JARS v44n2 - Phosphorus and Nitrogen Nutrition of Rhododendrons

Phosphorus and Nitrogen Nutrition of Rhododendrons
George F. Ryan
Tacoma, Washington

Since the publication of papers on phosphorus (P) by Myhre (28) and Myhre and Mortensen (29), there have been misunderstandings about the role of phosphorus in rhododendron nutrition, especially in relation to flower bud formation. The papers reported that application of phosphorus increased flower bud formation in young plants of 'Cynthia'. The largest number of buds was produced when phosphorus was made available to the plant by working it into the soil before planting or by drenching the root ball with a "starter solution" of dilute phosphoric acid immediately after planting.
This report apparently prompted many rhododendron growers to apply large amounts of phosphorus to plants that were not producing the desired number of flower buds. They soon found that in most cases they could not increase flower bud formation in this way. Other research did not show an increase in flowering from phosphorus application to the soil (19, 40) or as a spray on the plants (22). 'Cynthia' plants grown in a soil similar to the one used by Myhre did show a slight response to preplant application of phosphorus (32), but the increase in flower buds was small compared to the results of Myhre and Mortensen. 'Humming Bird' plants either did not respond or had fewer flower buds when the amount of phosphorus in the container mix was increased (32).
In a series of field and container experiments from 1969 to 1977, application of phosphorus did not increase flower bud formation except in one case (33). In that one experiment, plants that received 0.25 pound of treble superphosphate per cubic yard of growing mix produced more flower buds than plants with no phosphorus supplied to the mix. Increasing the amount of phosphorus did not affect flower bud formation. There was no correlation between the percent of phosphorus in rhododendron leaves and flower bud formation, except in two cases, where foliar nitrogen was also correlated with the number of flower buds. Foliar phosphorus sometimes increases along with an increase in foliar nitrogen (20).
Criley attributed an increase in flower bud formation to application of phosphorus (10). His source of phosphorus was magnesium ammonium phosphate (Mag Amp), so the rate of nitrogen application increased as rate of phosphorus increased. There was no way to distinguish which caused the flowering response. French and Alsbury found no significant correlation between phosphorus and flower bud formation in 'Pink Bountiful' (16).
In retrospect, it now appears likely that the soil in Myhre's experiments was highly deficient in phosphorus and that application of phosphorus merely overcame this deficiency. The response attributed to a large amount of phosphorus applied prior to planting may have been due to providing enough phosphorus to overcome this deficiency early in the growth of the young plants, rather than to the presence of a large amount of phosphorus. No pre-plant treatments were made with small amounts. The response to annual surface applications was smaller than to pre-plant or starter solution treatment, probably because of a delay in correcting the deficiency until the surface applied phosphorus moved into the root zone.
Part of the increased flower bud formation in response to phosphorus reported by Myhre and Mortensen was due to increased growth (Table 1). This is another indication that plants that received nitrogen but not phosphorus were restricted by a deficiency of phosphorus. The response to phosphorus was not specifically on flower bud formation. It was simply a response to enough phosphorus for normal plant development.
Johnson and Roberts reported that leaves of 'Cynthia' and 'Pink Pearl' contained 0.11% phosphorus at the first visible symptoms of deficiency (21). In surveys of nurseries in 1977 and 1978, plants rated good contained at least twice this much phosphorus, while some of the plants rated as poor were only slightly above this deficiency level (41, 42) (Table 2). The level of phosphorus was 0.11 to O.16% in vigorous plants of 'Victor', compared with 0.05% in plants that were not growing because there was no phosphorus in the nutrient solution (18) (Table 2). Similar relationships to phosphorus were reported for azaleas (36).

