Pennsylvania Corn, Soybeans: Mid-Season Crop Tissue Testing

    Clipping the newest soybean trifoliates at early flowering for tissue testing. Photo: Charlie White, Pennsylvania State University

    Why Take Tissue Samples?

    Tissue testing is a valuable diagnostic tool that can aid in managing soil fertility. Routine tissue testing on corn, soybeans, and other crops is often carried out mid-season to determine whether the crop has a sufficient nutrient supply.

    For farms that do routine tissue testing, it is usually a pre-planned activity that is part of the normal monitoring program; it is not necessarily done in response to visual deficiency symptoms or a suspected issue with nutrients. Rather, the purpose is to detect any nutrient problems that might have gone unnoticed, what we call hidden hunger, which should be addressed now or in the future.

    Tissue testing is also particularly useful for assessing the sufficiency of nitrogen, sulfur, and most of the micronutrients, such as iron, zinc, manganese, boron, and copper for which our soil test interpretations are not as reliable.

    How to Take Samples

    For routine tissue testing, sampling needs to be done at the correct growth stage and from the correct plant part to be able to compare the tissue test results with an interpretative table of sufficient nutrient levels. This is because sufficient nutrient levels vary over time as a crop develops and vary across different plant parts within the same growth stage.

    For mid-season corn, the appropriate growth stage and plant part to sample is the ear leaf at silking. Mid-season soybeans should be sampled prior to or at early flowering by clipping the uppermost fully-developed trifoliates. Tissue sampling soybeans after pod set is not recommended.

    For growth stage and plant part to sample for other crops, refer to Table 1.2-7 in the Penn State Agronomy Guide (reprinted here as Table 1) or the instructions for taking tissue samples from the Penn State Agricultural Analytical Services Lab. Tissue samples should be taken from at least 10 representative plants, but preferably 20 to 30 plants, in each field or sub-field area of interest.

    After collecting tissue samples in the field, samples should be placed into a breathable paper bag or envelope (the Penn State Agricultural Analytical Services Lab provides an envelope in its sampling kit) so that the tissue samples can begin to dry down while in transit to the lab, rather than rot or mold as might happen in warm weather if transported in a plastic bag.

    Be sure to provide all the requested information on the sample submission form so the lab can provide a report which references the nutrient levels in your tissue sample against the correct sufficiency levels for the crop type, growth stage, and plant part you sampled.

    Sufficiency Ranges

    Table 1 lists the sufficiency range for different nutrients based on the crop, growth stage, and plant part. If the nutrients are within these ranges, they should not pose a limiting factor for crop yields. If the nutrients are above the sufficiency range, the crop yield should not suffer due to lack of the nutrient, but it may indicate a fertility problem worth investigating.

    For example, excess levels of nitrogen (N) may suggest that there is an opportunity to reduce N fertilizer applications without sacrificing yield. Excess potassium (K) may indicate that soil test K levels have accumulated to above optimal levels, perhaps through historical manure additions.

    Excess K should be evaluated carefully if the crop is to be used as cattle forage because of issues with grass tetany and milk fever. Excess levels of iron and manganese may be an indicator that soil pH is getting low because these micronutrients become much more soluble as soil pH drops. If any nutrients are below the sufficiency range, it suggests that availability of those nutrients to the crop could be a limiting factor to yield.

    For corn that is tasseling, it may be too late to make a rescue application of nutrients due to clearance issues with sprayers or fertilizers. For soybeans, which are shorter and are more plastic in their growth and yield, rescue applications of nutrients may be more practical, especially for micronutrients which require only small doses that can be foliar applied.

    Even if a rescue nutrient application cannot be made, a tissue test with sub-optimal nutrient levels can alert you to problems that should be further evaluated and fixed for the next growing season.

    Table 1. Nutrient sufficiency ranges in selected agronomic crops for specific plant parts and growth stages. Reprinted from Penn State Extension Agronomy Guide, Table 1.2-7.

    Crop: Corn Corn Alfalfa Small grains Soybeans
    Plant part: Whole plant Ear leaf Leaves, top 33% of plant Uppermost leaves Uppermost full leaves
    Growth stage: ~12 inches tall Silking Bud to 10% bloom Before heading Prior to or early flowering
    Sufficiency range percent (%) Sufficiency range percent (%) Sufficiency range percent (%) Sufficiency range percent (%) Sufficiency range percent (%) Sufficiency range percent (%)
    Nitrogen 3.50–5.00 % 2.75–3.50 % 3.75–5.50 % 2.50–3.50 % 4.25–5.50 %
    Phosphorus 0.30–0.50 % 0.25–0.50 % 0.25–0.70 % 0.20–0.40 % 0.25–0.50 %
    Potassium 2.50–4.00 % 1.70–2.50 % 2.00–3.50 % 1.50–3.00 % 1.70–2.50 %
    Calcium 0.30–0.70 % 0.20–1.00 % 1.75–3.00 % 0.20–1.00 % 0.35–2.00 %
    Magnesium 0.15–0.45 % 0.20–0.60 % 0.30–1.00 % 0.15–0.60 % 0.25–1.00 %
    Sulfur 0.21–0.76 % 0.20–0.50 % 0.25–0.50 % 0.15–0.50 % 0.20–0.40 %
    Sufficiency range (ppm) Sufficiency range (ppm) Sufficiency range (ppm) Sufficiency range (ppm) Sufficiency range (ppm) Sufficiency range (ppm)
    Manganese 20–300 ppm 20–150 ppm 30–100 ppm 25–150 ppm 21–150 ppm
    Iron 50–250 ppm 20–250 ppm 30–250 ppm 20–250 ppm 50–350 ppm
    Boron 5–25 ppm 4–25 ppm 30–250 ppm 6–25 ppm 20–50 ppm
    Copper 5–20 ppm 6–20 ppm 10–30 ppm 6–25 ppm 10–30 ppm
    Zinc 20–60 ppm 20–70 ppm 20–70 ppm 20–70 ppm 20–50 ppm

    1Sufficiency range is valid only for the crop, plant part, and sampling time indicated.

    Tissue Testing Caveats

    First, diseased or pest damaged foliage should not be sampled for tissue testing because of the effect it can have on nutrient levels in the tissue. Similarly, crops that have experienced extremes of moisture, such as flooding or severe drought, should not be sampled.

    When crop growth is held back due to drought, nutrient levels in the tissue may become more concentrated. For severe drought stress or in flooded conditions, tissue testing may also have limited utility because nutrients are unlikely to be the most yield limiting factor in those years.

    Finally, while tissue testing gives you a snapshot of nutrients the crop was able to access from the soil, it doesn’t definitively tell you whether those nutrients are actually deficient in the soil. For instance, potassium deficiency can appear in the plant tissue even if the soil has optimal K levels already if there is an issue with root development (such as from sidewall compaction) or if the soil is so dry that diffusion and mass flow of nutrients to the root through soil water cannot occur.

    So, just because a tissue test shows a below optimal nutrient level does not always mean that the correct solution is to apply more nutrients to the soil. Rather, the underlying cause of the nutrient deficiency, and the appropriate solution for correcting it in the future, needs to be further analyzed through soil testing and evaluation of all the other environmental and management factors that may have contributed to the deficiency.

    Knowing how to use routine tissue testing, from sampling, to interpreting results, and ultimately making corrective actions if needed, can advance your crop management skills and allow you to potentially detect problems that you didn’t know existed, ultimately improving yields and increasing profits.

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