Using plant analysis as nutrient management tool
Plant analysis is an excellent “quality control” tool for growers interested in high yield crop production. It can be especially valuable for managing secondary and micronutrients, many of which don’t have high quality, reliable soil tests available, and also for providing insight into how efficiently applied nutrients are being used by the plants.
There are two basic ways plant analysis can be used by Kansas farmers: (1) monitoring nutrient levels at the end of vegetative growth as a quality control tool, and (2) collecting comparison samples for diagnostic purposes at any time. Monitoring is generally done at the end of vegetative growth and beginning of reproductive growth or grain fill, since most crops have accumulated the majority of the nutrients they will use at this time. Once grain fill starts, even though the plant may continue to take up nutrients, there will be a net flow of nutrients from the leaves and stem to the developing grain, and the nutrient content of the vegetation will steadily decline. With diagnostics, the absolute level of a given nutrient in the plant is less important, as we are looking for significant differences in nutrient content between good and bad samples from the same field.
Plant analysis for nutrient monitoring
For general monitoring or quality control purposes, plant leaves should be collected as the plant enters reproductive growth. Sampling under severe stress conditions for monitoring purposes can give misleading results, and is not recommended.
In the case of corn, 15-20 ear leaves, or first leaf below and opposite the ear should be collected at random from the field at silk emergence, before pollination, and before the silks turning brown.
In sorghum, the first or second leaf below the flag leaf at heading should be collected. Again, 15-20 individual leaves should be collected from the field at random.
In soybeans, the top, fully develop trifoliate leaflets should be collected when the first pods are ¾ to one inch long. The top fully developed trifoliate leaflets are normally the third set of leaves below the terminal bud on the main stem of the plant. They should be a dark green, and will likely be positioned at the top of the canopy, while developing/growing leaves will be a lighter green color and generally be below the fully developed leaves in the canopy. Collect 30-40 sets of leaflets at random, removing the petiole, or stem connecting the leaflets to the stem.
In wheat, the flag leaf is normally collected at heading. Since the flag leaves are small, 40-50 individual leaves will be needed to have enough dry plant material to have adequate material for analysis. Again, collect the leaves at random from the field or area which is being monitored.