Calcium: An Overlooked Blueberry Nutrient
This past year I noticed decreased vegetative growth in our older northern highbush blueberry planting made back in 1979. Plants are 6 to 7 feet tall, and are annually pruned to remove the oldest stems to maintain plants each with six to eight strong young to medium-age main stems — none of which is ever more than six to eight years old. The plant crowns are now more than 32 years old, so they are quite large, but I estimate that with good annual care, these plants should have at least another 30 years of good production.
We mulch these plants every couple of years or so as needed to maintain a 2- to 3-inch depth of wood chips in-row under them. We monitor soil pH annually and strive to maintain optimum blueberry soil pH at 4.8 to 5.1, using acidic fertilizer and occasionally agricultural sulfur as needed. Supplemental foliar nutrients also are used to help ensure good plant health. On our silt loam soil, higher soil pH results in chlorotic leaves with severe iron deficiency and slow plant growth.
Nutrient analysis of such leaves always shows not only iron deficiency, but also low calcium content that was below optimum sufficiency range. Calcium is essential for strong cell wall development, enzymatic activity, nitrogen uptake, and metabolism, plus it is important in other needed plant functions. For years and for some unknown reasons, we paid no attention to our calcium levels, forgetting that calcium is poorly absorbed as a foliar-applied nutrient. Perhaps, too, we confused calcium importance with pH maintenance.
During the 32-plus years of our time with this planting, it finally dawned on us that we have never added lime to these plants or much of any fertilizers containing any calcium as we worked to maintain an acid optimum blueberry soil pH. We know blueberries have only a very dense, fibrous root system that is very shallow in depth, with no tap root to plumb soil depths for calcium and other needed plant nutrients. From experience, we also know our blueberry root system does not generally extend outward beyond the dripline of the plant canopy of leaves. Thus it stands to reason that after a few years, these shallow roots likely have sought, obtained, and mined or extracted about all the calcium available to them!
The Power Of Gypsum
In late March this past year, I purchased agricultural gypsum — calcium sulfate, not building supply gypsum that may contain binding agents and/or other non-plant nutrients. Our local farm supply stores stock agricultural gypsum at a reasonable price. I applied at least a pound of ag gypsum around each plant in-row in late March as new growth was just beginning for the year. Agricultural gypsum contains a neat balance of both calcium and sulfur so that it has no effect to alter soil pH up or down. Agricultural gypsum contains 32.5% calcium oxide, or roughly 25% actual calcium, so a pound of ag gypsum per plant replaces about 4 ounces of depleted calcium, and is quickly available to the plant roots. Ag gypsum also benefits soil structure by loosening it to improve aeration and water infiltration. Thus it can aid somewhat deeper rooting of blueberries. We were delighted and amazed to see that by early summer, new shoot growth was abundant and very vigorous on all of these treated plants, so I began to sleep better!
We also considered the nutritional calcium in blueberry fruit that had been removed over many, many good harvest years. Blueberries contain about 1% calcium that is all supplied from the soil beneath the plants’ restricted, shallow root system. So if you remove 10 to 20 pounds of fruit per plant per year, for example, in a few years you have removed 100 pounds of fruit that contained 16 ounces of actual calcium per plant, which may never have been replenished by the well-meaning grower, the shepherd of the planting. Indeed, now may be your time too, to go for the ag gypsum. Both spring and fall are considered the best times to apply gypsum, and spring is coming!
In another, bigger commercial blueberry field in times past, we had always used a premium grade granular fertilizer that contained 6% calcium along with 5% sulfur plus several more minor nutrients. Yes, it was more expensive and was not available locally, but it had no effect on our blueberry soil pH, supplied needed calcium, and provided us with excellent annual growth for juvenile stem development and good solid fruit yields. I recommend that when you shop for your annual blueberry plant nutrients, check those labels for calcium too, not just the nitrogen, phosphorus, and potassium. Get an annual foliar leaf analysis right after harvest each year to help you stay informed of your plants’ nutritional needs.
A closing thought: What are the calcium levels in your other berry crops? Also, note to ole self: Practice more of what I preach!