Understanding plant analysis for crops

When plant sampling diagnostic tool was introduced, it was intended to either help diagnose nutrient related problems or track the nutrient status of high-yielding crops. In today’s agriculture, nutrient deficiencies are not common. Therefore, the use of plant analysis as a diagnostic tool has diminished. Nevertheless, the value of plant analysis as a monitoring tool remains.

Sampling the right plant part

Sampling the correct plant part at the correct time is critical to ensure accurate results. In addition, sampling multiple plants to form a single composite sample is crucial to ensure that the concentration number obtained from the lab is meaningful and represents a true average from the collection area. Table 1 shows a summary of which plant parts to sample. Samples can be air dried by placing them in a warm area with a fan blowing air across them. To lessen the risk of molding, store samples in paper bags prior to drying or sending samples to the lab. Do not store plant samples in sealed plastic bags.

Table 1: Recommended plant parts to sample based on crop growth stage

Crop Growth Stage Plant Part Minimum Number of Samples
Corn <12 inches All above ground plant parts 10
Corn Silking Leaf opposite and below ear 10
Soybean <12 inches All above ground plant parts 10
Soybean Mid to full bloom Upper fully developed trifoliate 30
Alfalfa Prior to 1/10 bloom Upper six inches of the plant 10
Grass Prior to heading Top leaves 50
Potato Tuber initiation to bulking Petioles 30 to 40
Small Grains Boot stage All above gorund plant parts 20
Sugarbeet Six to ten weeks Developed leaf and/or petiole 15

Taking plant samples early in the season

An analysis of nutrient concentration only, is usually not effective in diagnosing many problems. Calculation of nutrient uptake is a better choice. Why? Nutrients, even though one or more may be deficient, are usually more concentrated in stunted plants. For example, the concentration of nitrogen may be greater in plants that are 12 inches in height compared to plants that are much taller. The nitrogen is simply diluted by carbohydrates in plants that are much taller. Calculation of nutrient uptake is a better approach. To calculate nutrient uptake, make sure you:

 

1) dry the whole plants collected,

2) get an accurate weight, and

3) complete an analyses of the plant material.

 

To calculate nutrient uptake, multiply plant dry weight by nutrient concentration. Knowing the number of plants sampled, uptake for an individual plant can be determined. To measure nutrient uptake there must be access to an oven that will dry a sample rapidly and a scale or electronic balance that can measure small differences in weight. So, some planning is needed if there is intent to calculate nutrient uptake. In diagnostic situations, collect soil samples whenever and wherever you collect plant samples. Analysis of soil samples can often provide a good indication of nutrient deficiencies. By comparing the results of the analysis of soil samples collected, you can confirm or reject suspected nutrient deficiencies.

 

Taking plant samples late in the season

When collecting samples late in the season there is less emphasis on total uptake of nutrients and more emphasis on sampling plant parts that, when the nutrient concentration is compared, correlate well to final crop yield. For corn, sample leaves opposite and below the ear at silk emergence when pollen is falling. Timing of sample collection for corn is important. Collect samples before the silks turn brown. Nutrient concentrations decline substantially after this point in the life cycle and recognized standards cannot be used for comparison. For soybeans, a sample of the most recently matured trifoliates collected at early to midbloom is the standard.

Plant sampling as a diagnostic tool

When used as a diagnostic tool we expect plant analysis to identify a suspected nutrient deficiency. In these situations, we are usually faced with normal and stunted and/or off-colored plants in the same field. The normal tendency of individuals is to collect the stunted plants and conduct an analysis of the plant tissue. Plant sampling, however, is more complicated if we expect tissue analysis to be an effective diagnostic tool. Take three samples for successful identification. Collect one sample of whole plants from the stunted area. Next, collect a sample of whole plants from a marginal area where there is a slight reduction in growth or where the plants are slightly stunted. For the third sample, use plants that are normal and healthy.

Plant sampling as a monitoring tool for corn and soybean

Plant analysis can also be used to assess the nutrient status of plants in relation to the fertilizer program used. If used for this purpose, techniques for sample collection are different. This discussion will focus on corn and soybeans. Since the results of the plant analysis will be compared to known standards, parts of plants should be sampled at a certain stage of development. The results of the analysis of these tissue samples are compared to standards that are summarized in Tables 2 and 3.

Plant analysis, if used correctly, can be a useful management tool in modern agriculture. To get good information, stop and think before sample collection. Are you collecting samples to diagnose a problem or monitor the results of a fertilizer program?

