Although both calcium (Ca) and magnesium (Mg) are essential for plant growth, they are usually present in adequate amounts in calcareous, alkaline soils and in irrigation waters. Some deficiencies have been observed in very sandy soils or in acid soils where supplemental applications of Ca and Mg were needed to meet tuber growth
requirements. Exchangeable soil Ca concentrations less than 300 ppm indicate a need for supplemental Ca which can be met with preplant applications of 200 lb Ca/acre. Magnesium deficiencies can develop at exchangeable soil Mg levels below 100 ppm. Broadcast applications of 100 lb/acre as magnesium sulfate or potassium-magnesium sulfate, or band applications of 20 lb Mg/acre should satisfy crop requirements. Calcium and magnesium can be applied as dolomitic lime when increases in soil pH are desired.
Some tuber quality disorders such as internal brown spot are associated with Ca deficiencies. To improve tuber Ca uptake, Ca fertilizer should be placed in the zone of tuber formation to facilitate uptake by the stolon roots. Fertilizers such as calcium nitrate, calcium-ammonium nitrate and calcium sulfate can be used to supply Ca without significantly increasing the soil pH.
Calcium applied to foliage is not translocated to the tubers but may help satisfy some of the Ca requirement of the leaves when deficiencies develop. Immobility of Ca in soils and plants also limits the effectiveness of sprinkler-applied Ca. Foliar sprays of magnesium sulfate can be applied to correct Mg deficiencies when petiole concentrations are less than 0.3%.