The Effects of Fertilization and Severe Pruning on Canopy Size and Potassium Deficiency Severity in Sabal Palmetto

Field plantings of mature Sabal palmetto palms established in 2006 received no fertilizer or 4.9 g•m^-2 of N every 3 months from either 16N–1.7P–6.7K or 8N–0.9P–10K–4Mg fertilizer. Half of the palms in each fertilizer treatment were pruned annually by removing all but 3 of the youngest leaves (severe pruning) while the other half had only completely dead leaves removed.  After 3 years, there were no significant differences due to fertilizer treatment in the number of total leaves, the number of potassium-deficiency symptom-free leaves (green leaves), the percentage of leaves that were symptom free (percent green), or the potassium-deficiency symptom severity score (K score).  Severely pruned palms had significantly fewer total leaves, but had a higher percentage of green leaves and K scores than those having only dead leaves removed. These data suggest that severe pruning reduces the number of leaves in the canopy below that number that can be supported by existing K reserves in the palm. Exceeding this number of leaves results in visible K deficiency symptoms due to dilution of the palm’s limited K reserves. Pruning was discontinued in 2009 and thereafter half of all palms received 9.8 g•m^-2 of N every 3 months from 8N–0.9P–10K–4Mg for 3 years.  The other half received no fertilizer during that period. Fertilized palms had about 50% more total and green leaves than unfertilized palms, but percent green and K scores did not differ between fertilizer treatments. These data support our hypothesis that the severity of visual K deficiency symptoms will improve with fertilization until the palm attains its maximum genetically-determined canopy size.