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."
Use a large, clean paper bag or a clean plastic bucket to collect the sample, Urbanowicz advises. Remove dust or residue from the leaf surface with a clean, dry, soft bristle brush. Do not wash the sample.
"Samples should be dry to the touch and strongly wilted before you ship them," Urbanowicz says. "Wilting is not as critical if you ship overnight."
Understand the results. If your lab doesn’t provide an analysis of your results, ask your crop adviser to translate. "Numbers alone can be deceiving," Urbanowicz says.
"For example, in a dry year the soil might test high for a nutrient, but uptake might be poor. Or the uptake may be reduced because of an excess of another nutrient. Instead of applying more of the first nutrient, you might get the same result by applying less of the second one," he adds.
If visual symptoms, soil test results and plant tissue test results all point in the same direction, you can be confident you have a problem. As with the identification process, fixing the problem isn’t quite as simple as with major nutrients, Ferrie cautions.
"Among the micronutrients, sulfur, zinc and boron deficiencies are easy to correct because you can apply fertilizer to the soil, as you do with N, P and K," Ferrie says. "Correcting iron, manganese and molybdenum deficiencies requires foliar feeding."
If you’re new to foliar feeding, remember that iron, manganese and molybdenum do not move within the plant. "So you need to cover the entire leaf, as if you were painting a wall," Ferrie says. "New leaves that emerge over the next few weeks might be deficient, so you might have to make multiple applications during the season."
Bang for your buck. When choosing among micronutrient fertilizer products, consider water solubility. Sulfate products, such as manganese sulfate, are almost completely water-soluble and immediately available to plants, explains Spectrum Analytic senior agronomist Scott Anderson.
However, oxide forms, such as zinc oxide, are essentially unavailable to the growing crop.
Usually, oxides cost less, Anderson explains. Sulfates are the most expensive, and oxysulfates, a combination of the two, are in between.
Because of solubility differences, the cheapest product might not be your best buy. Researchers at Colorado State University and Alberta Agricultural Research Institute found it could take up to four times as much of a less-soluble fertilizer to equal the response of a highly soluble product.
If you haven’t applied micronutrient fertilizers before, remember you often will use micro-amounts, compared with N, P and K. "If you apply 50 lb. per acre instead of 5 lb., it can create serious problems," Ferrie says.
Understanding a micronutrient deficiency is the first step toward healthier plants. The three-pronged approach will help identify the problem. Then choose the best material to fix it.
Fertilizer Might Not Be the Long-Term Answer
Managing other nutrients or pH might be the best long-term solution to micronutrient deficiency problems.
"Most micronutrient deficiencies are caused by excessive amounts of something else," says Farm Journal Field Agronomist Ken Ferrie. "For example, iron deficiency is triggered by excessive calcium—it shows up when soil pH is 7.8 to 7.9 or higher. There might be sufficient iron in the soil, but it is ‘fixed’ in a non-soluble form, so applying iron to the soil won’t help the plants. Instead, apply iron by foliar feeding. Then try to correct the pH problem, if you can."
Applying large amounts of lime without incorporating can create high pH levels at the soil surface, which can lead to manganese and iron deficiencies, Ferrie notes. Overapplying calcium carbonate leads to deficiencies of manganese, zinc, iron and, sometimes, sulfur.
At the opposite end of the pH scale, if soil becomes acid, iron and manganese can become toxic. "Keeping soil pH in the sweet spot, 6.3 to 6.5, can solve or prevent a lot of problems," Ferrie says.
The price of fertilizer will prevent you from overapplying potassium. But just in case you ever do build up excessive levels, remember that too much potassium can cause boron deficiency.