Cork spot and bitter pit on my apples (a sign of insufficient calcium) has led me down the path of exploring calcium and micronutrient deficiencies. Calcium is an essential nutrient in our gardens and orchards. A quick web search on the function of calcium in plants indicates it plays a key role in cell structure and division, as well as disease and pest resistance. Advancing Eco Agriculture website indicates that calcium is critical for bud formation and pollination, as well as cell division. Calcium also helps with soil structure, particularly in compacted soils, and helps the soil microbiome capture nitrogen from the atmosphere converting it to a plant usable form.

I recently listened to an episode on the No Till Flowers podcast with Bryant Mason (AKA The Soil Doctor) with an extended discussion on the use of gypsum and also foliar calcium. Calcium is not transported well through the structures of a plant, hence his suggested use of both soil and foliar applications. Gypsum is recommended as a source of calcium where soil ph levels are neutral or higher, as is true for most soils in our region. As an additional benefit, gypsum also supplies sulfur, another essential nutrient. 

Apparently, Honeycrisp apples are notorious for developing bitter pit defects. In commercial production of the apples in Washington state, growers are applying 1 to 2 tons of gypsum per acre per year. Bryant uses gypsum in his peach orchards in Colorado at a rate of 4 tons per acre! A more manageable suggestion for home growers might be 2 shovels full around the drip line of each tree. In annual beds, his suggestion is to use about 5# per 100 square feet. Lighter soils in annual production often move more nutrients out of the root zone, and calcium is an essential nutrient for our veggies as well.

Gypsum is water soluble, and rain can carry surface applied gypsum into the soil. The calcium in gypsum does not generally leach out of the soil, unless there are exceptional levels of rain or irrigation, and instead will be available to the plant when the plant is able to take it up. This seems like a good time of year to apply the gypsum, and it can also be applied as a mid-season boost if needed.

Growing season foliar sprays of calcium can be more immediately effective, and can be purchased or apparently even homemade from oyster shell powder or eggshells and vinegar. I did not previously realize however, that a foliar spray should have a pH of 5 to 5.5 for good uptake by the leaf. Bryant’s recommendation is to use citric acid to bring the pH of the foliar to the proper level. He also stated that adding some carbon to the spray will help with uptake. HIs suggestions for carbon were a bit of molasses, liquid kelp, and/or compost extract to the foliar application to boost its effectiveness.

These are actionable and relatively easy suggestions! Now I have a plan…..!