By Hans Schmitz, Technical Assistance Specialist
Indiana Ag Nutrient Alliance
When corn and soybeans are firmly out of the ground, post-planting sprays are complete, and a little time opens up on the farm, crop scouting can begin in earnest. While keeping an eye out for yield-robbing insects and fungal diseases like tar spot are traditionally considered, keeping an eye out for nutrient deficiencies should also be on the table, especially when decisions were made before this season to limit or forego expensive spring nutrient applications. While leaf samples sent to the Purdue Plant and Pest Diagnostic Lab may be the best option for unknown issues, most soil testing laboratories today also will perform leaf sample analysis for macro and micronutrients.
If you are out walking corn and see consistent off-color or streaked leaves, take note of the growth stage of the crop. Generally, plants with less than 6-7 fully developed leaves require 15 or more entire plants to be shipped, with some labs suggesting up to 30 be taken. For older plants without ears, the uppermost fully formed leaf is suggested, while corn in reproductive stages suggest taking the leaf directly below and opposite the ear. In both cases, 15 leaves usually allows for full processing. This type of sample gives a good baseline for whole field or zone management knowledge of in-season nutrients. Problem areas, meanwhile, may not be consistent with this sampling protocol. Mobile nutrient deficiencies cause older leaves to suffer while the plant funnels what nutrients it has to younger developing leaves. Immobile nutrients will cause the irregularities to form on newer tissue. Wherein samples are being pulled for leaf analysis, grabbing a relatively healthy sample alongside a symptomatic one may give better data about the range of plant nutrients in the crop.
To pull samples in a soybean crop, one always takes the uppermost trifoliate, or set of three leaves. Because soybean leaves are smaller than corn, anywhere from 25 to greater than 50 trifoliates have to be taken. Local labs vary widely in recommendations, but each lab has a sampling guide with which to refer for proper sample collection and shipment. Labs also generally will accept a wide variety of plant species, serving specialty crop producers as well as traditional row crops. Testing fees don’t vary greatly from the type of fee one pays for a soil test. If you identify a nutrient deficiency, a rescue foliar treatment can be made in most situations. These foliar treatments also benefit from the drone infrastructure we have built in Indiana, as most liquid products can be applied to any growth stage without having to put wheels in fields and damage plants. Speaking of wheels in fields, those who spray from the side by side or put on a backpack for problem areas can still count as field scouts and report back issues or take samples.