Micronutrients | Interpretation of Plant Tissue Analysis Results for Micronutrients

Plant tissue analysis is the most useful tool the agronomist may have to detect minor element deficiencies that may not be evident by visual symptoms in the growing plant. Symptoms of toxicities will usually be evident visually; however, the plant tissue analysis is most useful in confirming this toxicity. True visual symptoms may be used as a guide, but very often minor element deficiencies may be marked by other deficiency symptoms.

In cases where analysis is being performed early in the growth of the plant, action may be taken to correct the deficiency. This is especially important in instances of borderline deficiencies where visual symptoms may not be detected early enough to take corrective action. Where samples of plant tissue are being analyzed at the mid-season stage, it may not be possible to correct the minor element shortage. The information gained may also be used to determine fertilizer treatment for future crops.

Soil tests for zinc and boron are quite reliable in forecasting the availability of these two elements in the soil, while tests for copper, iron, manganese, and molybdenum are less reliable as a basis for determining the concentration actually available for plant nutrition. Molybdenum is essentially restricted to plant tissue analysis for verification of nutrient level.

Interpretation of plant tissue analysis for micronutrient content requires complete background information as to the nature of the soil on which the crop is grown; weather, including temperature and rainfall since crop has been planted; applications of pesticides, foliar applications of chemicals, and limestone and fertilizer treatments; as well as the nutrient content of the major elements.

It is obvious that the requirements will vary from crop to crop as to the level of micro-nutrients required for optimum growth. There is an association between the time of sampling and the nutrient content. This factor of stage of growth is one which makes interpretation of results more difficult.

The portion of the plant selected for sampling is a critical factor. In all cases where plant samples are submitted for micro-nutrient analysis, it is absolutely essential that a composite soil sample from the same area be analyzed to determine the nature and fertility level of the soil (unless recent soil tests are available). An agronomist should never attempt to make an evaluation of plant tissue data without the supporting soil analysis as well as other information listed above. In considering the reasons why particular elements may be deficient in plants, we should take into consideration the soil conditions that will contribute to a possible deficiency.

In approaching the interpretation of micro-nutrient analysis results for plant tissue, the agronomist should certainly look at the concentration of the major (nitrogen-phosphorus-potassium) and secondary (calcium-magnesium-sulfur) elements. If the level of the major elements is deficient to the extent that they have retarded or reduced growth, there is little value in attempting to interpret the results of the micro-nutrients. Usually, a plant that is deficient in the major elements will show a normal content of the micro-nutrients. This may lead to an assumption that minor elements are not a factor contributing to the reduced growth of the plants.

In some cases the shortage of major elements or reduced uptake of major elements may be due to a minor element shortage. And, in some instances a shortage of minor elements may have retarded growth, and the content of major elements may be above normal. Because of the relationship between the major nutrients and the micro-nutrients, all of these nutrients have to be evaluated on the basis of their relationship and ration of one element to another.

Our Rating System
In plant tissue analysis work, three categories designate the level of nutrients in the plant:

Deficient — Level of the nutrient is below the suggested level for normal yields of the plant or crop. Additional testing is suggested to further confirm or deny the deficiency. This testing may be in the form of soil tests or trial applications of fertilizer material to supply the nutrient in question. If the deficiency appears to be strong in nature, a micronutrient recommendation may be offered on a definite basis.

Adequate — Level of the nutrient is within the range that is common for crops showing good growth and producing normal or high yields. No response to growth or yield by the application of the observed element is anticipated.

High — Level of the nutrient is above the normal range for that particular plant or crop. May indicate the need for additional observation to determine if element in question is having detrimental effect on growth of plant, especially if the level is at least 50% in excess of the adequate or normal range. The tolerance may be much narrower in some instances.

ZINC

Soil Factors: Soil factors that will lead to possible zinc deficiencies are:

  • Alkaline soil pH and also a strongly acid pH (below pH 4.8).
  • Coarse-textured soils that have been highly leached.
  • Soils that have been formed from parent materials that are naturally low in total zinc.
  • Soils that have been subjected to severe erosion, especially sheet erosion by water, will often be low in available zinc since the sub-soil is usually lower in available zinc than the surface.

