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Plant Health: Heed Nutrient Levels at Critical Growth Stages


Plant Health: Heed Nutrient Levels at Critical Growth Stages

Nutrient testing equals quality control, according to Dr. Dave Mengel, professor of Soil Fertility and Nutrient Management at Kansas State University.

"My daughter works in the food industry, in Quality Control," Dr. Mengel says. "I think we need to take some lessons from them — regular sampling. Both soil and plant analyses are really good tools to understand what is happening out in the field, and to help us find what the agronomists used to call 'hidden hunger.'"

By understanding the stages of plant growth, producers can spot when things are headed in the wrong direction, says Dr. Ross Bender, senior agronomist for Eastern North America at The Mosaic Company.

The first thing to consider is the soil, the growth medium itself, as an extension of the plant. Immediately after harvest, preparations begin for the next crop, so conduct a soil test to accurately plan for next season, experts say. But then, in-season plant testing, conducted at just the right growth stage and using the right plant part, is a must "to make sure your plants don't run out of gas, and to determine quality control and identify potentially limiting nutrients for future crops," Mengel says.

New research reveals telling nutrient information, according to Bender.

"What we have learned is that a very rapid rate of nutrient removal occurs for phosphorus. About 80 percent of the phosphate in the plant is in the grain at the end of the year," Bender reports. "And that's removed from the field. Gone for good. That's the same for both corn and soybean. For potassium the number is less — between 35 and 45 percent. But remember, at record high yields, you remove all macronutrients quickly."

So when do you test to make sure phosphorus (P) or potassium (K), or other nutrients, will not limit corn?

To classify corn growth using the V stages, one looks for leaves with a collar on the stalk. For each leaf with a collar, you count as one V stage. (E.g., six collared leaves mean the V6 growth stage.)

"Tissue tasseling for corn is growth stage-dependent," notes Bender. "Anytime below the V6 to V8 growth stage requires the sampling of the whole plant. When the tassel emerges from the plant, that is called VT — or vegetative tasseling. Between V6 and VT, you sample the uppermost collared leaf."

At V5 to V6, about 30–35 days after emergence in many environments, the plant develops immature ear shoots, and the maximum number of kernels per ear is determined, including both number of rows and kernels per row. This then sets the yield trajectory in that field, and while lots of things can reduce the final yield, the yield potential can never be any higher.

"Creating conditions that will maximize ear size is critical," Mengel says. "You want to avoid nutrient stresses in this period so you can set the greatest potential ear size and get the best yield. Stresses at this time put a cap on your yield potential from the very beginning."

The planting-time fertility program, in most cases based on soil testing, is key to carrying the plant through ear size determination. Mengel observes that potential ear size determination happens at an interesting time, right when the plant is shifting gears, from the seminal seed roots over to the crownal root system.

"The seminal root system lasts about 30 to 40 days, then fades out, and the crownal root system gets established," Mengel continues. "If you get stresses at this time, like hot, dry weather, that can slow down the transition process and cause some issues."

With the potential for high rainfall in the Corn Belt during the late vegetative growth period, V8 to tasseling, nitrogen (N) loss is a common problem. With the increase in high-clearance sprayers across the countryside, the ability to add additional N or make planned N applications at this time is becoming more common. Traditional plant analysis, normally using the top, fully developed leaf with a visible leaf collar, can be used to guide any subsequent N applications. Other valuable tools available as alternatives are chlorophyll meters or crop sensors.

Silking time is a key testing opportunity, according to Mengel, who instructs growers to take a tissue sample when the silks are just beginning to emerge, before they turn brown, and before pollination.

"You take either an ear leaf or the leaf directly below the primary ear on the corn plant, and analyze that for most of the essential elements," he adds. "There are well-established norms for levels you should see, which will help you sort out whether you have an issue developing. Low K, low N, low zinc or low sulfur, or whatever it may be," Mengel says.

One key point for growers to understand is that traditional ear leaf samples are true quality control samples. While it may be too late to make treatments for that crop, the real value is to serve as an early-warning system for hidden or developing nutrient issues. Another important thing to remember is that low levels in the leaf are not just caused by low nutrient levels in the soil. Dry weather, root-feeding insects, and drainage or compaction issues can all reduce availability of nutrients and nutrient uptake.

Then come the reproductive-growth stages. A farmer may take a tissue sample anytime during this period, right up to the time when a grower harvests the crop, Bender advises. Early on in the reproductive phase, testing can reveal a treatable deficiency. Later tests will help the producer tweak the nutrient levels in advance of next year's crop. When the plants begin shedding pollen, environmental stresses can cause major problems.

