With corn, plant analysis can be especially valuable for producers by providing insight into how efficiently they are using applied nutrients, and for managing secondary and micronutrients which don't have high quality soil tests available, says Dave Mengel, Kansas State Research and Extension soil fertility specialist.
Plant analysis is also an excellent diagnostic tool to help understand some of the variation seen in the field, he adds.
To sample for general monitoring or quality control purposes, producers should collect 15 to 20 ear leaves, or the leaf below and opposite the ear, at random from the field at silk emergence, before pollination, Mengel explains. Sampling under stress conditions for monitoring purposes can give misleading results, and is not recommended.
When using plant analysis to diagnose field problems, producers should try to take comparison samples from both good areas of the field and problem spots, Mengel says. While taking diagnostic plant samples, producers also should collect soil samples from the same good and bad areas, he said.
Sampling for diagnostic purposes should be done earlier than sampling for routine monitoring, he said.
"When trying to diagnose a problem, don't wait for tasseling or silking to take the first set of samples. Instead, collect whole plants from 15 to 20 different places in your sampling area early in the season, as soon as a problem is identified. Prior to the sixth to seventh leaf, or roughly a foot high, whole plant samples should be cut off at ground level. Later in the season, but prior to tasseling, collect 15 to 20 top, fully developed leaves — those with leaf collars visible," Mengel says.
Whether for monitoring or diagnostic purposes, the collected leaves should be allowed to wilt overnight to remove excess moisture, placed in a paper bag or mailing envelope, and shipped to a lab for analysis.
"Do not place the leaves in a plastic bag or other tightly sealed container, as they will begin to rot and decompose during transport, and the sample won't be usable. Most of the soil testing labs working in the region provide plant analysis services, including the Kansas State University lab," he notes.
In Kansas, producers should have plant samples analyzed for nitrogen, phosphorus, potassium, sulfur, zinc, chloride and iron. These are the nutrients most likely to be deficient.
"Normally the best values are the bundles or packages of tests offered through many of the labs. They can be as simple as nitrogen, phosphorus and potassium, or can be all of the 14 mineral elements considered essential to plants," he says.
Most labs will compare plant nutrient concentrations to published sufficiency ranges. "A sufficiency range is simply the range of concentrations normally found in healthy, productive plants during surveys. It can be thought of as the range of values optimum for plant growth. The medical profession uses a similar range of normal values to evaluate blood work," Mengel explains.
The sufficiency ranges change with plant age (generally being higher in young plants), vary between plant parts, and can differ between hybrids, according to Mengel.
"So a value slightly below the sufficiency range does not always mean the plant is deficient in that nutrient, but it is just an indication that the nutrient is relatively low. Values on the low end of the range are common in extremely high yielding crops. However, if that nutrient is significantly below the sufficiency range, then one should ask some serious questions about the availability and supply of that nutrient," he says.
Keep in mind also that any plant stress (drought, heat, soil compaction, etc) can have a serious impact on nutrient uptake and plant tissue nutrient concentrations, he added. A low value in the plant doesn't always mean the nutrient is low in the soil and the plant will respond to fertilizer, rather that the nutrient may not be available to the plant, he said.
Levels above sufficiency also can indicate problems.
"High values might indicate over fertilization and luxury consumption of nutrients. Plants also will sometimes try to compensate for a shortage of one nutrient by loading up on another. This occurs at times with nutrients such as iron, zinc and manganese. Plants will load up on iron at times, in an attempt to compensate for low zinc," Mengel says.
Source: Kansas State University