PDA issues new potash guidance for potato crops

Potatoes need a soil potash supply that satisfies two separate demands — the peak rate of uptake during early vegetative growth and the total amount the crop removes in late summer — according to new guidance from the Potash Development Association (PDA).

The association has published a set of fresh nutrient considerations for potato growers, and the core message is blunt: where soil reserves cannot meet either of those two requirements, growers must top up with potash in fertiliser or manure.

But there is a catch, and it matters for anyone hoping to fix a depleted field in one season. Applied potash, whether from fertiliser or manure, may not deliver potassium (K, the nutrient supplied in potash) as efficiently as the reserves already sitting in the soil.

Why fertiliser can't rescue run-down soils

Long-term experiments, cited by the PDA, show that yields from impoverished soils are unlikely to match yields on fertile soils — even when large amounts of potash fertiliser go on.

That finding has a hard commercial edge. A grower who has mined soil potash reserves over years of cropping without replacement cannot simply buy back full yield performance with a single heavy dressing. Recovery is slow.

The PDA notes it can take several years to improve a soil with low potash status once reserves have been run down. Think of the soil reserve as a bank account and the fertiliser as cash flow: the crop draws on both, but the account pays better interest.

What does calcium do for tubers?

Calcium is, in the PDA's words, a somewhat overlooked nutrient for arable crops — appreciated mainly for its role in soil structure and its link to the liming properties of limestone.

Potatoes are the exception. Here the importance of calcium is better understood, because of its direct impact on tuber quality.

Calcium maintains healthy cell walls, which reduces the incidence of internal brown spots within tubers, a disorder known as internal rust spot. That same cell-wall strength has three further payoffs:

• Improved skin finish on the tuber;
• Protection against physical damage at harvest and during storage;
• Better defence against pests and diseases, because thinner, weaker cell walls are more prone to disease attack.

The PDA also points to reports of beneficial effects from applying a non-liming source of calcium to potatoes — that is, calcium delivered as a nutrient rather than as a pH-correcting liming material.

Why magnesium deserves attention too

Magnesium sits at the centre of chlorophyll, the green pigment that drives photosynthesis, making it a key nutrient for all crops. It is also involved in the production and use of carbohydrates, and helps transport them from the leaves down to the tubers — the part of the potato plant growers actually sell.

Deficiency shows up first in older leaves near the base of the plant, because magnesium is mobile within the plant and moves to new growth when uptake runs short.

The more costly problem is invisible. According to the PDA, a magnesium deficiency can reduce crop productivity long before any visual symptoms appear. An inadequate supply cuts root growth and lowers the chlorophyll concentration of the plant, reducing the photosynthetic rate — a silent drag on yield.

When is magnesium too much of a good thing?

The guidance flags a less familiar risk: a potassium/magnesium imbalance.

There is evidence, the PDA says, that potash availability and uptake can be reduced on soils where magnesium levels are excessive — even where the level of soil potash itself is not limiting. This situation most frequently follows long-term use of magnesian limestone (a lime material containing magnesium) to correct soil pH.

The threshold is concrete. Where the soil concentration of magnesium is more than double that of soil K, potash applications may need to increase above the normal recommended rate to achieve adequate potash nutrition of the crop.

For growers on fields with a history of magnesian liming, that imbalance rule is the number worth checking against the latest soil analysis before next season's fertiliser plan is signed off.