Carolina Cotton Notes - June 30 2008

The N.C. Department of Agriculture & Consumer Services, in cooperation with N.C. Cooperative Extension, is recommending a new protocol for routine cotton tissue samples (Table 1). To monitor crop nutrient status, begin collecting samples the week before first bloom and continue weekly through the 3rd or 4th week of bloom. Only the prebloom sample should include both leaves and petioles.

To receive accurate recommendations, you must fill out the Plant Sample Information form properly. For each sample, indicate the current growth stage and week of the crop (Table 2), identify the plant part submitted, specify the desired tests and provide the correct fee. Two weeks after pinhead-square formation (E2) is approximately one week prior to first bloom (E4). The first week of bloom is when there are five open blooms per 25 row feet. Fruiting (F5) is assumed to begin the fifth week after the week of first bloom. Suggested sample times and designations are E4, B1, B2, B3 and B4.

Table 3 shows how the following hypothetical samples should be entered on the Plant Sample Information Form:

1st sample, collected one week before first bloom (E4), leaf blades and petioles;
2nd sample, collected during first week of bloom (B1), petioles only;
3rd sample, collected during second week of bloom (B2), petioles only;
4th sample, collected during third week of bloom (B3) , petioles only; and
5th sample, collected during fourth week of bloom (B4), petioles only.

Although it is not recommended as part of routine monitoring, if another sample is submitted the fifth week of bloom for diagnostic purposes, then F5 would be the proper designation (see Table 2).

For a prebloom leaf-and-petiole sample, the correct designation for PLANT PART is M (most recent mature leaf); for the subsequent petiole-only samples, use P. For both types of samples, check NO3-N under EXTRA TESTS. The fee is $7 per sample ($5 for total elemental content and $2 for NO3-N).

Table 1. Recommended cotton tissue sampling protocol for 2008

Sample Type and Growth Stage	Appropriate Plant Part	Cost
Routine: wk before 1st bloom (E4)	petiole* + leaf blade**	$7 per sample
Routine: 1st wk of bloom (B1)	petiole only***	$7 per sample
Routine: 2nd wk of bloom (B2)	petiole only	$7 per sample
Routine: 3rd wk of bloom (B3)	petiole only	$7 per sample
Routine: 4th wk of bloom (B4)	petiole only	$7 per sample
Problem: any time §		$7 per sample

* measures only NO3-N in petiole

** measures total elemental content (N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B) of leaf blade

*** measures total elemental content (see above) as well as NO3-N in petiole

§ If abnormal color or growth is observed, submit both blades and petioles for analysis. Collect one sample from the healthy area and another from the problem area. Also, submit corresponding soil samples to surface and subsurface depths. Collect surface samples to a depth of 8 inches for conventionally tilled soils and 4 inches for no-till or conservation-tilled fields. Collect subsurface samples to a depth of about 16 inches, or to the clay layer. Standard analyses will be provided unless otherwise requested.

Table 2. Consecutive growth stage and week designations for cotton tissue samples

S=Seedling, 4wks 1-4

E=Early Vegetative Growth 4 wks 1-4

B=Bloom, 4wks 1-4

F=Fruit, 4 wks 5-8

Table 3. Correct entries for completing each individual Plant Sample Information form

Sample Time	Crop	Growth Stage	Week	Plant Part	Position	NO₃-N
1st sample	Cotton	E	4	M	U	X
2nd sample	Cotton	B	1	P	U	X
3rd sample	Cotton	B	2	P	U	X
4th sample	Cotton	B	3	P	U	X
5th sample	Cotton	B	4	P	U	X

In North Carolina, sufficiency ranges for petiole P and K have not been established. NCDA&CS measures the concentrations, but it is up to the client to interpret the results. Table 4 provides some guidance. At early bloom, petiole P should be about 800 ppm (0.08%).

Table 4. Petiole K (%) sufficiency ranges used in other states (* estimated from a graph)

Growth Stage	Week	*Virginia	*Georgia	California	Arkansas
E	3
E	4	4.0	3.1-5.1
B	1	3.6	2.9-5.0	4.0-5.5	3.7
B	2	3.1	2.2-4.2		3.0
B	3	2.5	1.9-3.8		2.8
B	4	2.1	1.75-3.5		2.2
F	1	2.2	1.6-3.5	3.0-4.0	2.0
F	2	1.6	1.6-3.5		1.0
F	3	1.0	1.6-3.5	1.5-2.5

Accurate recommendations depend on good sampling technique, including 1) always collecting samples at the same time of day; 2) collecting the proper plant part (the most recent mature leaf which is usually the 4th leaf from the growing point on the main stem only); 3) separating the entire petiole from leaf blade before leaving the field but preferably immediately after collecting; and 4) providing all details—growth stage and week, planting date, fertilization history, environmental conditions, appearance, problems, disease/insect pressure—on the Plant Sample Information form.

Under good environmental conditions, daily fluctuations occur in petiole NO3-N and K concentrations (Fig. 1). Typically, there is a decrease in petiole nutrient concentrations from late morning to mid-afternoon. This is good and indicates assimilation of nutrients into sugars and proteins by photosynthesis. Samples collected in the morning had adequate petiole nitrate concentrations whereas the 1300 (1 p.m.) samples were below the sufficiency range for both growth stages. If misinterpreted, these data could lead to unnecessary fertilizer applications. Because of this, it is recommended that tissue samples be collected in the morning.

Stresses—such as drought, low light and high humidity—affect transpiration and photosynthesis and, therefore, nutrient concentrations in the petiole. During the 2007 drought (Fig. 2), petiole nitrate appears to “flatline” and barely reached the sufficient concentration in field 1 only. Water stress limited transpiration and nutrient flow into the petiole, not soil nitrogen. Such stresses make petiole-based recommendations unreliable. Trained field personnel must consider these factors when interpreting petiole data and deciding if fertilization is appropriate. The NCDA&CS laboratory personnel are not in a position to make these judgment calls.