We’ve been getting several calls over the last week from consultants and growers sharing similar observations regarding what appears to be symptoms of nutrient deficiency. Symptomology ranges but seems to reflect deficiencies in N and K primarily, and in some cases, S. Along those lines, most folks want to know if they should take corrective actions and apply fertilizer to alleviate the symptoms or prevent further progression of such.

First, it’s important to understand the likely causes of these nutrient deficiency symptoms. As everyone remembers, June was excessively hot and dry. Thankfully, July rains have been incredibly timely for most parts of NC, and more than sufficient as well. Growth resumed vigorously right as most of our crop entered the bloom period, and growth remained vigorous as we moved into August. Due to the excessively hot and dry June, most of the side-dressed or top-dressed fertilizer wasn’t really used until rains resumed and the crop began blooming. Another way of saying this is that fertilizer applied during the prebloom period was not lost, especially when compared to previous years when rains were plentiful during June. The assumption during early July was that nutrient depletion throughout the bloom period was not as likely as we have seen in previous years, or at least it would be significantly delayed. In most parts of the state, that has been accurate. Some areas however received excessive rains throughout parts of July, and more areas received excessive rains when Debby rolled through. The nutrient deficiency symptoms that became more pronounced over the past week are the cumulative result of both rainfall accumulated throughout July and early August and the heavy rains most folks experience during Debby. Therefore it’s not likely the result of Debby’s rainfall alone, even though symptoms appeared very recently, unless some fields or parts of fields were drowned or waterlogged. Some folks are reporting entire fields showing symptoms, while others are reporting spotty areas within fields. Symptomology ranges, however the illustrations below mirror most of them. In sandier soils, especially on high ground, deficiency symptoms are likely the result of leaching of nutrients below the root zone, whereas in bottom areas or low-lying areas where water accumulated, symptoms are the likely result of waterlogging and saturated soils. In the latter cases, plants experienced some degree of time when oxygen was not accessible by roots. Regardless of the cause, the end result is similar.

Secondly, it is important to remember the time of year relative to cotton growth. Although it varies from year to year, nutrient deficiency symtoms often appear at some point in August. It is more common when drought occurs in August, but it is nearly always more pronounced during this time of year, simply because of the accumulated boll load. Most fields reach a true cutout at some point during this time of year, and the developing boll load has very high demands for nutrients during this time. Regardless of whether the underlying contributor to these deficiency symptoms is leaching, waterlogging, or drought, the developing boll load will pull nutrients out of leaves to meet their demands, if such nutrients can not be accessed through traditional means (uptake through the root system). This year, our crop appears to be ahead of schedule in terms of development, likely due to higher-than-normal heat unit accumulation during June. Most of the crop is at or very near cutout right now, even in irrigated fields.

Should growers take corrective action to address these deficiency symptoms? In most cases, No. As mentioned above, this crop is slightly ahead of schedule, and most areas currently have a strong boll load. Research suggests that the probabililty of fertilizer applications actually translating into higher yields, rapidly diminishes after the 3rd week of bloom. Therefore, corrective action should be taken during the very early part of the bloom period in order to have a high probability of improving yields, assuming that a true deficiency exists during that time. Most cotton in NC at this point in time is well beyond this point, and any true deficiencies would have only occurred recently, with the exception of field that may have recieved enormous rains in early July. There may be a few cases of cotton planted in early June that MIGHT be right on the line of potentially improving yields, if a true deficiency exists. For earlier planted cotton, we are not at all surprised to see some leaves showing deficiency symptoms, due to the developing boll load.

It is also important to remember that WE WANT cotton to ripen during this time of year. For cotton planted on time (during May), with relatively timely and normal growth throughout the summer, we expect to see the heaviest boll load at about this time of year, and therefore some signs of nutrient depletion. We dont want to see these symtoms during July, but it is not surprising to see them at this point in time, and this should progress between now and mid/late September. Fertilizer applied now will likely keep leaves greener for longer than we want, and will make the crop more difficult to defoliate, and will likely make regrowth more of a challenge than necessary.