Interpretation of Plant Analysis for Soybeans

Table of Contents

1. Introduction
2. Sample Collection
3. Interpretation of Plant Analysis Results
4. Common Ontario Deficiencies
5. Updates to the Plant Tissue Values for Potassium
6. Use Accredited Labs for Consistent Results
7. References

Introduction

The interpretation of plant analyses for soybeans has recently been updated, particularly with regards to potassium concentrations. The information provided here replaces the values in Publication 811, Agronomy Guide for Field Crops.

Fertilizer recommendations for soybeans are normally based on soil test results, but plant tissue analysis can provide useful additional information. The most common use for plant analysis is to diagnose nutrient related problems with crop growth, either a deficiency or toxicity. In cases where the soil tests are adequate but there are deficiencies showing up in the plant tissue, it may indicate some problem reducing the ability of the plant to access the nutrients in the soil, such as diseases, insects or soil compaction impeding root function. Plant analysis can also be used to validate the fertilizer program, particularly where there is a comparison between different programs.

Sample Collection

Plant tissue analysis of soybeans is normally done on the top fully developed trifoliate (three leaflets plus stem) at the time of first flowering. This represents the time of the maximum rate of growth of the soybean plant, as well as the greatest extent of the root system, but is prior to the movement of nutrients from the leaves into developing seeds.

Samples for plant analysis should be taken from at least 20 plants, distributed throughout the area chosen for sampling. Each sample should consist of at least 100 grams of fresh material. Problem areas should be sampled separately. When taking samples for plant analysis take care not to contaminate the sample with soil. Even a small amount of soil will cause the results to be invalid, especially for micronutrients. Store samples in paper rather than plastic bags, to avoid condensation and mould damage. Sample both normal growth plants and affected areas for comparison.

The most common errors in collecting plant tissue samples are:

• not collecting enough material
• collecting chlorotic or dead tissue or insect damaged leaves
• collecting plant tissue contaminated with soil
• shipping the sample in plastic bags.

Samples of fresh plant material should be delivered directly to the laboratory. If they are not delivered immediately they should be dried to prevent spoilage. Samples may be dried in an oven at 65 C or less or dried in the sun provided precautions are taken to prevent contamination with dust or soil. Avoid contact of samples with galvanized (zinc coated) metal, brass or copper.

You may have to sample outside the recommended times to get a diagnosis. In this case, the nutrient contents will not correspond to the values at the standard times. Time of sampling has a major effect on the results since nutrient levels within a plant vary considerably with the age of the plant. Nevertheless, plants suspected of being nutrient deficient should be sampled as soon as a problem appears. Samples are best taken from a problem area rather than from the entire field, and it is often helpful to sample an adjacent area of healthy plants at the same time, for comparison. Do not sample dead plants but those from border areas (See Figure 1).

Figure 1. Collect diagnostic samples from healthy plants (good sample) and from plants in the margins of the affected area (poor sample). Avoid plants that are dead or dying (do not sample).

Plant analysis is most useful if combined with visual inspection of the crop and soil conditions, knowledge of past management in the field, and a current soil test to provide information about soil nutrient levels and soil pH.

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Interpretation of Plant Analysis Results

Table 1 shows the critical and normal concentrations of nutrients in soybean leaves collected at the recommended time (first flowering). Leaf tissue concentrations at or below the critical concentration indicate that the levels of that nutrient are insufficient for maximum crop growth and yield. Occasionally the crop will respond to added fertilizer (e.g. foliar manganese to correct a manganese deficiency), but more often the tissue results will point to management changes for future crops.

Plant tissue nutrient concentrations between the Critical Concentration and the Maximum Normal Concentration reflect the range where there is adequate nutrient supply for the plant. Concentrations above the Maximum Normal are not generally harmful, but they do indicate abnormal conditions. It may be luxury consumption of some nutrient that is available from the soil at high levels, or it may indicate that some non-nutrient factor is limiting the growth of the plant so there is less dilution of the nutrients in the tissue. Be aware that critical values for tissue concentration may be misleading because the concentration of nutrients in unhealthy plants may be high simply because there is not enough tissue to dilute the nutrients. Try to collect samples with a reasonable amount of growth.

Interpretation of plant analysis results is seldom as straightforward as the values in Table 1 imply, and requires some judgement to make good recommendations. The clearest situation is where one nutrient is low while all the others are in the adequate range, so we can reasonably infer that the low nutrient is limiting crop growth. Less clear is the situation where one nutrient is within the adequate range while all the others are at or above the maximum normal concentrations. This may indicate that the growth of the crop is limited by the supply of that nutrient to the point where the other nutrients build up in the plant tissue. In other cases, the concentration of all the nutrients will be low, which indicates that the plant is growing vigorously and diluting the nutrient concentration in the plant tissue.

