I showed M. the berry laden blueberry plants and ventured they would be ready for picking within a few weeks. But why are the leaves yellow? she asked and drew my attention, as she usually does, to what I had missed. Yellow leaves with green veins can indicate the plant is not receiving enough iron (iron chlorosis) since iron produces cholorphyll which makes the leaves green which enables photosynthesis, sugar production and growth. Blueberries thrive on acid soil (between 4 and 5) and if the soil is not sufficiently acid they cannot uptake iron.
Following my pH experiments last week I decided to sample the pH of my blueberry plants. I selected a sample from each of 3 plants, mixed each sample in a separate container with distilled water, and followed my testing protocol: 1) rinse meter in distilled water; 2) put meter in 4.0 buffer solution – it read 4.1 great; 3) rinse meter in distilled water; 4) put meter in 6.9 buffer solution – it read 6.9 great; 5) rinse meter; 6) put meter in soil sample 1 – it read 6.6; 7) rinse meter; 8) put meter in soil sample 2 – it read 7.4; 9) rinse meter; 10) put meter in sample 3 it read 6.3. Each immersion was for 1 minute and water temp around 65 deg F.
I was expecting lower pH readings since earlier in the year I had purchased and added sulfur powder to the soil of the blueberry plants. I mentioned to M. that when taking the samples I had noticed sulfur powder in the soil (it is very yellow and easily seen) and she said that was because it had not been absorbed. The sulfur powder I am using does not dissolve in water.
I mixed some sulfur powder with distilled water and the pH was 6.6 about what it would have been without the powder, so the sulfur powder was not interacting with the water. M. explained that sulfur loving bacteria, which are equipped with the necessary enzymes, are needed to transform the sulfur powder into soluble form. So the sulfur I added in spring had not yet been transformed and apparently this is a slow process which takes time. It would be quicker if we found some sulfur loving bacteria and added them to the soil, but this is an unlikely route.
So what to do in the meantime? There are synthetic iron chelates (a chelate is a soluble form of the mineral which makes it accessible to the plant) which will work quickly but this is at odds with my organic philosophy. The yellowing blueberry leaves do not presage a good blueberry harvest. Short term remedy – we researched and ordered an organic iron foliar spray which will work directly on the leaves and with some luck (if I don’t over apply or under apply) may get them normal looking and back to their photosynthesis work.
Also, if you look carefully at the leaves you may notice some red blotches which could indicate magnesium deficiency so I will provide an epsom salts drink to the plants as well.