The Importance of Timely Side-Dress Fertilizer Application and Proper Petiole Sampling (Collins, Edmisten, Crozier, & Hicks)
Timeliness of side-dress applications: As mentioned in other recent articles, some cotton has reached the squaring stage while other fields will soon be at this stage if moisture and heat unit accumulation continues at the current pace. First square marks the time in which uptake of, and demand for, nutrients such as nitrogen (N) and potash (K) will begin to increase as time elapses.
The purpose of this particular article with regard to soil fertility is to emphasize timeliness. Cotton fertilization is thoroughly discussed in the Fertilization chapter in the 2016 Cotton Information Book. As is discussed in this chapter, cotton utilizes very little N and K from planting until first square, and peak demand for nutrients like N and K occurs during the bloom period. However, most fertilizer sources require rainfall and variable amounts of time in order to be made available in soil solution for plant uptake. Therefore, side-dress applications need to be made at some point in the squaring stage, but should definitely be plant available by first bloom, which generally occurs at 55 to 60 days after planting in most years (could easily be later in 2016). This window generally give growers a 2 to 3 week window in which to make these applications. The “optimal” time of application is difficult to define due to the unpredictability of rainfall, however the sensitivity to application timing could be loosely predicted depending on several factors. The two primary factors that come to mind are 1.) the amount of N applied between pre-plant and first square, including starter sources, and 2.) the amount of rainfall that has occurred during this same time frame. Therefore, a particular field may be more sensitive to side-dress application timeliness if little N had been applied between pre-plant and first square and/or if substantial leaching rains occurred during this time frame, especially on deep sandy soils. Neglecting to make timely applications in these situations could run the risk of the crop experiencing some degree of deficiency as it enters the bloom period, which could lead to yield loss or suboptimal impact from fertilizer when it was finally applied (less “bang for the buck”). The reason that we want to emphasize this point is that there were several fields during 2015 in which side-dress applications were delayed beyond the ideal time frame, therefore deficiencies occurred when those nutrients were most needed which likely resulted in lower yields than if applications were made in a more timely manner.
Proper petiole sampling: Petiole and tissue sampling can be an effective tool for monitoring nutrient status throughout the season, and if conducted properly and in a timely manner, could allow for corrective action to be taken. There are several publications and online resources available for producers from NCDA&CS that provide information regarding sampling techniques, when to submit petiole versus tissue samples or both, fees, storage and handling, the sample submission process, how to interpret results, etc. A few are listed in the links below:
The primary purpose of this particular newsletter article is 1.) to illustrate the importance of knowing true sufficiency/deficiency ranges for various nutrients in the petiole and tissue as it applies to NC cotton, 2.) to help growers understand how environmental conditions can influence nutrient status, so that results aren’t misleading, and 3.) to illustrate proper sampling techniques.
For point #1, it is very important to be aware of accurate sufficiency/deficiency ranges for various nutrients in petiole or leaf samples for NC cotton. Failure to do so, could lead to unnecessary and costly applications of remedy products that may or may not be needed. Additionally, growers should know when to submit petiole versus leaf samples for various nutrients in order to obtain the most accurate information regarding crop status and potential needs. This information can be found in the links above.
For point #2, there are environmental influences on nutrient status in petioles/leaves, therefore collection of samples during conditions that should be avoided could result in costly and unnecessary applications of remedies. Such conditions are droughty or saturated soils (water-logged conditions) as well as cloudy, overcast days. All of these situations could cause artificial deficiencies in petioles and leaves. Many nutrients must move from the soil solution to various plant parts through root uptake and subsequent vascular flow and evapotranspiration. Naturally, in drought conditions, sufficient nutrients may be present in the soil, however, if soil moisture is poor, the uptake of these nutrients will also be poor. On the other hand, water-logged conditions cause root development and activity to be reduced due to the lack of oxygen, which could also lead to deficiencies in petioles and leaves. During cloudy, overcast days, nutrient uptake is slower and petiole nitrate nitrogen concentrations are less accurate. Therefore, producers should avoid collecting tissue samples during these conditions. In situations where collecting samples during suboptimal conditions is unavoidable, growers should indicate field conditions on the submission form so that those factors are taken into consideration when recommendations are given.
For point #3, Sampling techniques can have a significant impact on tissue analysis results, therefore care should be taken when collecting samples so that growers obtain the most accurate results. Such techniques are illustrated below.
Step 1. For cotton, it is important to collect and submit only the uppermost fully expanded mainstem leaf on the plant. This leaf is the youngest photosynthetically active mature leaf and is the best indicator of current nutrient status. Younger leaves (nearer the terminal) are naturally smaller and aren’t fully developed, whereas older leaves (further down the canopy) may be shaded or may even be beginning to deplete of nutrients. Additionally, this leaf is a mainstem leaf with a thick petiole that is directly attached to the main stalk (avoid subtending leaves which are found on fruiting positions of reproductive branches). Counting down from the first terminal leaf (size of a quarter), growers should collect the 4th or 5th leaf down the main stalk as illustrated below.
Step 2. Remove the leaf by detaching the base of the petiole from the main stalk.
Step 3. Immediately remove the leaf blade from the petiole. Make sure to remove the entire petiole by pinching the petiole at the very base of the leaf blade. Including partial petioles with leaf blades can lead to misleading results.
Failure to immediately remove the leaf blade from the petiole will cause misleading analysis. If the leaf blade is not removed rather quickly, the leaf will continue to transpire, which will pull nutrients from the petiole into the leaf, thereby resulting in artificially low nutrient status in the petiole. The photo below illustrates how the leaf will continue transpiration and subsequently wilt in just a short time if the leaf blade remains intact with the petiole.
Finally, growers can submit either Predictive (routine) or Diagnostic (troubleshooting) samples. For Predictive samples, growers submit one sample of 25-30 leaves (with detached petioles) beginning at match-head square through the third week of bloom. Diagnostic samples can be submitted any time during the growing season but producers should submit TWO samples: a “good” (healthy) sample and a “bad” (unhealthy) sample for comparison.