There is also a tendency for zinc to be fixed or inhibited by excessive levels of phosphorus in the soil.

There is also the possibility that excessive accumulations of trash mulch on the surface and high levels of organic matter in the soil will contribute to zinc deficiencies.

Plant Factors: Within the plant itself we find that, under certain conditions, an excessive uptake of phosphorus may inhibit the uptake of zinc. Or, the stimulation of additional phosphorus may create a need for zinc that the soil simply cannot meet.

A high level of nitrogen fertilizer usage may also create a zinc deficiency due to the growth stimulation that exceeds the ability of the soil to supply needed zinc.

We rarely find an excess of zinc in the plants, except in situations where an excessive application of zinc fertilizer has been made or where some other factors have limited the growth of the plant, which has resulted in an abnormal concentration of all micro-nutrients in the plant.

 

Zinc Ranges for Plants
Crop Deficient Adequate High
Corn (ear leaf) –
Silking Stage
0 – 20 ppm 21 – 70 ppm 71 + ppm
Grain Sorghum –
Flowering Stage
0 – 16 ppm 17 – 70 ppm 71 + ppm
Alfalfa – Bud
Stage
0 – 19 ppm 20 – 75 ppm 76 + ppm
Soybeans – Prior
To Pod Seed
0 – 20 ppm 21 – 50 ppm 51 + ppm
Wheat, Oats, Barley
— Prior to Heading
0 – 19 ppm 20 – 70 ppm 71 + ppm
Pasture Grasses –
Vegetative
0 – 15 ppm 16 – 80 ppm 81 + ppm
Clover – Bud
Stage
0 – 16 ppm 17 – 70 ppm 71 + ppm
Potatoes –
Blossom (Pre)
0 – 17 ppm 18 – 50 ppm 51 + ppm
Tomatoes –
Blossom (Pre)
0 – 20 ppm 21 – 100 ppm 101 + ppm
Cotton –
Vegetative
0 – 19 ppm 20 – 40 ppm 41 + ppm
Sugar Beets (Leaf
Petiole)
0 – 14 ppm 15 – 50 ppm 51 + ppm
Tobacco –
Vegetative
0 – 19 ppm 20 – 60 ppm 61 + ppm

MANGANESE

Soil Factors: Soil conditions that affect availability:

  • Manganese availability is very closely related to soil pH or soil reaction. Soils that are strongly acid (pH 4.0) are apt to have a very high level of manganese available to the plants. In fact, the manganese level may be within the toxic range for the plant.
  • As the pH approaches 6.5, the availability of manganese decreases, and at a pH of 8, it is highly probable that manganese will be unavailable to the plants. On some soils, manganese will actually become deficient at a pH of 7.5.
  • Organic matter content of the soil reduces the availability of manganese due to the fact that the organic matter reduces manganese to a less soluble form.
  • Soil bacteria, which are more active at pH’s of 6.5 to 7, will also tend to reduce the availability of manganese.
  • A high level of field moisture or poor drainage may also contribute to manganese deficiency problems.

Plant Factors: Cool, damp soil conditions during the early part of the growing season will reduce the uptake of manganese and virtually all of the elements. In other words, anything that is going to slow up the growth of the plant may have an adverse effect upon the uptake of plant nutrients.

In addition, excessive uptake of zinc, copper, or iron may reduce the uptake of manganese. Excess of sodium may reduce the manganese availability in the soil or the plant uptake. On the other hand, excessive uptake of manganese may actually result in a deficiency of iron.

