn) |
The normal range of Zn in most plants is between 20 to
100 ppm. Zinc deficiencies occur in a wide variety of plants when the leaf
level drops below 15 ppm. The critical Zn value for apple is about 14 ppm
with the first symptom of the deficiency being small fruit size. Zinc
deficiency in pecans occurs when the Zn leaf level is 30 ppm or less. In order to avoid Zn deficiency, Zn levels in most
crops should be maintained at 20 ppm or better, except for pecans when 50 ppm
Zn is the desired minimum. Zinc toxicity is an uncommon problem and does not
generally occur until the Zn level exceeds 200 ppm. However, in crops such as
peanuts, Zn toxicity has been reported in Georgia when tissue levels reach
220 ppm (Keisling and others, 1977). More recently (Parker and Walker, 1986)
reported that Zn levels up to 287 ppm did not adversely affect peanut yields
nor show any of the symptoms associated with Zn toxicity. However, the author
has observed plants exhibiting Zn toxicity symptoms, described by Keisling
and others (1977), with Zn concentrations of 117 ppm. Apparently, there are
other plant growth factors or nutrient relationships in addition to just the
Zn concentration that affect the manifestation of Zn toxicity. One such
relationship appears to be the Ca:Zn ratio in the tissue. Upon evaluating
unpublished data of Parker in which the Zn concentration in tissue varied
from 50 to 302 ppm, and Zn concentrations could not be related to Zn
toxicity, the author noted that when the Ca:Zn ratio was less than
approximately 45 to 50:1 Zn toxicity symptoms were evident. However, when the
ratio was greater, where the Zn concentration was 302 ppm, no toxicity
symptoms were detected. Continued research in this area should elucidate the
nature of this relationship. Excessive Zn also interferes with the normal
function of Fe in plants giving rise to symptoms similar to Fe deficiency. Zinc is not a very mobile element in plants, and
deficiency symptoms occur in the newly emerging leaves. Stunting is a
frequent symptom associated with Zn deficiency. Zn concentration in leaves
remains fairly constant with a fairly rapid increase at the end of the growth
cycle. |
|
Aluminum (Al) |
Aluminum is not considered a plant nutrient; therefore,
it is not required by plants. However, its presence in plants can affect the
normal function of some other elements. As with Fe, probably no accurate
measure of the Al status of the plant can be obtained unless the tissue is
free from dust and soil contamination. High Al in plants is usually an
indication of very low soil pH or poor soil aeration due to compaction or
flooding. Aluminum levels in excess of 400 ppm in young tissue or 200 ppm in
mature plants and leaves are undesirable. |
|
Molybdenum (Mo) |
Molybdenum deficiencies occur in many plants when the
plant concentration is less than 0.10 ppm. Toxicity levels in plants have not
been established. Molybdenum is quite toxic to animals if the forage being
consumed contains more than 15 ppm |
|
Nutrient |
Whole Plant 24-45 Days* |
3rd Leaf, 45 – 80 Days† |
Earlef Green Silks‡ |
Earleaf Brown Silks$ |
|
N, % |
4.0-5.0 |
3.5-4.5 |
3.0-4.0 |
2.8-3.5 |
|
O, % |
.49-.60 |
.35-.50 |
.30-.45 |
.25-.40 |
|
K, % |
3.0-5.0 |
2.0-3.5 |
2.0-3.0 |
1.8-2.5 |
|
Ca, % |
.51-1.6 |
.20-.80 |
.20-1.0 |
.20-1.2 |
|
Mg, % |
.30-.60 |
.20-.60 |
.20-.80 |
.20-.80 |
|
S, % |
.18-.40 |
.18-.40 |
.18-.40 |
.18-.35 |
|
B, ppm |
6-25 |
6-25 |
5-25 |
5-25 |
|
Cu, ppm |
6-20 |
6-20 |
5-20 |
5-20 |
|
Fe, ppm |
40-500 |
25-250 |
30-250 |
30.250 |
|
Mn, ppm |
40-160 |
20-150 |
20-150 |
20-150 |
|
Zn, ppm |
25-60 |
20-60 |
20-70 |
20-70 |