Plant analysis is a nutrient management tool
By David G. Hallauer
Meadowlark Extension District Agent
It’s not uncommon for us to see areas of a field that are pale or discolored, or sometimes turning all kinds of colors that we can attribute to nutrient deficiency. What’s not always easy to tell is why, and that’s where plant analysis can be an excellent ‘quality control,’ particularly for managing secondary and micronutrients as well as your efficiency of application.
If you are doing a simple monitoring process, you’ll usually test at the beginning of reproductive growth (sampling under stress conditions is not recommended). For corn, collect 15 to 20 ear leaves at random at silk emergence and before the silks turn brown. For soybeans, collect the top, fully developed trifoliate when the first pods are 3/4 to one inch long. Collect 25 to 30 sets of leaflets at random, removing the petiole, or stem connecting the leaflets to the stem.
Diagnostic sampling can be done at just about any time. Simply collect samples from both good/normal areas of the field, and problem spots. It’s also good to collect soil samples from the same good and bad areas since physical problems such as soil compaction often limits the uptake of nutrients otherwise present in adequate amounts. For plants less than 12” tall, collect the entire plant. For larger plants, collect the top fully developed leaves.
For shipping and handling instructions, be sure to plan ahead with your lab.
Tomato leaf-spot diseases
Now is a good time to start scouting tomatoes for two common leaf-spot diseases: Septoria leaf spot and early blight are both characterized by brown spots on the leaves.
Septoria usually shows up first, producing small dark spots. Early blight spots are much larger and often have a distorted “target” pattern of concentric circles. Heavily infected leaves eventually turn yellow and drop. They tend to start at the bottom of the plant and work up.
You can reduce the incidence by mulching, caging, or staking to keeps plants off the ground. Better air circulation allows foliage to dry quicker than in plants allowed to sprawl. Mulching also helps prevent water from splashing and carrying disease spores to the plant. You’ll tend to see it worse in areas where you’ve planted tomatoes or related crops (peppers, potatoes, eggplant) in the same area for several years.
If you end up with a problem, fungicides are often helpful. Be sure to cover both upper and lower leaf surfaces, and reapply fungicide if rainfall removes it. Plan to apply when the tomato fruit is about the size of a walnut or when the first sign of disease is seen (it’s almost impossible to stop on heavily infected plants). Chlorothalonil is a good choice for fruiting plants because it has a 0-day waiting period, meaning that fruit can be harvested once the spray is dry. Mancozeb is another good product. Be sure to follow all label directions.