Manganese Ranges for the Plants
Crop Deficient Adequate High
Corn (ear leaf) –
Silking Stage
0 – 19 ppm 20 – 150 ppm 151 + ppm
Grain Sorghum –
Flowering Stage
0 – 19 ppm 20 – 150 ppm 151 + ppm
Alfalfa – Bud
Stage
0 – 29 ppm 30 – 100 ppm 101 + ppm
Soybeans – Prior
To Pod Seed
0 – 20 ppm 21 – 100 ppm 101 + ppm
Wheat, Oats, Barley
- Prior to Heading
0 – 19 ppm 20 – 150 ppm 151 + ppm
Pasture Grasses –
Vegetative
0 – 19 ppm 20 – 120 ppm 121 + ppm
Clover – Bud
Stage
0 – 29 ppm 30 – 120 ppm 121 + ppm
Potatoes –
Blossom (Pre)
0 – 39 ppm 40 – 250 ppm 251 + ppm
Tomatoes –
Blossom (Pre)
0 – 49 ppm 50 – 150 ppm 151 + ppm
Cotton –
Vegetative
0 – 24 ppm 25 – 500 ppm 501 + ppm
Sugar Beets (Leaf
Petiole)
0 – 20 ppm 21 – 400 ppm 401 + ppm
Tobacco –
Vegetative
0 – 39 ppm 40 – 350 ppm 351 + ppm

IRON

Soil Factors: There are some misconceptions about iron. The quantity of iron present in most of our mineral soils is very high as far as total amount is concerned, but the actual amount available for plant usage may be reduced or tied up by the soil.
Soil factors that are most often a cause are:

  • An alkaline soil pH.
  • Excessive level of phosphorus.
  • In many of the soils, the iron deficiency is directly attributed to an excess of calcium in the soil.
  • In soils that have a high pH, the iron is precipitated as an insoluble hydroxide.
  • An excess of zinc, manganese, or copper in the soil may also reduce the availability of iron to the plant.
  • Poor drainage, which also results in poor aeration of the soil, will contribute to iron deficiencies.
  • Excesses of iron are primarily due to strongly acid pH of the soil.

Plant Factors: An excess uptake of zinc, manganese, or copper may result in a deficiency of iron within the plant. Essentially, it is an imbalance situation. On the other hand, an excess of iron in the plant may-indicate strongly acid soil pH or a deficiency of zinc.

Iron Ranges for Plants
Crop Deficient Adequate High
Corn (ear leaf) –
Silking Stage
0 – 20 ppm 21 – 250 ppm 251 + ppm
Grain Sorghum –
Flowering Stage
0 – 20 ppm 21 – 250 ppm 251 + ppm
Alfalfa – Bud
Stage
0 – 29 ppm 30 – 250 ppm 251 + ppm
Soybeans – Prior
To Pod Seed
0 – 20 ppm 21 – 200 ppm 201 + ppm
Wheat, Oats, Barley
- Prior to Heading
0 – 20 ppm 21 – 200 ppm 201 + ppm
Pasture Grasses –
Vegetative
0 – 29 ppm 30 – 100 ppm 101 + ppm
Clover – Bud
Stage
0 – 29 ppm 30 – 250 ppm 251 + ppm
Potatoes –
Blossom (Pre)
0 – 64 ppm 65 – 300 ppm 301 + ppm
Tomatoes –
Blossom (Pre)
0 – 30 ppm 31 – 300 ppm 301 + ppm
Cotton –
Vegetative
0 – 79 ppm 80 – 300 ppm 301 + ppm
Sugar Beets (Leaf
Petiole)
0 – 99 ppm 100 – 150 ppm 151 + ppm

 

COPPER

Soil Factors: Soil conditions that will contribute to a deficiency of copper are:

  • A high level of organic matter. Most organic soils (by an organic soil, we mean one having 20% or more organic matter) are deficient in copper, although occasionally we will find some of our mineral soils high in organic matter which are also deficient in copper.
  • Occasionally, some of the sandy soils that are deep in nature and low in organic matter will also be deficient in copper.
    Soil pH will have an influence on the availability of copper, but it seems to have less influence than does organic content of the soil.
  • Excessive levels of other nutrients in the soil may also contribute to reduced availability of copper.

Plant Factors: As far as the plant is concerned, excessive uptake of phosphorus or high level of phosphorus in the soil may reduce the uptake of copper.

