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A pH test night be advisable first. If your Seattle soil is acidic as it is here in Nova Scotia iron deficiency is not likely to be the problem. Chlorotic Rhododendrons here are commonly treated with iron chelate but it nevers solves the problem; our soils are rich in iron. Usually we try epsom salts (potassium sulphate @ 2Tbs/gallon) first to address a possible magnesium deficiency and a good shot of nitrogen. Usually you find the owner has been using an acid plant fertilizer loaded with sulphur to counteract alkaline soils thus making the acid soil here even more acidic thus causing nitrogen and magnesium lockup. If the epsom salts and nitrogen don't work then we give them a few handsful of dolomitic lime - I know what you're going to say but remember our soils are oftentimes far too acidic, a common occurrence where trees are predominantly coniferous. Only near new foundations do we ever see alkaline problems, fresh cement and mortar rubble the culprits.
What we need here - and in Seattle - is a plant fertilizer formulated for acid plants on acid soils. There was once the perfect cure-all fertilizer called Magamp but although it is still made in Texas and/or Japan by Sumitomo it is unobtainable. Sumitomo never returns calls or replies to queries, I hear the Dutch nurserymen have the market cornered. Instead when required we just use 6-12-12, a common potatoe fertilizer used here.
johnw
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johnw coastal Nova Scotia
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