Table 1. Effects of nitrogen and phosphorus treatments on flower bud formation and growth of 'Cynthia' rhododendron z
Treatment Flower buds
per plant y
Number of
shoots x
length x
lbs/acre 1959 1960 1961 cm
0 0 1a 11a 44a 45a 1318a
40-80 w 0 2b 12a 54b 48a 1386a
80-120 v 0 2b 15a 58c 48a 1405a
40-120 u 105 8c 26b 82d 61b 1735b
z Means followed by the same letter within columns are not significantly different according to Duncan's Multiple Range Test (5%).
y Myhre and Mortensen.
x Unpublished data from the same plants; totals for 1958-61.
w 40 in 1958, 1959, 1960; 80 in 1961.
v 80 in 1958, 1959, 1960; 120 in 1961.
u 40 in 1958; 80 in 1959, 1960;, 120 in 1961.

Excess Phosphorus: Can too much phosphorus be applied? Excessive phosphorus application may produce iron or zinc deficiency. Iron deficiency may result from tie-up of iron by excess phosphorus in the soil (35), or it may occur within the plant as a result of excessive absorption of phosphorus if an over-abundance of phosphorus is available in the soil (11). Iron deficiency was observed in rhododendrons where annual applications of superphosphate had been made in October "for bloom stimulation" (31), or where a phosphoric acid solution had been applied in the fall in an effort "to aid winter hardiness and to induce better bloom" (17). Leaves showing iron deficiency symptoms (chlorosis) often contain as much or more iron than green leaves. Very often the phosphorus content of the chlorotic leaves is greater; the ratio of phosphorus to iron apparently determines whether the leaf will appear chlorotic or healthy (11).
Ticknor reported that where treble superphosphate was applied before planting, flower bud formation was decreased in three out of five cultivars (40). Where iron chelate was applied with the phosphate, flower bud formation was stimulated in those same cultivars. It appears that applied phosphorus tied up iron in the soil or in the plant enough to interfere with flower bud formation. Where iron chelate was supplied with phosphorus, enough iron was available in the plant for normal metabolic activity leading to flower bud formation. In other words, in the first treatment, the effect of phosphorus may have been entirely indirect through a depressing effect on iron. A small amount of chlorosis appeared in plants of that treatment. In the second treatment, phosphorus probably had no effect, but the iron chelate applied with it had a positive effect on flower bud formation.

Relationship to Flower Bud Formation: In the first experiments reported by Myhre and Mortensen, there was an increase in flower bud formation when nitrogen (N) was the only fertilizer applied (29). In their other experiments, nitrogen was always applied with phosphorus, so there was no way of distinguishing effects of either nitrogen or phosphorus separately.
Other research showed an increase in flower bud formation with application of nitrogen (19). Smith reported maximum flower bud formation without foliage injury on 'Roseum Elegans' with l.60 to 2.06% nitrogen in the leaves (37) (Table 2.) Analysis of his published data shows a significant positive correlation between number of flower buds and foliar nitrogen within the range of 0.92 to 2.06% nitrogen. Foliar damage and/or growth reduction occurred above that range(2.14 to 2.42% N).
In four of five container and field experiments from 1969 to 1977, Ryan found the nitrogen levels in 'Mrs. G. W. Leak', 'A. Bedford' and 'Anna Rose Whitney' were closely correlated with flower bud formation (33). Foliar nitrogen levels for good flower bud formation (1.6 to 2.1 %) were the same as reported by Smith (Table 2). Fewer buds were formed with nitrogen levels of l.2 to 1.7%.
Ticknor and Long surveyed 15 nurseries and found plants rated good had foliar nitrogen levels of 1.6 to 2.1 % (42), the same as reported by Ryan and by Smith for good flower bud formation (33,37) (Table 2). They found about the same nitrogen content (1.6 to 1.9%) in plants rated good the following year (41) (Table 2). Similar optimum leaf levels of nitrogen were reported for azaleas 'Formosa' (12, 13), and 'Ambrosia' (2) (Table 2). French and Alsbury found a significant positive correlation between number of flower buds and foliar nitrogen level, and no significant correlation with any other nutrient (16).