Table 2: Expected range in nutrient concentrations for corn ear leaves collected at 50 percent silk

Nutrient Expected Range
Nitrogen (N) % 2.7 to 3.5
Phosphorus (P) % 0.2 to 0.4
Potassium (K) % 1.7 to 2.5
Sulfur (S) % 0.1 to 0.3
Calcium (Ca) % 0.4 to 1.0
Magnesium (Mg) % 0.2 to 0.4
Boron (B) ppm 4 to 15
Copper (Cu) ppm 3 to 15
Iron (Fe) ppm 50 to 200
Manganese (Mn) ppm 20 to 250
Zinc (Zn) ppm 20 to 70

Table 3: Expected range in nutrient concentrations for soybean trifoliate samples at early to mid-bloom

Nutrient Expected Range
Nitrogen (N) % 4.01 to 5.50
Phosphorus (P) % 0.26 to 0.50
Potassium (K) % 1.71 to 2.50
Sulfur (S) % 0.21 to 0.40
Calcium (Ca) % 0.36 to 2.00
Magnesium (Mg) % 0.26 to 1.00
Boron (B) ppm 21 to 55
Copper (Cu) ppm 10 to 30
Iron (Fe) ppm 51 to 350
Manganese (Mn) ppm 21 to 100
Zinc (Zn) ppm 20 to 50

Tissue Analysis for Potato

The recommended tissue used for nutrient analysis in potato is the petiole (leaf stem and midrib) of the fourth leaf from the shoot tip (Figure 1). It is critical to collect tissue at this stage because younger or older tissue will have different nutrient concentrations and can lead to erroneous interpretations.

For sampling, collect 30 to 40 leaves from randomly selected plants within a field. Strip the leaflets and discard them. Most diagnostic criteria for tissue analysis are based on a sample taken during the tuber initiation through the tuber bulking stage. Samples taken too early in the season or soon after a fertilizer application may not accurately reflect the true or potential nutritional status of the crop if uptake of applied fertilizer by roots has not yet occurred.

For irrigated potato, tissue analysis should begin when rows are about ¾ closed and at least four days after a fertigation and then continue at 10 to 14 day intervals through the bulking stage. If petiole nitrate-N falls below the suggested range, then 10- 20 lbs N/A should be applied through the irrigation system. Higher rates of N applied during tuber bulking may cause misshapen tubers and hollow heart in some varieties like Russet Burbank.

Table 4: Expected range in nutrient concentrations for whole leaf potato tissue (leaflets plus petioles collected from the 4th leaf from the top of the shoot during the tuber bulking stage.)

Nutrient Expected Range
Nitrogen (N) % 3.5 to 4.5
Phosphorus (P) % 0.25 to 0.50
Potassium (K) % 4 to 6
Sulfur (S) % 0.30 to 0.45
Calcium (Ca) % 0.5 to 0.9
Magnesium (Mg) % 0.25 to 0.50
Boron (B) ppm 20 to 40
Copper (Cu) ppm 5 to 20
Iron (Fe) ppm 30 to 150
Manganese (Mn) ppm 20 to 450
Zinc (Zn) ppm 20 to 40

Table 5: Expected range in nutrient concentrations for potato petioles collected from the fourth leaf from the top of the shoot during the tuber bulking stage and for petiole nitrate-N at three growth stages

Nutrient Expected Range
Nitrogen (N)
--at tuber initiation % 1.7 to 2.2
--at tuber bulking % 1.1 to 1.5
--at maturation % 0.6 to 0.9
Phosphorus (P) % 0.22 to 0.40
Potassium (K) % 8 to 10
Sulfur (S) % 0.20 to 0.35
Calcium (Ca) % 0.6 to 1
Magnesium (Mg) % 0.30 to 0.55
Boron (B) ppm 20 to 60
Copper (Cu) ppm 4 to 20
Iron (Fe) ppm 50 to 200
Manganese (Mn) ppm 30 to 300
Zinc (Zn) ppm 20 to 40

Whole leaves can also be used for analysis; however, different diagnostic criteria need to be used for interpretations. Petioles are generally preferred as the tissue to use for predictive purposes, because they more accurately reflect the immediate nutritional status of the plants and whether they are currently taking up sufficient nutrients. Nutrients are ultimately transported from the petiole to the leaflets and the whole leaf provides a more integrated nutrient status since nutrients tend to accumulate in the leaflets. Therefore, leaves are better indicators of the cumulative nutritional status of plants and whether nutrient uptake has been adequate up to the present point in time.

A comparison of nutrient sufficiency ranges for petioles vs. whole leaves is provided in Tables 4 and 5. Note that K requirements are much greater in petioles compared with whole leaves. Also note that total N is used for whole leaves, while nitrate-N is used for petioles. Most N in petioles is in the nitrate form and measurement of nitrate-N is a more straightforward procedure than total N; however, there is less nitrate-N in leaflets and total N provides a more accurate measurement of N status for whole leaves.

