Avoiding nitrogen deficiencies, particularly late in the growing season, is the primary reason for using cotton petiole analysis. But, there are several other instances where petiole monitoring can be helpful, says Steve Hodges, North Carolina State University soil scientist.
“Petiole monitoring can help us when we're making manure applications, when we're heavily irrigated and where we have very high management conditions and can supplement nitrogen in-season,” says Hodges. “It also can help us where we have unusual fruiting patterns. Petiole monitoring can tell us a lot about what's going on in the plant.”
Petiole monitoring also can be beneficial when farmers face unexpected leaching conditions, he adds. “It can let us know what's going on in the field before we see yellow leaves and before we lose yield.
“It also can be beneficial when we've had low leaching conditions during the previous season and we have residual nitrogen in the soil. Petiole analysis can be a very effective tool in helping us to predict that situation. And, it can be helpful whenever we're planting into new fields — it can help us to determine if our nitrogen rates are on target,” says Hodges.
Studies have shown, says the agronomist, that good cotton yields can be achieved in fields with no new nitrogen applications. “We don't expect this to happen every year, but it's possible in certain cases. In one of our tests, with cotton planted in organic soil, we made 1,216 pounds of lint per acre with no nitrogen added. In many cases, we can pick up residual nitrogen with petiole monitoring.”
Knowing how the cotton plant takes up nutrients is important in understanding petiole monitoring, says Hodges.
“Cotton is a deep-rooted plant. It's difficult to know whenever we do have leaching conditions if the nitrogen is truly out of the root system. It may be temporarily out of the root system, and the roots might reach down later and pick up the nitrogen.
“The cotton plant also is very sensitive to low-dissolved oxygen levels. Whenever we have leaching rains, and the rain stays in the field for several days, it doesn't take very long for those dissolved oxygen levels to be reduced dramatically, causing root slumping.”
Petiole monitoring, notes Hodges, recognizes that cotton internally competes for all of the building blocks, including water, nutrients and photosynthates.
“Vegetative growth, root growth and reproductive growth all are occurring at the same time. In breeding cotton varieties, especially the newer ones, we're compressing the fruit-set period. We're seeing higher yields, but it's coming at the expense of a higher harvest index. We have less vegetative material and more reproductive material.
“The developing fruit starts at the bottom of the plant, and those fruits have first call for water, nutrients and photosynthates. The top of the plant may be starved in conditions where we have very active fruit set occurring at the bottom of the plant. That competition becomes most fierce at about first bloom.
“At that time, we want to start seeing how nitrates are being partitioned in the plant. Taking the upper most mature leaf and petiole from that plant would give us a very good indication of how nitrogen is being shifted around in the plant system.”
Leaf analysis, he says, is the traditional method of monitoring a cotton plant, and it reflects what has happened to that leaf as it has expanded and grown.
“Leaf analysis tells us what has occurred in the previous 14 to 30 days. It's a very good indicator of what was available to the plant during that prior period. Unfortunately, the leaf analysis is calibrated only prior to the bloom period or at early bloom. Beyond the early bloom period, we don't have good numbers for interpreting the leaf analysis.
“If the analysis is low, we know we definitely have a problem. If we have a deficiency in the leaf, it's pretty certain that we have a plant deficiency and that it has been there for 14 to 30 days before we ever picked up those low nitrogen levels.”
On the other hand, he adds, if there's an adequate nitrogen level in the leaf tissue, that's no guarantee of sufficient levels for new positions on the plant. “We still could have leaching conditions or unusual demand. Leaf analysis is not a good indicator of current uptake.”
The root system of the cotton plant, explains Hodges, takes up nitrogen and moves it into the stem. From the stem, nitrogen moves into the petiole and from the petiole into the leaf.
Whatever is in the petiole is a function of what is moving through the root system, up through the stem and into the petiole at that particular time, he says.
“When the petiole nitrate level is low, and there's adequate moisture, that's a strong indication that nitrogen availability is low. A response to a nutrient addition is very likely under those conditions. This response can change from week to week. The magnitude of those changes is another key indicator.
“We've looked at various tests and found that as petiole nitrate levels increase, there's an increase in relative yields up to a certain point. Then, those yields begin to decline and we see things such as excessive vegetative growth and boll rot. Yields do decline whenever we use excessive rates of nitrogen. That's especially important to remember now that nitrogen is so expensive.”
North Carolina State University has developed a sufficiency curve to help growers do a better job of interpreting petiole analysis results, says Hodges.
“We have problems with petiole interpretations because petiole is a direct indicator of what the roots are seeing. If the roots don't see a problem, neither will the petiole. Some of the typical problems we see are when leaf numbers are adequate and petiole levels are low. The leaf shows what happened in the previous two to three weeks while the petiole shows what is happening today.
Dry weather impact
“In some cases, the leaf will be low and the petiole number will be adequate. This can occur where we have dry weather during the formation of the leaves followed by rainfall. The root systems begin to regrow, and petioles begin to see that nitrogen. The leaf, however, still isn't picking it up.
“We've also seen situations where leaf and petiole levels are low, and other nutrients are low in the leaf analysis. This can be the case if we have extended water-logging conditions. The root system is slumping, and the leaf or petiole aren't getting the needed nutrients.”
It's important, says Hodges, that petiole samples are taken at the proper times.
“During the first or second week of bloom, the interpretation is totally different than for the third week of bloom or pre-bloom. As a general rule, I don't recommend petiole nitrate testing for every field and every situation. But, I would look at representative fields for your soils and your rotations.
“Look at the upper-most mature leaf, and sample a week or two before first bloom — certainly no later than the week prior to first bloom. After that, weekly samples are best. We can see weekly changes and do a better job of predicting deficiencies.
“If you're going to look at a limited number of samples, look at the week prior to first bloom plus one, and then bloom plus two. Then, go back as needed if those numbers are in the adequate zone but trending downward. Generally, I don't recommend sampling much beyond the fifth week of bloom. There's very little we can do in terms of corrective treatments so late into the season.”