Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Several disorders of potatoes are associated with localized calcium deficiencies in the tubers. This includes internal rust spot, internal browning, heat necrosis, hollow heart, and bruising. Calcium is a component of plant cell walls and the pectin in the middle lamella that cements cells together. Local deficiencies of calcium during the development of potato tubers can cause collapse of cells leading to these disorders.
In plants, calcium moves from the soil exchange sites into soil water and to plant roots by diffusion and mass flow. At plant roots, the calcium moves into the xylem (water conducting vessels), mostly from the area right behind root tips. In the xylem, calcium moves with the transpirational flow, the movement of water from roots, up the xylem, and out the leaves through stomata. Calcium is taken up by the plant as a divalent cation, which means it has a charge of +2. It is attracted to negatively charged areas on the wall of the xylem, and for calcium to move, it must be exchanged off the xylem wall by other positively charged cations such as magnesium (Mg++), potassium (K+), ammonium (NH4+), or other calcium cations (Ca++). This cation exchange of calcium in the xylem requires continuous movement of water into and up through the plant. It also requires a continuous supply of calcium from the soil. The main sink for calcium is developing shoot tips.
In potatoes, tubers develop below ground on the tips of underground plant stems called stolons. Because calcium movement in the plant is driven by transpiration there is limited movement of calcium through root uptake to the developing tubers. It is therefore necessary to have adequate available (exchangeable) calcium around stolons as tubers are formed and adequate moisture to maintain calcium levels in the soil water. To get the full benefit, calcium needs to be in the zone of tuber development and moisture needs to be maintained at optimal levels critically during early tuber development (cell division) and then through the growing season.
To have adequate calcium it is important to apply lime to bring the pH to acceptable levels; however, this often is not sufficient for potatoes. One reason is because of the need in scab susceptible varieties to keep soil pH below 5.4 to control this soil-borne disease.
To deal with this issue, additional calcium is often added in the form of gypsum (calcium sulfate) at the rate of 500-1500 lbs per acre. Gypsum supplies calcium without changing the soil pH. An application of 900 lbs of gypsum will supply approximately 200 lbs calcium/A. Foliar applications of calcium and sidedress applications of calcium nitrate (at rates commonly applied on Delmarva) do little to provide adequate calcium to prevent disorders. Gypsum can be applied pre-plant or during hilling. Calcium needs to in the tuber development zone during the cell division stage. Once tubers reach ¼ inches in diameter there is very little new cell formation. For Ca to be able to get in the tuber it needs to be available between the hook and initiation stages (see figure).
Photo showing hook and tuber initiation stages in potatoes.