The basal stalk nitrate test for corn

The basal stalk nitrate test is a tool that can help farmers combat higher prices for fertilizer N and concerns for environmental quality. This analytical test was developed and refined by faculty at Iowa State University. It is a diagnostic - not a predictive - test. It was not intended to and cannot predict the amount of fertilizer N needed for the next time that corn is in the rotation. However, its use does allow for a closer evaluation of the rate of fertilizer N used in the year that the corn was grown.

What’s measured?

In this analytical test, a 6-inch section of the corn stalk starting at 6 to 8 inches above the soil surface is analyzed for NO3 -N. Leaves are not included. The results are compared to standards developed from field research. For best results, collect the sample after formation of black layer in the kernel. Waiting until after harvest to collect the sample could easily lead to inaccurate results.

What’s in the sample?

Use the base of the corn stalk for this test. Take a six-inch-long sample starting six to eight inches above the soil surface, including the bottom node of the plant. Remove leaf and sheath tissue. Collect at least 15 stalks from the area of interest for a representative sample. Some advisors have worked with farmers to compare the impact of various rates of nitrogen fertilizer across the landscape. For these comparisons, this test, in addition to yield, would be an added feature in the evaluation of nitrogen rates. This test could also be used in the evaluation of management zones.

Handling the sample

Once the sample is collected, it should be split vertically parallel to the length of the corn stalk. Splitting each stalk into four sections would be ideal. Splitting into two sections is absolutely necessary. The splitting is necessary to assure rapid drying.

Once split, dry the sections as rapidly as possible. Use an oven or place in front of a fan blowing warm air for rapid drying. Once dried, submit samples to the laboratory.

Table 6: Interpretation of the basal stalk nitrate test for corn.

NO3-N Level Interpretation
0 to 250 ppm Low Nitrogen was probably deficient during the growing season
250 to 700 Marginal Nitrogen shortage may have limited yield
700 to 2000 Adequate Nitrogen level did not limit yield
2000+ Excessive Nitrogen rate was too high or some production factor caused a yield reduction

Interpreting results

As mentioned, this is a diagnostic, not predictive, test. Interpretation of the results is given in Table 6. When interpreting the basal stalk nitrate values, it’s important to remember that factors other than excessive use of N fertilizer can lead to high values. Anything that can cause a severe reduction in yield such as hail damage or drought can lead to high values.

Table 7: Summary of basal stalk nitrate averages for multiple nitrogen rates at two locations in Minnesota.

Nitrogen Rate Site 1 Site 2
0 lbs. N/ac 10 ppm 10 ppm
120 lbs. N/acre 12 ppm 595 ppm
200 lbs N/acre 2100 ppm 3263 ppm
300 lbs. N/acre 4711 ppm 4548 ppm

Table 8: Summary of individual replication data for basal stalk samples collected at Site 2 in Table 7 for 120 lb. N/acre application rate.

Replication Nitrate Nitrogen Level
Rep. 1 7 ppm
Rep. 2 9 ppm
Rep. 3 468 ppm
Rep.4 1896 ppm

Concerns

1. The results of this test are diagnostic and not predictive. Do not make a management change base only on the test results.

2. The results from different samples can be quite variable. See Table 7 and Table 8. The results from the sample area in a field managed the same way have a range of values from 7.4 to 1896 ppm.

3. Choosing the representative location to sample in a field may be difficult at best

4. Any stress, such as drought, can cause the test results to be greater than expected and thus affect the interpretation.

 

The Laboratories: The University of Minnesota Soil Testing Laboratory as well as some commercial laboratories will analyze these stalk samples. All use the same analytical procedure. Submit the samples in paper, not plastic, bags. Get the samples to the laboratory as soon after collection as possible. Speed in getting the sample to the laboratory will help to insure accuracy of analysis.

Summary

Remember, as with taking any plant sample proper planning is crucial for obtaining the best results. In addition, samples represent only a single point in time. Stress on plants can significantly affect nutrient uptake and results obtained. There is no guarantee plants will not recover and resume normal growth for the rest of the season. With any plant tissue test there are no fertilizer recommendations associated with the results given. Fertilizer recommendations should be made based on soil samples since these tests are correlated to crop response and calibrated to aid in making decisions on rates of fertilizer to apply.

Daniel E. Kaiser, Extension nutrient management specialist and Carl J. Rosen, Extension nutrient management specialist and Department Head

Reviewed in 2018

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