Early season field scouting is essential to ensure good emergence, detect potential weed and insect pressure, and monitor the effectiveness of your fertility program. The earlier an issue is detected, the better chance there is to correct the issue. One tool to help detect potential fertility problems is plant tissue analysis. However, to get useful results back from the lab, the proper plant part must be collected for the current growth stage of the crop.
The proper method for collecting early season corn tissue samples is to collect 15 or more whole plants to comprise a single sample. This is only referring to the above ground portion of the plant. This method is appropriate for corn up to 12 inches tall, or approximately V4 to V5. However, corn can be sampled too early to provide useful data for making decisions. During the first three weeks after emergence, much of the nutrient content of the young plant is not coming from the soil, but from the embryotic tissues within the seed. The young plants also have a very small seminal root system during the first few weeks which are not able to access the nutrients in a large volume of soil; the main role of this root system is to get water to the seed and new leaf tissues. Nutrient deficiencies during the first few weeks are likely caused by environmental conditions and do not necessary reflect low nutrient levels in the soil. An example of this situation is young corn plants turning purple. Purpling of young corn plants can be a symptom of a phosphorus deficiency, but can also occur on soils with adequate phosphorus levels when nighttime temperatures are low. Cool spring nights slow metabolic processes in the plant resulting in the buildup of anthocyanins, which appear purple. Tissue sampling of corn should be delayed until 3 to 4 weeks after emergence, or until the plants have developed a functional nodal root system in order to ensure that the tissue analysis is representative of the nutrients that are available to the plant.
Image 1: V4 corn plant showing purpling. Source: purdue.edu
Similar to corn, soybeans can also be sampled too early. Proper tissue sampling for all growth stages for soybeans is collecting 25 or more of the most recently mature trifoliates without petioles. The first leaves to appear on a recently emerged soybean plant are unifoliates, or cotyledons. The nutrient content of the cotyledons, sometimes referred to as seed leaves, does not accurately represent the nutrients available in the soil. Tissue sampling in soybeans should be delayed until the V2 growth stage. The V2 growth stage is reached when the second trifoliate has completely unrolled. It will generally take a minimum of 3 to 4 weeks after emergence to reach this stage. At this point, the first trifoliate is considered mature, and can be collected for tissue analysis.
Image 2: V2 soybean plant. Source: clemson.edu
Spring tissue sampling of winter wheat can be a very useful management tool. The timing of wheat sampling does not correspond to a specific growth stage though. The important factor when determining the appropriate time to sample wheat is that the wheat has broken dormancy and is actively growing again. Generally, wheat will be at a growth stage of Feekes 3 or 4 when this occurs. The appropriate method for collecting wheat samples at this stage is to collect 25 or more whole plants from ½ inch above the soil surface. One of the benefits of early season wheat sampling is to fine tuning a “green-up” nitrogen applications based on the nitrogen content of the plant at Feekes 5 (please visit the Purdue Extension News Release for more information).
Image 3: Feekes 5 wheat. Source: Kansas State University
Accurate plant tissue testing begins with proper sample collection and handling. Make sure to collect the proper plant part for the current growth stage of the crop, and collect the proper number to make the sample. This information can be found on the plant analysis page at algreatlakes.com. Always avoid soil contamination in your plant samples. Package samples in paper bags. If shipping is delayed, store samples in a cool location, but do not freeze. Never include roots with a plant sample. If you have any questions on proper plant tissue sampling, please contact the lab for assistance.