Fruit mineral analysis during the growing season can guide producers in their fertilizer programs and help them improve fruit quality.
Dr. Bill Wolk, quality development manager with the B.C. Tree Fruits Cooperative in British Columbia, Canada, said research on fruit mineral analysis was done at the East Malling research center in the United Kingdom in the 1970s.
Researchers and horticulturists in British Columbia began to take an interest during the 1980s and 1990s, with the idea of predicting storability of fruit delivered from different orchards at harvest. This was important in British Columbia, where packers were -receiving fruit from many different small orchards.
“This originated as a packing house storage thing,” Wolk told members of the International Fruit Tree Association during their annual meeting in Kelowna, British Columbia. “We weren’t even thinking about this to help growers grow better fruit in the orchard.”
Now, it is used primarily as a guide for growers. Wolk said the cost of the analysis—at $40 to $60 per sample—is nothing compared to the value of the fruit the orchard will produce and the information and guidance it can give growers in their fertilizer programs. “It’s really cheap,” he said. “If you’re not doing it, you should give it serious consideration.”
Working with Drs. Sam Lau and Gerry Neilsen with Agriculture and Agri-Food Canada in Summerland, Wolk conducted research initially with McIntosh, Spartan, and Golden Delicious apples. The fruit were tested for minerals, as well as for weight, density, and dry matter.
It was assumed that fruit analysis would give the best prediction of fruit quality when done at harvest. The scientists wondered how long before harvest fruit quality could be accurately predicted. Wolk and his colleagues tested fruitlets nine, six, and three weeks before harvest and at harvest.
They then stored the fruit for different periods in commercial regular and controlled-atmosphere storage and looked for correlations between the results of the fruitlet analysis and postharvest fruit quality, such as firmness, sugar levels, and storage disorders.
They were surprised to find that, for all three varieties, fruitlet analysis gave a better prediction of postharvest quality when done six weeks before harvest, rather than nearer harvest. The scientists developed nutrient recommendations for growers based on the actual storage data.
Over the years, they added more varieties to the fruitlet analysis database, and found that each one has different -quality-mineral correlations. The program is currently testing Ambrosia, -McIntosh, Gala, Honeycrisp, and -Spartan.
The recommended calcium level differs from variety to variety more than any other mineral, Wolk pointed out. For example, for Gala, the minimum recommended level when fruit is tested six weeks before harvest is 10 milligrams per 100 grams of fruit weight, compared with 7.0 mg for Fuji, 6.0 for Jonagold, and only 5.5 mg for Braeburn (see “Minimum calcium levels”).
Statistical analysis showed that in Jonagold, for example, there was a level of calcium—4.9 mg of calcium per 100 g of fruit weight—below which there was a “tectonic shift,” and the percentage of bitter pit increased dramatically, Wolk said.
Often though, producers can “eyeball” the data and see how ranges in calcium levels relate to the percentage of bitter pit. Bitter pit is particularly high in Jonagold with less than 4.0 mg of calcium per 100 g.
Although there’s not a perfect relationship between the calcium level and amount of bitter pit, Wolk said fruitlet analysis can be used to improve the odds in favor of the producer.
For example, a grower or packer who has Jonagold apples averaging less than 4.0 mg per 100 g might want to wait to pack that fruit until symptoms of bitter pit appear so it can be sorted out before marketing (see “Recommended calcium in Jonagold”).
The most telling indicator of all-round fruit quality is the ratio of calcium to nitrogen, Wolk said. In Honeycrisp, neither nitrogen nor calcium alone correlates well with fruit quality, but there’s a strong relationship between the -nitrogen-calcium ratio and the percentage of bitter pit, for example.
Potassium has a positive effect on fruit color, which is why hundreds of millions of dollars of potassium fertilizer have been sold to growers over the years, Wolk said. He has seen this positive effect on color regularly in Spartan and Jonagold, but not so much in other varieties.
It is often held that you cannot have too much phosphorus in fruit, Wolk said. While it is desirable in fruit because it can help enhance firmness, it can also increase bitter pit in sensitive varieties if the level is high, particularly if it is out of balance with the calcium level.
Because British Columbia soils are low in boron, fruit levels were expected to be low. But most fruitlet samples had high levels, indicating that growers were applying too much boron. High boron levels can lead to early maturity (including stem bowl splits), fruit drop, and poor storability. “Growers are unaware of how easy it is to get boron into the tree and also unaware how sensitive the fruit can be to high boron,” Wolk said.
The density of the fruitlets, which is easy to measure, is related to firmness, bitter pit, stem-bowl splitting, and visual appearance.
“We haven’t used the density a lot,” Wolk said. “But I’ll tell you something: high-density fruit is really good fruit.”
Small fruit tends to be more dense, he said. “But if you have two size 88 apples and one is more dense than the other—I’m assuming it has smaller cells and more cells—that fruit is just going to be all around higher quality than the lower density fruit.”
Fruit weight (size) is related to firmness, internal breakdown, and bitter pit. Big fruit usually has lower calcium. A powerful predictor of fruit quality is the weight of the fruit divided by the milligrams of calcium per 100 grams of fruit weight.
As well as helping individual growers, fruitlet analysis can give the industry a heads-up about the overall storage potential of the crop and indicate if it’s going to be a high-calcium year or a low-calcium year. In addition, it can can red-flag grower lots that have specific problems, Wolk said. •