Plant tissue concentrations show what the plant has been able to absorb, but they do not necessarily reflect the nutrient supply in the soil. Any disruption to nutrient uptake, by (for example) low soil moisture content or damage to root systems by fertilizer burn, acid soil, insects, disease or soil compaction, will reduce the nutrient concentration in the plant tissue. Identifying and correcting the underlying problem will be more beneficial than simply adding more fertilizer.

¹Yield loss due to nutrient deficiency is expected with nutrient concentrations at or below the "critical" concentration.
² Maximum normal concentrations are more than adequate but do not necessarily cause toxicities.

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Common Ontario Deficiencies

Although a range of soybean nutrient deficiencies have been reported in Ontario the most common are listed below:

1. N - Nitrogen deficiency is due to a lack of proper nodulation. This usually occurs in fields where soybeans are being grown the first time and proper nodulation did not occur. Apply 50kg/ha of N at first flower as urea, ammonium sulphate or calcium ammonium nitrate when the foliage is dry to supply the crop with N. Plant symptoms will include stunting, and a pale green colour to the plant, particularly in the older leaves.
2. P - Soybeans respond to adequate supplies of phosphorus from the soil, but do normally respond to starter P like corn or cereals. There are no obvious visual symptoms of phosphorus deficiency, although the plants may be stunted and darker green colour relative to plants with ample P supply.
3. K - If a leaf deficiency has been identified take a soil sample to determine the level of K that should be added to the field for future crops. There is no way to supply enough K during the growing season to improve yields the year symptoms occur. Visual symptoms will generally show up in the older leaves as chlorosis (yellowing) followed by necrosis (browning) of the leaf margins.
4. Mn - Manganese is the only micronutrient deficiency commonly found in Ontario. Symptoms are pale green to yellow leaves developing on the upper part of the plant with veins remaining dark green. A foliar application of Mn is recommended when visual symptoms occur (See http://www.omafra.gov.on.ca/english/crops/pub811/4fert.htm#manganese). Soil application is not successful because of the large amounts required.

Updates to the plant tissue values for Potassium

A study undertaken by Xinhua Xin and Dr. Tony Vyn at Kirkton, Belmont and Strathroy Ontario (1998-2000) looked at the response of soybeans to potassium fertilizer placement. Soybeans frequently responding to potassium fertilizer at leaf tissue levels higher than the published critical value for soybeans (1.2%) in the Agronomy Guide for Field Crops (Publication 811). The data collected during that study formed the basis for updated critical and normal values for potassium in soybeans.

Relative yield, as shown in Figure 2, is the yield without added fertilizer as a percent of the fertilized yield in each of the plots. A relative yield of 100% indicates that there is no response to added fertilizer. Below a leaf K concentration of 20 grams per kg (2.0%), most of the plots showed a response to added K fertilizer. Above this level, most of the plots were unresponsive.

Figure 2: Relative yield of soybeans at various leaf K concentrations. (Note: 10 g kg-1 = 1%)

Based on the results of these experiments and other similar studies the Critical Concentration for K in soybean tissue was revised upward from 1.2% to 2.0%, and the Maximum Normal Concentration from 2.5% to 3.0%.

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Use Accredited Labs for Consistent Results

There is no formal accreditation process for plant tissue analysis. However, the performance of the accredited labs in plant tissue analysis is monitored, and any of the accredited soil test labs can do a good job of analyzing plant tissue samples.

A list of labs that are accredited to perform soil tests for pH, buffer pH, P, K, Mg and Nitrate-N on Ontario soils can be found on the OMAFRA website.

References

Vyn, Tony J., Xinhua Yin, Tom W. Bruulsema, Chung-Ja C. Jackson, Istvan Rajcan, and Sylvie M. Brouder. 2002. Potassium Fertilization Effects on Isoflavone Concentrations in Soybean [Glycine max (L.) Merr.]. J. Agric. Food Chem. 2002, 50, 3501-3506.

Yin, Xinhua and Tony J. Vyn, 2002. Soybean Responses to Potassium Placement and Tillage Alternatives following No-Till. Agron. J. 94:1367-1374.

Yin, X.H., and T.J. Vyn. 2002. Residual effects of potassium placement and tillage systems for corn on subsequent no-till soybean. Agron. J. 94:1112-1119.

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