JARS v41n4 - Is There A Relationship Of Soil And Foliar Boron Levels In Rhododendron Nurseries?

Is There A Relationship Of Soil And Foliar Boron Levels In Rhododendron Nurseries?
R.L. Ticknor and J.E. Long
North Willamette Experiment Station
Oregon State University, Aurora, Oregon

Among the suggested research projects in the 1987 list of the ARS Research Committee was one on the relationship between the boron levels in the soil and in the foliage of rhododendrons growing in those soils.

Foliar nutrient content of 11 elements including boron (B) was determined for 'Blue Ensign', 'Jean Marie de Montague', 'Roseum Elegans', trilby', 'Unique' and 'Vulcan' in 15 nurseries in 1977 and were reported (7). In general plants rated good were higher in nitrogen and boron than poor plants. This was also true in the 1978 survey of 'Jean Marie de Montague', 'Roseum Elegans', 'Unique' and 'Vulcan' in 24 nurseries (8). The 1979 survey data included soil as well as foliar analysis of boron for 'Anna Rose Whitney', 'Cynthia', 'Sappho' and 'Scintillation' which have not been published and provide an answer to the Research Committee question.

Low foliar boron levels 15-20 ppm (3, 4, 5, 9) result in marginal yellowing with stunting and distortion of the leaves. The terminal bud may die and weak side shoots develop.

Excess boron first causes a slight chlorosis followed by marginal burn and necrotic spots (3, 5,6). This shows in older leaves first as boron accumulates in a leaf throughout its life. Boron found in some fertilizers is not shown on the label (1). It is also found in some irrigation water (6). Irrigation water with less than 0.3 ppm was safe and water with 0.5-1.0 ppm can be used with leaching for container grown azaleas (6).

Leaching will remove excess boron in time but removing leaves showing marginal burn reduced boron injury in the next growth flush of container grown 'Lee's Dark Purple' rhododendrons (2). Spraying the 'Lee's Dark Purple' plants with copper sulfate 3.6 grams/liter then drenching them with lime water 3.6 grams/liter of hydrated lime Ca(OH) ₂ increased recovery from excess boron.

When soil boron levels were plotted against the foliar levels for 'Anna Rose Whitney', 'Cynthia', 'Sappho', and 'Scintillation' found in the 1979 survey there was no apparent relationship between them (Table 1 reports the levels found). We did observe boron toxicity (marginal burn of the leaves) when the soil boron level was 1 part per million or higher and foliar boron was above 100 ppm. Boron toxicity pictured was the result of using gypsum to supply calcium and sulfur to the plants without changing the soil pH. Unfortunately it was not noticed that the bag was labeled "Borated Gypsum". Since only very small amounts of boron, less than 1 pound of actual boron is needed per acre for many crops, boron is sometimes combined with other products to make spreading easier. Normal gypsum would not cause a problem. Soluble forms of boron are often applied as a foliar spray to uniformly distribute the small amounts.

Table 1. The Relationship Soil and Foliar Levels of Boron in Oregon and Washington Field (F) or Container (C) Nurseries
Nursery	Growing Method	Soil pH	Soil Boron PPM	Leaf Boron PPM
				'Anna Rose Whitney'	'Cynthia'	'Sappho'	'Scintillation'
1	F	5.1	.50				29
3	F	5.1	.35	23	30	25	26
4	F	4.4	.37	13	8	7	6
5	F	5.6	.33	41		29	69
6	F	5.5	.20	21		21	37
8	F	5.0	.61	22	23	36	31
9	F	5.7	.58	50		50	48
12	F	5.9	.65	27		29	34
13	F	5.5	1.04	81		171	161
21	F	5.6	.52	47	46		66
23	F	5.2	.27	17	34	27	33
24	F	5.4	.39	16		27	33
27	F	5.5	.38	28		21	34
28	F	5.7	.46	26	27		20
29	F	4.6	.70	20		50	52
30	F	6.5	1.07	118		44
32	F	5.3	.45	32		47	28
32A	F	5.5	.58	25		60
2	C	5.2	.37	27	30	48	59
11	C	5.7	.84	28	27	17	56
31	C	5.6	.29	.34	19		28

We did not observe boron deficiency symptoms in this survey despite very low foliar readings in one nursery. This nursery had almost pure sand soil, relatively high rainfall and low soil pH. These factors may explain the low foliar B levels despite 0.37 ppm boron in the soil. Foliar boron levels in other nurseries with soil boron near this level were in the sufficiency range.

Soil tests should be in the 0.4-0.8 part per million range while foliar analysis should be 25-70 ppm. If boron is below .4 ppm in the soil or 25 ppm in the leaf, boron should be applied. An application of 0.4 lbs of actual boron per acre to the soil should restore it to a desirable level in most cases.

There are a number of products which can be used to supply boron including Boric acid (H ₃ BO ₃ , (17.5% B), Solubor (20% B), Borated Gypsum (1.5% B), Borax (11.3% B). Boron is also available in a number of trace element mixtures such as F.T.E. 503, S.T.E.M., Micro Max, and Esmigran.

Reminder, a little boron is necessary but more could be a disaster. So follow directions and measure the product to be applied and the area carefully.

Literature Cited
1.  Gilliam, C.H. and E.M. Smith. 1980. Sources and symptoms of boron toxicity in container grown woody ornamentals. J. Arboric. 6:209-212.

2.  Gilliam, C.H., E.M. Smith, S.M. Still, and W.J. Sheppard. 1981. treating boron toxicity in Rhododendron catawbiense. Hort. Science 16:764-765.

3.  Hale, M.C. 1986. Mineral nutrient deficiencies and toxicities. pp 46-48 in: Compendium of Rhododendron and Azalea Diseases . Coyier, D.L. and M.K. Roane, Editors, APS Press, St. Paul, Mn.

4.  Johnson, C.R. and A.N. Roberts. 1969. Deficiency symptoms of Rhododendron studied. Ore. Orn. & Nursery Digest 13(2)1-3.

5.  Kofranek, A.M. and O.R. Lunt. 1975. Mineral nutrition, pp 36-36 in: Growing Azaleas Commercially . Kofranek, A.M. and R.A. Larson, Editors. Univ. of Calif. Sale Publication 4058.

6.  Lunt, O.R., H.C. Kohl, and A.M. Kofranek. 1957. Tolerance of azaleas and gardenias to salinity conditions and boron. Proc. Amer. Soc. Hort. Sci. 69:543-48.

7.  Ticknor, R.L. and J.E. Long. 1978. Mineral content of rhododendron foliage. Quar. Bui. Amer. Rhodo. Soc. 32(3):150-158.

8.  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.

9.  Twigg, M.C. and C.B. Link. 1951. Nutrient deficiency symptoms and leaf analysis of azaleas grown in sand culture. Proc. Amer. Soc. Hort. Sci. 57:369-375.

Doctor Ticknor, professor of Horticulture at Oregon State University, has long been active in rhododendron and azalea research.