Ion sensitive potassium (K) electrode-based meters with flat sensors are now available in pocket-size form for nutrient monitoring in cotton and other crops. In cotton, petiole sap extracts are used to measure K concentration. This offers an immediate reading on K level in cotton, avoiding the delays associated with drying petioles and sending to a laboratory for analysis. In recent years, with the increased emphasis on K nutrition of cotton, many crop advisors are looking at these meters as a new management tool they can use.

To use these compact meters, however, calibration data will be needed to correlate sap readings to dried petiole samples. Preliminary results by Hodges (1993) indicated that cotton petiole sap concentrations for K were 10 times lower than were found in dried petiole samples. Also, results by Morse et al (1992) and Hodges (1993) indicated that the slope of the correlation between sap and dried sample readings may change as the cotton plant ages. Moisture conditions may also affect the ability of obtaining sap samples as noted by Hodges (1993).

In Alabama in 1992 two replicated field studies and five farmer fields were used to determine the usefulness of these hand-held meters under field conditions. The first replicated site was a dry-land field that contained areas that had received 0, 60 and 120 lb K₂O/A each year since 1985 in a complete randomized block design with 4 replication. The second replicated site was an irrigated field that had received 0, 30 and 60 lb K₂O/A each season since 1990 in a completely randomized block design with 5 replications. All 5 farmer fields were irrigated and on similar soil types.

At all sites 25-30 petioles were collected beginning at one week after bloom and every 10 to 14 thereafter until cutout. Petioles for sap readings were taken into an air conditioned room and cut into pieces and squeezed in a garlic press. One to 2 drops of squeezed sap were placed on the sensor of the compact meter and the reading recorded. Two readings were made for each sample. The compact ion meter was calibrated before each run and standards were used after each 10 samples. The remaining cotton petioles were dried and sent for lab analysis.

Weather conditions were unusual in 1992 in that rainfall was adequate through-out the growing season. Cotton plants were never in moisture stress during any part of the sampling period.

Generally K levels measured in the sap and dried petioles increased with increasing fertilizer rates of K in both replicated field studies. Sap K measurement, however, were much lower than dried samples by 12 to 15 times. Average K measurement in sap and dried petioles showed similar trends during each sampling period and generally had lower K values as the season progressed. However, correlation of sap reading K to dried petiole K was poor (R²=0.29 or lower) during all sampling periods in both tests. Slopes of the correlations also changed greatly between sampling periods. Correlations were best during the first sampling period.

In the 5 farmer field studies average K values in dried petioles and sap did not decline as sharply during the season as seen in the replicated trials. Again, trends for average sap K and average dried petiole K were similar. However correlations for sap K and dried petiole K were very poor (R²=0.08 or less) in all but the first sampling period (R²=0.53). Sap petiole K values were again 12 to 15 times less than K values found in dried petioles and the slope of correlations between sap K and dried petiole K changed between samplings.

Given the good soil moisture conditions in 1992 we didn't expect problems in obtaining sap extract, however, sap extract was progressively more difficult to obtain as the cotton plant matured. This probably accounts for the varied slopes of the correlations between sap and dried petiole by sampling dates.

From this data it appears that calibration data for sap readings on these portable K meters may be difficult if not impossible to obtain. Variability of sap readings from field to field and by sampling period will be difficult to over come. The best use of sap readings appear to be during early season while the petioles are more succulent. Even then a large number of samples should be averaged to compare fields of similar maturity and soil moisture.

Literature Cited

Hodges, Steven C. and Shelby Baker. 1993. Correlation of plant sap extracts of nitrate-N and K with dried petiole extracts. Proceeding 1993 Beltwide Cotton Conferences. 1335-1337.

Morse, S.G., J.P. O'Connor, M. Zick. 1992. The efficacy of diagnostic tools as tissue testing, selective ion meters, and plant mapping to determine the need and timing of foliar K-Power applications. Proceedings 1992 Beltwide Cotton Conferences. p. 1158-1161.