When you’re talking tree fruit nutrition, both soil and plant nutrients play important roles. Taking a soil sample analysis and adjusting nutrients accordingly does not translate directly into the optimum nutrient availability for fruit production and quality. Many variables play a role in how and when soil nutrients are made available in the tree, where those nutrients get distributed and whether or not the growing fruit has what it needs to be a high-quality crop.
“Trees, being perennials, are uniquely different from annuals,” Joseph Heckman, Rutgers University professor of soil science, said during a recent virtual orchard meeting, hosted by Rutgers Cooperative Extension. “The physiology is special, and it means that you need to manage them with that in mind.”
The “Ask the Expert: Orchard Soil Fertility and Tree Nutrition” presentation featured Heckman and Robert Crassweller, Penn State tree fruit specialist.
Orchard Soil Fertility
The primary concern with orchard soils is balancing the cation ratios to prevent deficiencies and excesses. Magnesium and calcium are the two primary concerns in tree fruits, and “balance is very important with respect to calcium, magnesium and potassium. If you put an abundance of one of these cations in the soil, it suppresses the uptake of the other,” Heckman said.
Liming is important in order to maintain soil pH. Tree fruit orchard soil pH levels of about 6.5 are lptimal. For new orchard establishment, liming to obtain appropriate pH levels should be done prior to any planting.
While magnesium deficiencies aren’t that common in orchards, they can be a problem if the liming program isn’t adequate. Excessive potassium levels will inhibit magnesium uptake. If magnesium soil levels are too low, using a dolomitic limestone product, high in magnesium, can rectify the situation.
Calcium is typically the cation of concern in tree fruit production. It’s used in cell wall and cell membranes inside the trees themselves and also in the fruit. Calcium also plays an important role in soil structure. Moisture levels in the soil can impact the availability of calcium. For soils where the pH is adequate, but soil calcium levels are low, gypsum can be used instead of limestone. Limestone with higher levels of calcium, rather than magnesium, is typically the best bet for orchard soils when pH adjustment is needed.
“The calcium/magnesium ratio is very important for crop quality,” Heckman said. “Calcium should be the dominant cation on your colloid.”
Soil colloids in orchards should be comprised of 68% calcium, 10% magnesium and 5% potassium. The soil cation exchange capacity can be used to calculate the amounts of each cation that should be on the soil colloid.
It is difficult to get calcium into fruit, as calcium moves up the tree with transpiration and becomes immobile, not moving once it’s incorporated. Calcium goes more readily to leaves and growing shoots because these parts transpire more than fruits.
Foliar application of calcium sprays enables fruit growers to attempt to correct low levels in fruits. These applications need to start very early in fruit formation, and be performed regularly in order to be effective. A soluble form of calcium must be used. Potassium should not be applied when plant cells are actively dividing, as it will decrease calcium availability to the fruit. Certain cultivars have more issues with calcium deficiency in the fruit than do others.
Bark splitting in trees indicates a calcium deficiency, and bitter pit occurs when calcium levels in the fruit are low.
Crassweller emphasized the importance of leaf analysis when seeking to enhance fruit quality and yield. Leaf analysis provides a better look than does soil testing at what is actually available in the plant. Moisture levels, crop load, season, temperature, cultivar and rootstocks are some variables to consider. These can influence how soil fertility impacts tree fertility.
“There’s always been a link between leaf analysis and yield. It’s imprecise. It’s probably because there is different rootstocks, and there is also different soil. And supply from the soil is not the sole influence on leaf concentration,” Crasswell said.
In tree fruit orchards, the nutrients are recycled through the seasons. During autumn, the tree’s nutrients move back into the roots, twigs, shoots and buds and are stored until they are needed in spring to start growth.
From a horticultural perspective, nitrogen, potassium, phosphorous, calcium, boron, zinc and soil pH are the factors to be actively managed in fruit trees, Crasswell said. “Really what we’re looking at is a balance of calcium, nitrogen, potassium and magnesium. Those are all positive cations.”
Nitrogen is important for vigor and crop load, and impacts the red color of fruit. For each 0.1% increase in leaf nitrogen, a corresponding 5% decrease in red color of fruit occurs, Crasswell said. Large trees have more nitrogen reserves, which is important to consider as smaller trees – planted in high-density orchards – are now the norm. Higher root density and decreased soil volumes probably do impact available nitrogen in soils and trees, although the effect of this has not yet been ascertained.
Too much nitrogen is more damaging than too little. The actual nitrogen available per acre each year for fruit trees should be between 25 – 40 pounds. Organic matter correlates to nitrogen levels in soil, with 20 – 30 pounds of nitrogen per acre for every 1% of organic matter, although only 40% of that amount will be available in the trees.
Large fruiting apple cultivars, processing apples where quality of fruit is not as important but tonnage is and cultivars which set heavy crops will require more nitrogen for best performance. Early maturing varieties need less nitrogen than those with later maturation dates.
Foliar nitrogen sprays via urea applications can provide the flower buds with nitrogen and are beneficial for trees with low nitrogen levels. Urea also breaks down leaves, which helps to suppress disease such as apple scab and Marssonina leaf spot, Crassweller said.
Foliar potassium is not recommended in tree fruit unless there is a known deficiency. Leaf levels of potassium should be between 1.2% and 2.5%. In the soil, 150 – 250 ppm of potassium is ideal. Excessive nitrogen will lower the availability of potassium.
The ratio of nitrogen to calcium in the fruit is key to fruit quality. Foliar calcium application of four to 14 pounds of actual calcium is the best way to enhance tree fruit quality, as applying calcium to the soil will increase calcium in leaves rather than the fruit. Rootstocks can also impact calcium uptake, Crassweller said.
A substantial decrease in acid rain – which historically provided sulfur – has left some orchards deficient. Sulfur is important to disease resistance and can enhance the flavor of foods, Heckman said. Elemental sulfur will lower soil pH. Sulfur can also be found in ammonium sulfate, calcium or magnesium sulfate or potassium sulfate. Compost and manures are also good sources of sulfur.