Table 2. Foliar nitrogen and phosphorus in rhododendrons and azaleas
Source of data Basis for rating
Nitrogen (%) Phosphorus (%)
Ref. Plants Good plants Poor plants Good plants Poor plants
42 6 cultivars Qual. & fl. buds 1.6 - 2.1 1.3 - 1.8 0.21 - 0.29 0.14 - 0.23
41 4 cultivars Qual. & fl. buds 1.6 - 1.9 1.1 - 1.5 0.21 - 0.29 0.19 - 0.24
18 Victor Growth --- --- 0.11 - 0.16 0.05
37 Ros. Elegans Flower buds 1.6 - 2.1 0.9 - 1.7 0.15 - 0.21 0.18 - 0.23
33 3 cultivars Flower buds 1.6 - 1.9 1.2 - 1.7 0.12 - 0.20 0.11 - 0.19
12 Formosa Growth, quality 2.0 --- --- ---
13 Formosa Growth, quality 1.6 1.2 --- ---
2 Ambrosia Growth 2.2 --- .18 ---

Source of Nitrogen: Spencer, from his research with R. ponticum , stated that nitrogen is not available to rhododendrons in the nitrate form (38). He recommended fertilizing with dried blood or tankage to allow gradual availability of ammonium-nitrogen for plant absorption before it is converted to nitrate. Tod found that growth of R. macabeanum seedlings in sand culture was poor and severely chlorotic when nitrogen was supplied as sodium nitrate (43). Plants grew well when it was supplied as ammonium sulfate. R. ferrugineum plants showed the same response.
Azaleas grown with nitrate as the nitrogen source were chlorotic, and the chlorosis was corrected when ammonium-nitrogen was supplied (4, 7, 39). There was some improvement from using ammonium nitrate where half of the nitrogen was in each form (4, 39), but growth was much better with at least 3/4 of the nitrogen in the ammonium form (7).
Not all studies with rhododendrons have shown ammonium to be the best source of nitrogen. Plants of 'Cunningham's White' made more fall growth with calcium nitrate than with ammonium sulfate (3). Excellent growth of three azalea cultivars occurred at pH 4.6 with nitrate as the nitrogen source (24). Ammonium-nitrogen was recommended for use with a pH in the 4.5 to 5.5 range (25). Oertli reported azaleas grew equally well with either nitrogen form at pH 4 when iron was supplied in chelated form (30). Growth was slightly greater with ammonium-nitrogen at pH 6. Wallace and Mueller also found that azaleas grew well with nitrate when chelated iron was supplied (45). On the other hand, Maleike reported iron chlorosis from nitrate in four out of five azalea cultivars, despite a supply of chelated iron (26). There is evidence from his data that the form of nitrogen supplied may be less important if azaleas are grown in the shade or reduced light intensity.
Severe necrosis of 'Roseum Elegans' (R. catawbiense hybrid) leaves occurred after 5 or 6 weeks of daily application of ammonium-nitrogen or urea, and healthy growth with nitrate-nitrogen (15). Maleike observed necrosis in R. catawbiense album from ammonium-nitrogen, and healthy growth from nitrate (26). Iron was supplied in chelated form in both cases. Since only one rate of nutrient application was used in each experiment, there is no way of knowing whether either form of nitrogen was applied at the optimum rate under those experimental conditions. At what may have been an excessive rate of application for R. catawbiense , ammonium would cause injury. It is more readily taken up than nitrate by ericaceous plants, and an excess of ammonium in the plant can be harmful. As noted earlier, 'Roseum Elegans' showed severe foliage damage and/or reduced growth from fertilizer treatments (high rates of Osmocote) that resulted in foliar levels of nitrogen above 2.06 (37). Rhododendron catawbiense and some of its hybrids such as 'Roseum Elegans' may be less tolerant of a high level of ammonium-nitrogen than some other rhododendrons. Maleike pointed out that R. catawbiense is native in drier upland soils where nitrification may be higher than in the native soils of many of the evergreen azaleas, and it might therefore be less adapted to ammonium-nitrogen than the azaleas.
The research cited above did demonstrate that nitrate sometimes may be a satisfactory source of nitrogen for rhododendrons and azaleas if enough chelated iron is used to prevent the iron deficiency associated with nitrate in ericaceous species. Because ammonium-nitrogen is not leached immediately from soil, and because it is readily absorbed by rhododendrons and azaleas, excessive use of ammonium-nitrogen can cause injury. However, ammonium sulfate and urea have been recommended and used successfully by rhododendron and azalea growers for many years, and generally have been found more satisfactory than the nitrate form (5, 6, 8, 9, 23). Successful experimental use of nitrate-nitrogen for growth of rhododendrons and azaleas at a pH above 4 depended on supplying iron in chelated form.
Organic sources of nitrogen such as cottonseed meal are safe because they break down gradually. They slowly and continuously release ammonium-nitrogen that can be taken up by the plant without an excess to cause burning, or an excess in the soil to cause salt problems (14, 23). Because of the delay in release of nitrogen, timing of application has to be different with organic sources than with soluble inorganic sources such as ammonium sulfate.