  • Heavy applications of nitrogen may intensify the deficiency of copper.
  • We seldom find an excessive level of copper in the soil, except where plants are grown on soils that have received repeated applications of chemicals high in copper. For example, in some cases where copper sulfate has been used frequently as a chemical treatment, such as in an orchard, there may actually be an excessive level of copper in the soil.
  • High levels of copper by plant tissue analysis sometimes reflects the presence of dead or diseased plant tissue in the sample. It may also indicate some contamination of the sample.
Copper Ranges for Plants
Crop Deficient Adequate High
Corn (ear leaf) –
Silking Stage
0 – 5 ppm 6 – 20 ppm 21 + ppm
Grain Sorghum –
Flowering Stage
0 – 5 ppm 6 – 20 ppm 21 + ppm
Alfalfa – Bud
Stage
0 – 10 ppm 11 – 30 ppm 31 + ppm
Soybeans – Prior
To Pod Seed
0 – 9 ppm 10 – 30 ppm 31 + ppm
Wheat, Oats, Barley
- Prior to Heading
0 – 9 ppm 10 - 30 ppm 31 + ppm
Pasture Grasses –
Vegetative
0 – 4 ppm 5 – 15 ppm 16 + ppm
Clover – Bud
Stage
0 – 10 ppm 11 – 30 ppm 31 + ppm
Potatoes –
Blossom (Pre)
0 – 4 ppm 5 – 30 ppm 31 + ppm
Tomatoes –
Blossom (Pre)
0 – 2 ppm 3 – 40 ppm 41 + ppm
Cotton –
Vegetative
0 – 10 ppm 11 – 50 ppm 51 + ppm
Sugar Beets (Leaf
Petiole)
0 – 5 ppm 6 – 45 ppm 46 + ppm
Tobacco –
Vegetative
0 – 5 ppm 6 – 30 ppm 31 + ppm

BORON

Soil Factors: There appears to be a direct relationship between the soil pH and boron availability. Soils with a high pH or high level of calcium will tend to be deficient in boron. Coarse-textured soils or soils low in organic matter may also be deficient in boron.

Boron availability is also sensitive to drought. Even on soils with good levels of boron, crops may suffer from a deficiency of boron during periods of drought.

An excessive level of potassium occasionally contributes to boron deficiencies.

Plant Factors: Excessive uptake of calcium or an excessive level of calcium in the soil will reduce the uptake of boron.

Excessive uptake of potassium may also result in boron deficiency within the plant.

Excessive level of boron in the plant or boron toxicity is rather unusual. It is usually due to an excessive application of boron fertilizer or unusual type of soil that may be high in boron salts.

Boron Ranges for Plants
Crop Deficient Adequate High
Corn (ear leaf) –
Silking Stage
0 – 5 ppm 6 – 25 ppm 26 + ppm
Grain Sorghum –
Flowering Stage
0 – 5 ppm 6 – 25 ppm 26 + ppm
Alfalfa – Bud
Stage
0 – 30 ppm 31 – 80 ppm 81 + ppm
Soybeans – Prior
To Pod Seed
0 – 20 ppm 21 – 55 ppm 56 + ppm
Wheat, Oats, Barley
- Prior to Heading
0 – 5 ppm 6 – 25 ppm 26 + ppm
Pasture Grasses –
Vegetative
0 – 5 ppm 6 – 20 ppm 21 + ppm
Clover – Bud
Stage
0 – 30 ppm 31 – 80 ppm 81 + ppm
Potatoes –
Blossom (Pre)
0 – 9 ppm 10 – 40 ppm 41 + ppm
Tomatoes –
Blossom (Pre)
0 – 30 ppm 31 – 90 ppm 91 + ppm
Cotton –
Vegetative
0 – 20 ppm 21 – 80 ppm 81 + ppm
Sugar Beets (Leaf
Petiole)
0 – 29 ppm 30 – 50 ppm 51 + ppm
Tobacco –
Vegetative
0 – 20 ppm 21 – 60 ppm 61 + ppm

MOLYBDENUM

Soil Factors: The soil pH has considerable influence on the availability of molybdenum. As the soil becomes acid, the level of available molybdenum declines; it increases as the soil is limed or becomes alkaline. Soils that are high in phosphorus and sulfur tend to be well supplied with available molybdenum. Organic matter is also an important source of available molybdenum.

Excessive levels of molybdenum may be attributed to over-liming or to heavy application of fertilizer materials that include molybdenum as a natural by-product.

Plant Factors: Molybdenum is mobile within the plant. Excessive uptake of molybdenum is more often associated with shortage of potassium in the plant on mineral soils. Shortage in the plant is more often associated with strongly acid soil.

