Managing mineral nutrition

Ever-evolving apple production systems in a diverse Western Cape environment require a continued focus on optimising tree nutrition. By Anna Mouton.

Mineral-uptake characteristics aren’t a primary consideration in rootstock selection, but growers should be aware of the interactions between rootstocks, orchard systems, and site-specific factors, according to soil scientist and fruit-tree nutritionist Nelius Kapp of Soil2Root Technologies.

“If we look at the effect of the rootstock on mineral nutrition, we need to put it in the context of a very diverse production environment,” he said. “And within that environment, we must decide how to manage calcium. The Western Cape mostly has acid soils, with associated low soil-calcium levels.”

Kapp highlighted the implications of big-picture changes in apple production systems for mineral nutrition. “We’ve transitioned to high-density orchards, so we’re filling our space and canopy quicker and need less growth per season.”

These orchards need less nitrogen, but their increased bearing potential increases their potassium and phosphate requirements. “More recently, I’ve seen that we need to look at increased magnesium for more vigorous rootstocks,” he added.

Protective netting also increases tree vigour and, therefore, reduces nitrogen requirement.

Improved fruit size on newer rootstocks such as the Cornell Geneva range can put calcium supply under pressure because calcium is diluted in larger fruit. However, bigger fruit sizes allow for a reduction in potassium, and less potassium decreases competition with calcium.

“Something that people aren’t aware of is slightly more soil-calcium fluctuation in high-density orchards,” said Kapp. He attributes this to farming in a smaller soil volume and applying lime and nutrients to a smaller area.

“There’s more competition for calcium and more leaching, which leads to acidification, which causes fluctuation in calcium levels,” he explained.

The rootstock effect

Although rootstocks affect mineral uptake and nutrition, this isn’t always obvious at the orchard level because other factors may strongly influence the tree’s requirements.

“A rootstock’s influence on mineral nutrition will manifest through the vigour, crop load, and fruit size it induces, and its sensitivity to replant disease,” said Kapp. “As soon as you have a replant-sensitive rootstock, you have poor rooting and nutrient uptake.”

He differentiates three nutrition phases in an orchard’s lifespan: phase 1 is a young tree without fruit; phase 2 is a growing canopy with increasing crop load; and phase 3 is a filled canopy with a stable crop load. More vigorous rootstocks or higher-density plantings condense phase 2. Rootstocks influence the orchard’s nitrogen requirements by affecting growth.

Less vigorous trees — for example, M.9 planted too widely on replant soil or a windy site — will stay on a young-tree nutrition programme for six years and continue receiving high nitrogen for the subsequent two years. “Eventually, by year nine or ten, it’s hopefully filled its space, or you’ve given up,” said Kapp.

Normally growing trees — for example, more vigorous rootstocks or high densities on dwarfing rootstocks — would spend two years on a young-tree nutrition programme and continue receiving high nitrogen for a subsequent two years before nitrogen is reduced to optimal levels.

“Then there’s the extreme scenario — G.778 that fills its space in one season,” said Kapp. “It’s only on a young-tree programme for one year, and then you bring its nitrogen down immediately, only giving it very little in autumn for the rest of its life.”

In the case of potassium, the rootstock’s influence manifests through bearing potential. “In the poor-growing scenario, the crop load and potassium requirement will only increase gradually,” said Kapp.

The potassium requirement will increase early and rapidly in high-cropping scenarios, such as precocious rootstocks or high-density plantings that fill their space and bear quickly.

Potassium in practice

Kapp presented data on the potassium nutrition of three full-bearing Gala orchards on different rootstocks (Table 1).

Table 1: Potassium nutrition in three commercial Gala orchards.

*for 2018–2023

Similar amounts of potassium were applied to the M.9 and MM.109 orchards, but the MM.109 trees had meaningfully higher leaf potassium levels. This fits other research showing that more vigorous rootstocks tend to take up more potassium.

The M.9 and G.778 orchards had similar yields and leaf potassium levels, even though less potassium per tonne of fruit was applied to the G.778 trees.

Kapp observed similar trends on another farm, which typically uses little potassium. “When they started planting M.9, I noticed we had to apply much more potassium to maintain leaf levels.”

He analysed data for Cripps Pink, Fuji, and Gala orchards on M.9, M.7, MM.106, and M.793 and found that M.9 needed more potassium per tonne of fruit but still had lower leaf levels, indicating that this rootstock takes up less potassium.

The calcium context

“I’m not saying that the rootstock is not important, but with calcium management, you need to focus on the environment around the rootstock,” said Kapp. “Multiple variables influence the leaf calcium, and we don’t measure all of them.”

He listed the most common causes of calcium-related defects as increased vigour, poor light distribution, low crop load, elevated potassium, and low soil pH, noting that these factors tend to go together.

“You need to manage nitrogen, potassium, and calcium collectively,” emphasised Kapp. “You manage the availability of calcium by liming and crop load and vigour by horticultural manipulation, so you can direct the flow of calcium to the fruit and reduce competition for calcium.”

The key to adjusting fertiliser applications is measuring the orchard’s response to fertilisation and soil pH to liming. Kapp warned that there is no one-size-fits-all recipe for lime and nutrient applications. Every situation is unique.

He thinks most growers should apply more lime and aim for a soil pH (KCl) of 6.0–6.5. Adequate annual liming is critical. Growers must not prioritise correcting soil pH variability within orchards to the extent of underapplying lime annually.

Kapp’s other recommendations for calcium management are to sample at least every two years — annually in bitter pit-prone blocks — and to sample 40–50 centimetres deep, soil permitting. He also advised growers to use laboratories with accredited exchangeable acidity measurements.

Crop load and vigour management are crucial. “Any reduction of more than 15% in crop load is a signal that you need to consider adapting potassium and reducing nitrogen,” said Kapp. “Excess nitrogen isn’t good for fruit quality.”

Commenting on rootstocks in general, he stated that postharvest defects are more likely with vigorous rootstocks and alternating yields. Dwarfing rootstocks such as M.9 and G.202 need more nitrogen and potassium than vigorous rootstocks, and high-density or precocious orchards need higher and earlier potassium, phosphorous, and magnesium fertilisation on the acidic soils of the Western Cape.

Watch Kapp’s presentation on the Hortgro YouTube channel.