Species Sensitivity to Nutrients
The idea that some rhododendrons may be more sensitive than others to ammonium-nitrogen was suggested above. Cox noted that members of the Neriiflora subsection and its hybrids and the Taliensia subsection are very sensitive to excess nitrogen (8, 9). They may suffer from leaf burn or even death from a dose suitable for other species. Miwa and Ozaki reported that R. quinquefolium was severely injured by high nitrogen rates that were not harmful to other native Japanese rhododendron species tested. This was regardless of nitrogen form or soil pH (27).
Van Veen commented that R. griersonianum hybrids cannot tolerate much fertilizer (44), and R. forrestii (Neriiflora subsection) also apparently is sensitive. 'Elizabeth' is a hybrid between these two species. Its response to high rates of applied nitrogen and potassium was investigated (34). With nitrogen applied at a medium or high rate (N content of leaves 1.6 to 2.0%), there was severe leaf burn only where potassium was supplied as potassium chloride, not where it was supplied in the sulfate form. 'Vulcan', another R. griersonianum hybrid, responded in the same way (Ryan, unpublished). These two cultivars are among the ones that "will not tolerate fertilizer" (1). More information is needed about this effect. It may be a response to chloride specifically, or to salt accumulation and sensitivity of these species and their hybrids to high soluble salts. This and other aspects of rhododendron nutrition are subjects for future articles.

Summary and Conclusions
Enough phosphorus is needed by rhododendrons and azaleas for normal, healthy growth. Supplying additional phosphorus beyond this amount apparently does not help in the formation of flower buds. Excess application of phosphorus may result in iron deficiency chlorosis as a result of iron tie-up in the soil or in the plant.
Nitrogen is the nutrient used in largest quantity by plants. A level of nitrogen for producing good healthy growth appears to be optimum for flower bud formation. Nitrogen can be supplied in organic or inorganic form. Among inorganic sources, the ones that traditionally have been considered best for rhododendrons are ones that supply nitrogen in the ammonium form, such as ammonium sulfate and urea. Because the ammonium form is readily absorbed by rhododendrons it must be used with care to avoid a toxic excess in the plant. Nitrate nitrogen is less readily taken up by rhododendrons, but may be a satisfactory source. However, it usually induces iron deficiency in the plant unless enough iron is supplied in chelated form. The deficiency is expressed as chlorosis and reduced growth. Some species and hybrids require less fertilizer than others, and are more sensitive to over-supply of nitrogen and other nutrients.