 

Molybdenum Ranges for Plants
Crop Deficient Adequate High
Corn Always sufficient
Grain Sorghum Always sufficient
Alfalfa 0.0-0.9 ppm 1.0-5.0 ppm 5.1+ ppm
Clover 0.0-0.9 ppm 1.0-5.0 ppm 5.1+ ppm
Soybeans 0.0-0.9 ppm 1.0-5.0 ppm 5.1+ ppm
Wheat, Oats, Barley 0.0-0.6 ppm 0.7-1.5 ppm 1.6+ ppm
Grass 0.0-0.5 ppm 0.6-1.5 ppm 1.6+ ppm

CHLORINE AND COBALT

Chlorine and cobalt are also recognized as being essential micro-nutrients. However, data on both of these elements is quite limited. Chlorine is rarely deficient and, in some instances when combined with sodium or calcium in large amounts, it may actually be toxic to the plant. Cobalt functions primarily as a “stimulator” for some of the plant growth functions, and cobalt is actually more important to the animal consuming the plant than to the plant itself. Concentrations of cobalt in excess of 0.1 ppm within the plant may prove damaging or toxic to the growing plant.

Antagonistic Reactions
In considering the plant tissue values, the following antagonistic or competitive reactions should be kept in mind:

  1. Zinc, copper, iron may reduce the uptake of manganese.
  2. Boron uptake is reduced by excessive calcium.
  3. Manganese availability or plant uptake may be reduced by an excess of sodium.
  4. A deficiency of copper may be intensified by heavy applications of nitrogen fertilizer.
  5. Zinc, copper, and iron may become deficient in the presence of excessive phosphorus
  6. Excessive levels of zinc, manganese, and/or copper may result in a deficiency of iron.
  7. The utilization of molybdenum may be reduced by an excess of copper or sulfates.

Reasons for Values Being above or Below the Adequate Range

Following is a brief summary of the more common reasons for values being above or below the adequate range. Some of this information is based on the criteria developed by Dr. J. B. Jones in his work at Ohio State University.

Manganese
  Less than adequate More than adequate
1.) Soil pH neutral or alkaline Acid soil
2.) Low level of manganese Soil or dust contamination in soil.
3.) Commonly associated with peat and muck soils. Heavy application of phosphorus and/or nitrogen on acid, low organic soils

 

Iron
1.) Alkaline soil Soil or dust contamination
2.) Poor drainage Zinc deficiency
3.) Calcareous soil May be due to strongly acid soil

 

Boron
1.) Low level of boron available in soil. Excessive application of boron.
2.) Associated with alkaline, sandy or low organic matter soils Contamination of sample from glue, or adhesive in plant sample, or shipping container
3.) Extremely dry soil conditions  

 

Copper
1.) Low level of available copper in soil Contamination from metal equipment
2.) Associated with peats, mucks, or light-colored upland soils Old, dead or diseased tissue
3.) Excessive application of phosphorus fertilizer  

 

Zinc
1.) Low level of available zinc in the soil Contamination from metal equipment
2.) High soil pH, excessive level of soil phosphorus, eroded soil. Also common to peat and muck soils. Old, dead or diseased tissue
3.) Excessive application of phosphorus fertilizer  

 

Molybdenum
1.) Low level of molybdenum in soil Potassium shortage
3.) Strongly acid soil Alkaline soil pH


In the final review of the plant tissue analysis results for micro-nutrients, the factor of contamination of the plant tissue sample during sampling, shipping, processing or analysis must always be considered. Strict quality control must be exercised at all times over all phases of the plant tissue analysis process.

Whenever values fall outside the accepted ranges on tissue from a normal-appearing plant, the sample should be retested or rechecked and supporting information accompanying the sample re-checked for errors or evidence of contamination. Following are just a few of the more common problems encountered.

