Effects of Nitrogen and Potassium on Leaf Burn of
G. F. Ryan, Puyallup, WA
The rhododendron cultivar 'Elizabeth' is noted for being subject to leaf burn under some growing conditions. This effect sometimes has been attributed to high rates of fertilizer application, and particularly to high potassium [K] rates.
Reported levels of K in rhododendron leaves vary according to species or cultivar analyzed. Johnson and Roberts  reported 2.2 to 3.9% K in leaves of healthy 'Cynthia' and 'Pink Pearl' plants. Tod  found .27 to .95% K in leaves of healthy plants of various species. In his later study of mineral deficiencies in rhododendrons , plants with less than about 0.50% K showed deficiency. In the absence of K deficiency symptoms, the leaf content of K ranged from 0.56 to 1.05 except for 2.08% in R. kyawii. Ticknor and Long  reported a range of 0.41 to 1.02% K in leaves of six cultivars. In 5 of the 6, the K content was lower [0.41 to 0.81 %] in plants rated good quality than in those rated poor quality [0.53 to 1.02%]. In 'Unique' the K content was slightly higher in good quality than in poor quality plants [0.71 vs. 62%]
This experiment was conducted to study effects of N and K fertilizer rates and sources of K on leaf burn in 'Elizabeth' in relationship to leaf content of N and K.
Methods and Materials
Plants of 'Elizabeth' started from cuttings in July 1972 were transplanted into gallon cans in July 1973 in a mix of Douglas fir and hemlock bark, sphagnum peat, and sand [6:1:1 ratio by volume]. Treble phosphate included in the mix at 1.12 lb per cu yd provided 0.22 lb of P per cu yd, at 6 lb per cu yd. Dolomite was added at 6 lb per cu yd.
Nitrogen was supplied by periodic application of ammonium sulphate in 100 ml water at 320, 640 or 1280 ppm N. Potassium was supplied as potassium chloride [KCl] or potassium sulphate [K2SO4] in water at 266, 532 or 1063 ppm K. N was applied twice a month and K once a month until mid October 1973. There were four plants per treatment.
The experiment was moved into a shaded plastic house for protection during the winter. N and K applications were begun again in February with longer intervals between applications the first two months. While still under lath shade, the plants were transplanted into 2 gallon cans in June 1974. Two weeks later the concentrations of N and K in the fertilizer solutions were increased at the medium and high rates, and the amount applied was increased to 200 ml. The plants were moved out into full sun again on July 25.
K was supplied only as K2SO4 all plants in 1975. All N and K rates were increased. The final concentrations were 640, 1420 and 4320 ppm N, and 532, 1196 and 3590 ppm K. Total amounts of N and K applied per plant each year are shown in Table 1.
Leaves were collected for N and K analysis in August 1974 and May 1975.
Table 1. Amounts of nitrogen and potassium applied per plant.
Total amount applied, grams per plant Fertilizer rate 1973 1974 Jul to Oct Feb to Oct Nitrogen Low 0.22 0.80 Medium 0.45 2.05 High 0.90 5.44 Potassium Low 11 0.53 Medium 0.21 1.44 High 0.42 3.99
Results and Discussion
Severe leaf burn occurred in 1974 on plants supplied with a high rate of N [leaf N was 1.9 to 2.0%] and a high rate of K as KCl, but not on plants that received high N and a high rate of K as K2SO4 [Table 2]. Leaf burn was moderately severe at medium N rates [leaf N was 1.6%] with KCl as the K source, but only slight with K2SO4. The amount of leaf burn was variable in the low N plants and did not appear to be related to rate or source of K.
No leaf burn was observed in 1975 when all plants received K in the sulphate form. Leaf analysis showed as high as 1.2% K in some treatments in 1975 [data not shown].
Table 2. Effects of K source and N and K rates on foliar analysis and leaf burn of 'Elizabeth' rhododendron, 1974 Rate of fertilizer application
Rating of leaf burn1, 2
N K KCl K2SO4 KCl K2SO4 Nitrogen Low Low 1 .5 abc 4.0 de 0.59 0.53 1.23 Low Medium 3.0 cde 2.3 bcd 0.67 0.82 1.10 Medium Low 2.0 abcd 0.5 ab 0.44 0.50 1.56 Medium Medium 1.0 ab 2.0 abcd 0.44 0.59 1.59 Medium High 3.8 de 1.5 abc 0.59 0.59 1.57 High Low 0.3 a 0.3 a 0.47 0.50 2.04 High Medium 1.3 abc 0.3 a 0.64 0.82 2.01 High High 4.8 e 0.3 a 0.73 0.82 1.90 Means, Medium and High N Low K 1.1 a 0.4 a Medium K 1.1 a 1.1 a High K 4.3b 0.9 a
1 Severity of leaf burn was rated on a 0 to 6 scale; plants rated 6 had some leaves showing more than 50% burn. 2 Means followed by the same letter do not differ significantly according to Duncan's multiple range test.
Within the range used in this experiment, high rates of N and K did not cause serious foliage burn except when K was supplied in the chloride form. The chloride ion is known to cause leaf burn in other plants, and its presence in some fertilizers may account for leaf burn in 'Elizabeth' and other sensitive rhododendrons.
Johnson, C. R. and A. N. Roberts, 1969, Deficiency symptoms of rhododendron studied., Ore. Ornamental and Nursery Digest 13: 1-2. [Also in Quart. Bull. Amer. Rhod. Soc.24: 110-112]
Ticknor, R. L. and J. L. Long, 1978, Mineral content of rhododendron., Quart. Bull. Amer. Rhod. Soc. 32:150-158.
Tod, Henry, 1959, Rhododendrons and lime., Rhododendron and Camellia Yearbook, 1959: 19-24.
Tod, Henry, 1961, Mineral deficiencies in rhododendron, Rhododendron and Camellia Yearbook, 1961: 38-41.