Some micronutrient deficiencies, such as low levels of sulfur, zinc and boron, can be corrected with soil applications. But iron, manganese and molybdenum deficiencies require foliar feeding.
Plant tissue testing is part of a three-step diagnosis
It’s a fairly simple process to correct deficiencies of major nutrients—nitrogen (N), phosphorus (P) and potassium (K). Deficiencies of micronutrients, such as iron and manganese, are trickier. You can solve the mystery of micronutrient problems, though, and bump your yield, by following sound procedures.
First, determine the problem. It’s a three-step process, which begins with identifying visual symptoms.
"Most micronutrients do not move in the plant," says Farm Journal Field Agronomist Ken Ferrie. "So look for deficiencies in the newest growth, at the top of the plant. This is different from major nutrients. There, you find deficiencies at the base of the plant because the plant removes nutrients from the oldest part and sends them to the newer part."
The tricky aspect of visual diagnosis is that one deficiency can be mistaken for another. Carry a good scouting manual to the field, Ferrie suggests.
If you think you have a deficiency, have your soil tested. "Soil tests for micronutrients are not as precise as tests for macronutrients, but they get us in the ballpark," Ferrie says.
Pull your soil samples based on soil type. "Don’t mix sand and silt loam or clay soil," Ferrie says. "The levels of major nutrients and calcium affect the availability of micronutrients. Some micronutrients, such as boron and molybdenum, are more prone to leaching out of lighter soils."
Make sure you understand your lab’s testing procedure and what constitutes high, medium and low values. "There can be significant value differences from one lab to another, depending on the test each one uses," Ferrie says.
The final step is to collect plant tissue and have it analyzed.
"Only plant analysis can identify the nutrient status of a plant or crop," says agronomist Bill Urbanowicz of Spectrum Analytic, a lab in Washington Court House, Ohio. "While soil testing identifies the level of nutrients available to the crop, plant analysis tells you how well the plants used the soil nutrients and the applied nutrients. In other words, plant analysis lets the crop tell you what nutrients it needs."
Sampling protocol. Collecting plant tissue samples is a precise skill. "Before you head to the field, check the sampling protocol by phoning your lab or visiting their website," Ferrie says. "Most labs will want the uppermost mature trifoliate [the youngest leaf that is as large as it will get]. They will want to know what stage the plant was in, from R1 to R4 because that makes a difference in what values should be present."
"The most common mistake we see farmers make is sending the trifoliate from only one plant," Urbanowicz says. With soybeans, his lab wants 20 or 30 trifoliates, collected from just inside the abnormal area of the field.
"Another common mistake is sending an entire plant instead of only the most mature leaf," Urbanowicz continues. "If the leaves dry out, the old and new leaves get crumbled together."
Protect samples from contamination. "If dust from your fingers gets on the leaf, it can throw off a tissue test," Ferrie says. "If I see extremely high iron numbers on a tissue test, I immediately suspect the sample was contaminated by lying on the dashboard of a truck or in a break room."
- October 2011