1.  Anonymous. 1962. Rhododendron Notes. Quart. Bull. Amer. Rhodo. Soc. 16(2):23.
2.  Bik, R.A. I972. Influence of N, P, and K rates on mineral composition of the leaves of azalea 'Ambrosia'. In Colloquium Proceedings: The nutrition of protected crops. No. 2:99-104.
3.  Bjerkestrand, E. 1971. Nitrogen fertilizer as a top dressing for container-grown trees. Gartneryrket 6:501-502, 504-505.
4.  Bradley, G.A., R.L. Meyers, and J.W. Fleming. 1961. Growth and chemical composition of azaleas as influenced by soil type, organic matter, acidification, chelated iron, and nitrogen source. Proc. Amer. Soc. Hort. Sci. 78:507-520.
5.  Clarke, J. Harold. 1956. Rhododendron Culture. Rhododendrons. Amer. Rhodo. Soc. pp. 5-12.
6.  Clarke, J. Harold. 1982. Getting Started with Rhododendrons and Azaleas. Reprint Edition. Timber Press. Beaverton, OR.
7.  Colgrove, M.S., Jr. and A.N. Roberts. 1956. Growth of the azalea as influenced by ammonium and nitrate nitrogen. Proc. Amer. Soc. Hort. Sci. 68:522-536.
8.  Cox, Peter A. 1973. Dwarf Rhododendrons. Macmillan Publishing Co.
9.  Cox, Peter A. 1979. The Larger Species of Rhododendrons. B.T. Batsford Ltd. London.
10.  Criley, R.A. 1969. Controlling rhododendron flower bud initiation. Flor. Rev. 143 (3715):36-37,93.
11.  DeKock, P.C. and A. Wallace. 1965. Excess phosphorus and iron chlorosis. Calif. Agric. 19(12):3-4.
12.  Dickey, R.D. and R.T. Poole. 1965. A comparison of slow release with readily available fertilizers on growth and chemical composition of R. indicum 'Formosa'. Proc. Fla. State Hort. Soc. 78:393.
13.  Dickey, R.D., R.T. Poole and J.N. Joiner. 1967. Effect of nitrogen, phosphorus, and potassium levels on growth and chemical composition of Rhododendron indicum 'Formosa' and Viburnum suspensum . Proc. Amer. Soc. Hort. Sci. 91:762-770.
14.  Dirr, Michael A. 1980. Cold-hardiness, nutrition, mycorrhizae and companion plants as they relate to the Ericaceae. Quart. Bull. Amer. Rhodo. Soc. 34(1):20-26.
15.  Dirr, Michael A., Allen V. Barker and Donald N. Maynard. 1973. Growth and development of Leucothoe and Rhododendron under different nitrogen and pH regimes. HortScience 8(2):131-132.
16.  French, Christopher J. and James Alsbury. 189. Comparison of controlled-release fertilizers and application methods for production of Rhododendron 'Anna Rose Whitney'. J. Amer. Rhodo. Soc. 43(1):12-18.
17.  Green, Eldred. 1969. Iron deficiency on acid soil. Quart. Bull. Amer. Rhodo. Soc. 23(1):18.
18.  Havis, John R. and John H. Baker. 1985. Phosphorus requirement of Rhododendron ' Victor' and Cotoneaster adpressa praecox grown in a perlite-peat medium. J. of Environ. Hort. 3(2):63-64.
19.  Heft, T. 1968. Dungungsprobleme in der baumschule dargestellt an versuchsergebnissen bei grossblumi-gen rhododendron-hybriden. Dtsche. Gartnerborse. 68:241-246, 251.
20.  Herath, M.E. and G.W. Eaton. 1968. Some effects of water table, pH, and nitrogen fertilization upon growth and nutrient-element content of high bush blueberry plants. Proc. Amer. Soc. Hort. Sci. 92:274-283.
21.  Johnson, C.R. and A.N. Roberts. 1969. Deficiency symptoms of Rhododendron studied. Ore. Orn. and Nurs. Dig. 13(2):1-3.
22.  Leach, David G. 1958. A report on the effectiveness of a chemical treatment for inducing the formation of flower buds. Quart. Bull. Amer. Rhodo. Soc. 12(1):28.
23.  Leach, David G. 1961. Rhododendrons of the World. Charles Scribner's Sons. New York.