  1. Contamination of plants in the field by soil or road dust or chemical residue.
  2. Contamination to sample by person taking sample, such as with cigarette ashes or using a rusty knife to cut sample.
  3. Washing sample with water high in iron or other minerals before shipping to laboratory.
  4. Allowing sample to be contaminated with foreign materials during the “air drying” procedure before shipping to the laboratory.
  5. Including improperly packaged soil sample in container with plant sample.
  6. Improper handling in the laboratory during drying and grinding operation; for example, using plant tissue grinder with brass screen.
  7. Contaminated glassware or improper glassware (must use boron-free for boron testing).
  8. Failure to use water that is mineral-free or chemically-pure reagents in the laboratory analysis.
  9. Poor operator technique. This is especially important in the operation of direct-reading emission spectrograph
  10. Poor housekeeping and uncleanliness in operation of the laboratory which result in accidental contamination of sample somewhere during the analysis process.

When properly performed and interpreted, the plant tissue analysis is by far the most useful guide available to the farmer (and everyone else concerned with plant nutrition) in making decisions as to the micro-nutrient status of his growing plants and crops.

INTERPRETING TISSUE TESTS FOR MICRONUTRIENT FIELD PROBLEMS

 

First of all we should understand that the tissue test report we receive back from the laboratory is the end product of a process that you began in the field by taking the leaves or stems from a plant or representative plants. From that initial point each step in handling, cleaning, drying, packaging, grinding and storage has contributed to the accuracy of the tissue test report. Many good articles are available which can help you manage the portion of the sampling process that is under your control, that is, from the selection of the sample until it is packaged and shipped to the laboratory. Choose a reputable laboratory and they will be skilled in caring for your sample once it arrives.

When you receive your tissue analysis report the numbers may be confusing since one laboratory reports in percent and another in part per million. Usually, however, they include a letter code of “adequacy,” “deficiency,” etc. which can help you understand the significance of the report. Some laboratories print a second column of figures which give you a basis of comparison between your plant’s nutrient levels and those that are between your plant’s nutrient levels and those that are acceptable as a general standard of sufficiency. These are helpful guides but not always the key to your field’s problem.

First of all we should know that it is seldom possible to completely diagnosis a field problem from a tissue test alone. Visual observation of the growth and color characteristics and particularly the abnormalities are essential elements in reaching a sound interpretation of the tissue test results. Everyone working in the field of crop production, who is concerned for trouble spots and problem fields should own a good camera loaded with color film. Do not rely upon your notes or memory to try to recall ten days later, when you have the test results, just what the plant looked like when sampled. The field may have grown completely out of the condition initially noticed but a color photograph will refresh your memory.

The most common error in interpretation of a tissue test report is to assume that because all nutrient levels are above the stated sufficiency levels then all must be well with the plant. Remember, it is often not the total amount of an element present that is of major importance, it is rather the relative amount as compared to several other elements that are metabolically interrelated. Also, the poor growth of any crop may be conditioned by outside factors more than nutritional characteristics. Insect, disease, and environmental weather factors may be affecting the crop much more than nutrition. You need to know, however, that any outside problem that affects the crop adversely for any length of time will also be reflected in a changing nutritional balance in the plant.

In order to help you better interpret your tissue results you can refer to the following ratios which are applied to several crops. This will emphasize the fact that plant nutrient balance is important and that absolute amounts of an element present is sometimes meaningless unless we know the environmental factors and have a good idea of the affected plants’ growth characteristics.

INTERRELATIONSHIPS* NORMAL RATIOS MICRONUTRIENTS
  N/Zn ppm P/Zn ppm
Ca/B ppm Fe/Mn ppm S/Zn ppm S/Mn ppm K/Mn ppm
Fe/Cu ppm Fe/Cu+Zn ppm
Crop  
Corn 1,000 100 300 2 80 30 400 12.5 3.5
Soybeans 900 90 500 1 100 40 200 8.0 2.0
Surghum 800 125 400 2 80 50 400 10.0 2.5
Wheat 750 140 600 0.5 100 30 350 4.0 1.0
Alfalfa 1,000 130 750 1.5 70 50 550 6.0 2.0
Sugar beets 1,200 110 350 1.5 130 30 225 13.0 3.0
*at flowering time

It should be understood that these ratios are tentative and based on the best information available at this time many people will be critical of them, and perhaps rightly so, but they are presented here as a guide to help you assess your tissue test report.


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