Tamara ScullyNo matter what kind of tree fruit you are growing, many factors influence the end-quality. Managing the nutrient levels during differing stages of crop development is one important aspect that can strongly influence fruit size, firmness and quality whether you are growing pome or stone fruits.

John Kempf, of Advancing Eco Agriculture, recently presented a webinar discussing trials involving the timing of foliar applications of various nutrients and the impact on tree fruit quality.

“We can actually shape fruit size, fruit firmness, and aroma and flavor a substantial degree, just before and during the blossoming and pollination period, and the cell division period, and later on during the fruit filling period as well,” all by managing nutrition, Kempf explained.

Trials were conducted at Simonian Fruit Company in California on nectarines, peaches and plums as well as on cherries at Omeg Family Farms in Washington on cherries. Plant sap analysis was used to measure the level of nutrient availability at various stages of growth on trees where foliar applications of calcium, potassium and magnesium were selectively applied to correspond with the stage of fruit development, versus the standard fertilizer applications. Thinning and other practices remained unchanged from the farms’ standard practices across all blocks, so data — size, quality, yield — could be compared to previous harvests.

“Cations antagonize each other inside of the plant,” Kemp explained, and a surplus of one can lead to a “deficiency of another even though there may be a generous supply of the deficient nutrient.” The relationship between calcium and potassium, in excess of either, at the wrong developmental stage, can limit fruit size, firmness and quality.

Often, calcium deficiency concerns such as bitter pit in apples, bitter rot in stone fruit, or blossom end rot are not due to a lack of calcium, but an excess of potassium. This excess prevents the needed calcium from entering the fruit, so the crop cannot benefit from the calcium, no matter how much is applied.

Calcium is the key element needed for cell membranes, and limiting calcium availability during cell division, due to excess potassium being present at this stage of growth, is detrimental to fruit size. Boron and manganese are also needed at this time. Boron increases calcium mobility, and manganese is a potassium regulator, allowing calcium freedom to translocate into the buds as potassium is not competing for that space.

Stages of development

Just after pollination, rapid cell division occurs, lasting from 10 to 14 days. During this stage, the rapid expansion of the total number of cells inside the embryo, is the primary activity taking place. The number of cells that form during this stage determines the potential fruit size: the more cells produced, the larger the fruit has the potential to be. By extending this cell division window as long as possible, and by insuring the largest number of cells are produced, growers optimize the fruit’s maximum size potential.

It is during this stage that the availability of calcium is crucial. Potassium levels inside the plant need to be low at this time, to allow the calcium to be accessed. Likewise, during fruit fill, it is potassium that is the key cation, influencing cell expansion and determining the actual fruit size, up to the maximum potential set during cell division. During fruit fill, sugars are produced and moved into the fruit.

“Potassium is the sugar transport locomotive,” Kempf explains. “How much sugar the plant can produce and move into the fruit,” depends on the plant’s ability to uptake available potassium. Fruit fill is the time for a foliar potassium application.

Looking at the cation ratio at the different stages of plant growth is key, and “will have a huge impact on fruit quality and firmness,” Kempf emphasizes. “You can produce really, really firm fruit by managing the potassium and nitrogen applications later on, once you have provided adequate calcium early on. We want to get calcium on early and then focus on potassium later.”

The actual harvest quality and quantity of fruit is obviously influenced by many factors, and is not the sole result of potassium and calcium ratios at various development stages. However, sap analysis during crucial cell division and fruit fill can demonstrate the importance of having the ratio of these cations in balance.

Post-harvest management

Foliar application of these elements at crucial, targeted times is one key management strategy, and fall preparation has shown to be helpful in setting the stage for the next season. “Post-harvest applications…can be a very important piece for producing an exceptional quality crop the following spring,” Kemp said.

Moving nitrogen and zinc into the plant in the fall, prior to dormancy, is important to the development of large vegetative buds, resulting in larger spur leaves in the spring. Spur leaves are believed to produce a larger proportion of sugars that end up in the fruit, according to Kempf. Increasing spur leaf size the following spring can be done by applying urea, zinc and magnesium in the fall, via foliar application post-harvest.

Photosynthesis is one process that is often not optimized in tree fruit production, Kempf says. “Most fruit trees are producing about 25 – 30 percent of their inherent photosynthetic potential.”

Increasing that rate would mean more sugar production, and more potential to move that sugar into the fruit. Key nutrients needed for optimal photosynthesis include magnesium, manganese, iron and nitrogen. Magnesium is a component of chlorophyll, while manganese is needed for water hydrolysis, and iron is used to build chlorophyll. Nitrogen is not normally a limiting factor in photosynthesis, but the other three elements are often lacking.

By understanding the requirements for fruit production, and the role key nutrients play during various stages of development, as well as how those nutrients interact, fruit quality can be enhanced. Profitability — due to increased fruit size, yield and pack out — can be significant.