Care with plant nutrient analysis
This season, many climatic and soil factors are causing problems with corn and soybean growth. Sometimes plants also exhibit nutrient deficiency symptoms. A natural questions is whether the plants are actually nutrient deficient. If they are expressing deficiency symptoms, then yes they are. However, the reason may not be due to insufficient available nutrients in the soil, but rather poor uptake due to climatic or soil factors affecting growth. This was recently discussed in an ICM News article for potassium (K) in corn and soybean. If the symptoms are clearly present, then there really is no need for plant sampling and analysis as the plant is indicating the deficiency.
click image to zoomFigure 1. Relationships between relative corn yield response to P and K fertilization and the nutrient concentration of small plants or leaves (at V5-V6 or silking) across several Iowa field trials. Relative yield represents the yield without fertilization expressed as the percentage of the maximum yield achieved with fertilization. Taking care with plant analysis
Issues with plant analysis, especially whole plants but also leaves, arise from various factors. The tissue nutrient concentrations often differ across hybrids and varieties and as plants develop during the season. So interpretation of test results often needs to be specific to hybrids or varieties and to the plant part sampled and the growth stage. Also, the nutrient concentration increases or decreases as a result of growth and amount of dry matter in response to climatic and soil conditions. There can be a dilution, where rapid growth, even with adequate nutrient supply, causes the concentration to be low. Or it can be the opposite, where reduced growth increases concentration when in fact the nutrient could be deficient in the soil. Partly due to these issues, as well as other factors, correlation of nutrient concentrations to fertilizer response for small whole corn and soybean plants or leaves at midseason has been poor.
click image to zoomFigure 2. Relationships between relative soybean yield response to P and K fertilization and the nutrient concentration of small plants or leaves (at V5-V6 or R2-R3 stages) across several Iowa field trials. Because of these issues the usefulness of plant analysis is limited, and it is commonly suggested to sample specific plant parts at specific times that provide the best possible correlation to yield response and critical levels. For corn this is the ear leaf or leaf opposite and below the ear leaf at tassel to silk emergence; for soybean the upper fully developed trifoliate leaves, petioles discarded, prior to pod set; and for alfalfa the top 6 inches of stems at the early bud growth stage. Sampling at these plant stages are too late in the corn and soybean growth cycle for remediation in the current year, but can be quite helpful for alfalfa where nutrients can be applied after any cut during the growing season.