According to the Potash Development Association (PDA), calcium is a somewhat overlooked nutrient for tillage crops.

Calcium plays a role in soil structure and through its link with the pH-enhancing properties of limestone.

However, potatoes are the one crop where the importance of calcium is better understood, through its impact on tuber quality.

Calcium plays a role in the maintenance of healthy cell walls, thereby reducing the incidence of the internal brown spots within tubers known as internal rust spot.

The maintenance of healthy cell walls also helps to improve skin finish and protect against physical tuber damage at harvest and during storage.

Another important role of calcium, again linked to cell wall strength, is the protection against pests and specifically disease as thinner, weaker cell walls are more likely to lead to attacks.

There are reports of beneficial effects from the application of a non-liming source of calcium to potatoes. 

All of this is significant due to the increasing use of integrated pest management strategies where the growing of all crops are concerned.

The number of available herbicides, insecticides, and fungicides available to production agriculture and horticulture is fast declining.

And, as a consequence, growers are relying on more natural methods to keep crops healthy.

Magnesium

Magnesium is a key nutrient for all crops due to its role in photosynthesis, being part of the chlorophyll molecule.

It is also involved in the production and use of carbohydrates, helping transport them from the leaves down to the tubers. 

Deficiency symptoms are likely to develop in the older leaves nearer the base of the plant as the nutrient is mobile in plants and is therefore able to move to areas of new growth when uptake is limited.

A deficiency of magnesium can reduce crop productivity long before visual symptoms can be seen.

An inadequate supply will result in reduced root growth as well as lowering the chlorophyll concentration of the plant, reducing the photosynthetic rate. 

There is some evidence that potash availability and uptake can be reduced on soils where magnesium levels are excessive, even though the level of soil potassium (K) may not be deficient.

This situation is most frequently associated with long term use of magnesian limestone to correct pH.

It is suggested that where the soil concentration of magnesium is more than double that of soil potash, then potash applications may need to be increased above the normal recommended rate to achieve adequate potash nutrition of the crop.