24.  Leiser, AT. 1959. Nutrition and root anatomy of azalea. Ph.D. Thesis. Univ. of Calif., Los Angeles.
25.  Leiser, AT. 1965. Azalea nutrition why acid soils? Statewide conference on role of acid soils in crop production. Div. of Agric. Sciences. Univ. of Calif.
26.   Maleike, Raymond R. 1973. Some aspects of nitrogen metabolism of selected Ericaceae. Ph.D. Thesis. V.I.P. and State Univ. 60 pp.
27. Miwa, S. and H. Ozaki. 1976. The effect of nitrogen fertilizers and soil pH on the growth of some native Rhododendron species. Bull, of Shizuoka Agric. Exp. Stn. 21:67-68.
28.  Myhre, Arthur S. 1963. Increasing bloom production in rhododendrons by fertilizing practices. Proc. Internat. Plant. Prop. Soc. 13:294-299.
29.  Myhre, Arthur S. and W.P. Mortensen. 1964. The effect of phosphorus on rhododendron flower-bud formation. Quart. Bull. Amer. Rhodo. Soc.18(2):66-71.
30.  Oertli, J.J. 1960. Diewirkungvon ammonium und nitrationenauf das wachstum und die mineralstoffzu-sammensetzung von azaleen. Die Gartenbauwissenschaft 25:293-301.
31.  Orr, P.B., Jr. 1972. Iron deficiency in rhododendrons. Quart. Bull. Amer. Rhodo.Soc.26(2):108-109.
32.  Ryan, G.F. 1970. Effects of succinic acid 2,2-dimethyl hydrazide and phosphorus treatments on rhododendron flowering and growth. J. Amer. Soc. Hort. Sci. 95(5):624-626.
33.  Ryan, G.F. 1979. Effects of nitrogen and phosphorus on flower bud formation in rhododendrons. Coll. of Agric. Res. Cent. Bull. 872. Wash. State Univ.
34.  Ryan, G.F. 1979. Effects of nitrogen and potassium on leaf burn of Rhododendron 'Elizabeth'. Quart. Bull. Amer. Rhodo. Soc. 33(2):76-78.
35.  Scarseth, George D. and Norman J. Volk. 1941. Why do plants starve? Chap.1 in Hunger Signs in Crops. Amer. Soc. Agron. and Nation. Fert. Assoc.
36.  Shanks, J.B. and C.B. Link. 1961. The ratio and intensity of nitrogen, phosphorus and potassium fertilization of azaleas for green house forcing. Proc. Amer. Soc. Hort. Sci. 78:496-506.
37.  Smith, Elton M. 1971. The effect of fertilizer treatments on Rhododendron . Nursery Notes. Ohio State Univ. IV(6):1-2.
38.  Spencer, E.L. 1929. The nutrition of Rhododendrons. Nursery Disease Notes. N.J. Agr. Exp. Sta. 2(3):1-9.
39.  Stuart, Neil W. 1947. Some studies in azalea nutrition. Nat. Hort. Mag. Oct. pp. 210-214.
40.  Ticknor, R.L. 1969. Influence of fertilizers and growth regulators on flower bud production of field-grown rhododendrons. Comb. Proc. Inter. Plant Prop. Soc. 19:305-309.
41.  Ticknor, R.L, M.H. Chaplin and J.E. Long. 1978. Foliar and soil nutrient values in rhododendron nurseries. Proc. 29th Northwest Fertilizer Conf. 127-136.
42.  Ticknor, R.L and J.L. Long. 1978. Mineral content of rhododendron foliage. Quart. Bull. Amer. Rhodo. Soc. 32(3):150-158.
43.  Tod, Henry. 1969. Nitrogen sources for rhododendrons. Cdnrs' Chron. 165(11):26.
44.  Van Veen, Ted. 1970. Increasing rhododendron enjoyment. Quart. Bull. Amer. Rhodo. Soc. 24(1):33-35.
45.  Wallace, Arthur and Roland T. Mueller. 1966. Inorganic nutrition of azalea. Current Topics in Plant Nutrition. Arthur Wallace. Edward Bros., Inc. Ann Arbor, Mich.

George Ryan retired in 1983 after 16 years with Washington State University at Puyallup, where he did research on horticultural problems of woody ornamentals, especially rhododendrons. A long time ARS member, he is also active in the Rhododendron Species Foundation.