"From a nutrient standpoint, you are looking at a time when you have pretty much accomplished all the vegetative growth," Mengel says. "You have taken up a lot of nutrients, you have produced a lot of vegetation. The plant is there. Tissue testing will help you assess what's going on: Have you had situations where you have lost a lot of nitrogen, for instance? Are you on a very heavy soil, or a sandy soil, so you are getting leaching or denitrification? If your plants are running out of nutrients, it will limit the number of kernels that are actually set. We have high-clearance sprayers now, so we can go into the field at the 16- or 18-leaf stage, just before pollination, and give it a shot of nitrogen. This step will sure give you a nice return on your investment." But the corn plant's nutritional needs continue on into the reproductive phase.

"After pollination, the most important leaf on the corn plant is the one that is attached to the ear," Bender says.

"There are typically anywhere from 16 to 18 leaves on the plant, but the one that is the most critical for corn yield is the ear leaf. Like other nearby leaves, it uses assimilates from photosynthesis and feeds the developing ear. The plant nutrients originally put in these leaves can also be repartitioned into the nutrient-rich kernels. Anytime after the tassel emerges, these leaves become particularly important."

Soybean plants make their leaves into the larder, where they store the macro- and micronutrients, according to Mengel.

As with corn, soybeans have two growth phases: vegetative and reproductive. Over the long vegetative phase, which can last three to four months in the more southerly zones of the Soybean Belt, the plant constantly moves nutrients up into the leaves.

"Soybeans have nodes on the main stem," Bender explains. "At each node, there is a set of leaflets connected to that main stem by a structure called the petiole. Every time a new trifoliate [i.e., three leaflets] becomes fully developed, that constitutes a new vegetative growth stage. Assume you notice you have five sets of petiole-plus-leaflets — that's a plant at the V5 growth stage."

Though the fully grown plant will have anywhere from 15 to 18 nodes, flowering may begin before the completion of vegetative growth and as early as around V8. At that point, the plant has begun R1, or the beginning of reproduction.

So what triggers the transition from vegetative to reproductive growth? The answer is the length of the day, Bender says.

"Because the length of day is different depending on where you grow soybeans, the varieties and their maturities are quite regional-specific in nature," he explains. "A variety grown in Canada is not one I would want to grow in Texas, and vice-versa. If I grew a Texas variety in Canada, it would likely not mature before a killing frost. If I grew a Canadian variety in Texas, it might be mature in July — it grows really fast. Soybean varieties have been adapted for different growing conditions and lengths of daylight."

Bender and Mengel agree that this phased, or indeterminate, approach is seen more often in northern soybean varieties. Often, longer-maturity southern soybean varieties follow a straighter sequence from the vegetative to reproductive stages. Mengel notes, however, that when a soybean plant in any zone experiences environmental stress, it may abort flowers or pods, continue to grow vegetatively, and when conditions improve, may make another attempt to reproduce.

When the soybean plant moves into its reproductive phase, major changes happen quickly, according to Mengel.

"As soon as the soybean plant starts to fill that seed, you get a rapid movement of nutrients from the leaves and stems into the developing grain," he says. "It's the natural senescence that you see. The leaves are there to carry out photosynthesis, but also to serve as reservoirs of nutrients, which can be quickly translocated to the seed.

"The seed is very high in protein, so there's a high demand for nitrogen," Mengel explains. "It's also high in oil, which takes a lot of carbohydrates. So the plant sends triggers to the roots, to say, 'Hey, we need all this carbohydrate to fill this seed, we need all this nitrogen; we can't afford to send much carbohydrate down to the root system and to the nodules. So we are going to shut them off.' So what you will find is that once you really get seed fill started, and kicking into high gear, the nodule function ceases. The root system begins to senesce. The nodules stop functioning and begin to senesce. This is well before the plant begins to show signs of maturity."

Knowing about and tracking the morphological changes in the soybean plant gives the grower an indication of when nutrient sufficiency is critical to yield success.

"Take, for example, potassium," Mengel says. "There is a lot of potassium in a soybean seed, and there is a lot of potassium in a soybean leaf. When the pods are about an inch long, before the seeds have really started to develop much, we suggest that is the appropriate time to do routine plant analysis in soybeans.

"Take leaf samples to see what nutrient levels are," he continues. "The leaf potassium level should be around 1.7 to 1.9 percent, up to 2.2 percent — that's what's called the sufficiency range. Once it drops to 1.7, that's the breaking point, or critical value. Below there, you are deficient and you are going to reduce yield. It happens very quickly, within two weeks, that seed begins to develop and the drain of potassium becomes very, very real. So the key time to gauge the effectiveness of a nutrient management program in soybeans is when the pods are small; an inch or less in size, and before the seed begins to fill."