Interpreting plant tissue reports can be challenging due to variation in tissue concentrations from one field to the next. Plant tissue sampling has been used for years to help diagnose potential nutrient deficiencies in fields. Variations in nutrient concentrations in plant tissues can be impacted by many factors; some which can be controlled and some cannot.
When planning tissue sampling, there are a few factors you should consider to get the most out of the information you receive.
1. Don’t sample too early or too late in the growing season: Proper sample timing is critical to ensure that you get accurate data which can be utilized to help diagnose issues in the field. Tissue concentrations vary over time and knowing when to sample is important if the samples are being compared to a known sufficiency value. The optimal time to start taking plant tissue samples is near the point where rapid growth occurs through early reproductive growth stages. For corn, this means sampling between V5 through silking (R2).
2. Sample the correct part of the plant: Nutrient concentrations are not consistent across the various plant tissues. Sampling the correct tissue is important to help determine if nutrients are deficient. Nutrients vary in their mobility in the plant. Nitrogen, phosphorus and potassium are mobile. The plant will translocate these nutrients to new tissues if the nutrients are limited in the soil. Sulfur and micronutrients are not mobile and newly developing tissues will show deficiencies first. If leaves are being sampled, it is best to sample the newest, fully-developed leaf which is not the uppermost leaf on the plant. When collecting samples, it is best to contact the lab where the samples will be submitted in order to determine which part of the plant they recommend sampling.
3. Collect soil samples: If you are diagnosing problem areas in fields, soil samples can help further determine if there is an issue which must be addressed now or in future years. Soil sample results are better calibrated to help determine how much fertilizer should be applied. Plant tissue samples are not calibrated, making it difficult to know what to do once the sample results are returned. Tissue results can help to determine if there is a problem but will not give information on why the problem is occurring.
4. Handle samples properly: Biological processes do not stop once a sample is collected. Nitrogen reduction can occur in plant tissues which have not been dried. Rapid drying of samples is needed to stabilize the samples, ensuring that reported values are an accurate representation of field conditions. Keeping samples cool will slow biological processes, but getting samples dried or to the lab quickly is best to ensure accurate results.
5. Know what your results can and cannot tell you: Plant tissue testing was never meant to be used to determine nutrient applications in annual cropping systems. There are very few guidelines that recommend using tissue tests to help manage nutrients in-season. For corn and soybean, tissue testing has long been suggested as a diagnostic tool but is increasingly being proposed as a management tool where foliar application of nutrients are suggested in-season. Foliar application of most nutrients should be viewed as a last resort – only in cases where severe nutrient deficiencies are seen. Research on foliar applications of micronutrients has not shown that yield can be increased in corn and soybean, even if tissue results indicate a deficiency.
With current low commodity prices, caution should be exercised when using plant tissue testing. It is also important to understand that sufficiency ranges for most nutrients are fluid and vary by crop species, hybrid/variety, and growth stage. Our research has consistently shown significant variations in nutrient concentrations among cultivars within, and across, locations. This variation among nutrient concentrations has no relationship to final yield. Plant tissue analysis was meant to be a diagnostic tool to help determine a problem exists, but it is only one tool in the toolbox and should not be used alone to guide nutrient management decisions.