Scientists have determined there are 16+ essential mineral elements needed for tree fruit growth. The amount varies by element. In most cases our soils are adequately supplied with these elements. However there are five elements in orchards that should be monitored with more scrutiny because shortages can develop. These minerals are nitrogen, phosphorus, potassium, calcium and boron.
The Major Minerals
These major minerals all perform different functions in the trees.
Nitrogen is essential for growth and is the foundation for amino acids, proteins, nucleic acids and other compounds. Nitrogen can be loosely monitored by observing annual shoot growth. Pome fruit trees that are bearing a crop should average about 12 to 18 inches of shoot growth. Bearing stone fruit trees should average about 18 to 24 inches of shoot growth. Nitrogen is taken up primarily in the form of nitrate nitrogen that occurs naturally in the soil solution and can easily be leached from the soil. The ammonium form can also be absorbed by the trees but is usually found in a much lower concentration in the soil.
Phosphorus on the other hand is not very soluble in the soil solution. Phosphorus has a high affinity to bind to iron and aluminum when the soil pH is low. Phosphorus is important in energy transport, storage compounds and nucleic acids. It is especially important during periods of high metabolic activity particularly immediately after planting when young roots are emerging. Root growth is enhanced when phosphorus levels are adequate. Roots infected with naturally occurring mycorrhizae can show enhanced phosphorus uptake.
Potassium is the third element to monitor and is required in large amounts in the fruit. The actual potassium levels in the soil can be quite high but its availability in the soil solution can be very low due to binding to the structure of many soil minerals. In the fruit potassium concentrations exceed all other mineral nutrients. Like nitrogen potassium is required in high quantities by the trees. However, unlike nitrogen it is not recycled as much because fruit are harvested and removed from the orchard; where much of the nitrogen can be recycled with the abscission and decay of leaves and the chopping of brush from pruning. I would expect that with the dry season we are having that potassium levels for leaf samples this year will be low unless you have a light or no crop situation.
Calcium, it goes without saying, is important in the cell wall structure and we know that low levels of calcium in the fruit can result in bitter pit and/or corking in apples. Calcium is usually very abundant in Pennsylvania soils. However, it is preferentially moved to actively dividing tissues primarily through the xylem cells. Movement in the phloem cells is restricted and therefore tissues more dependent on obtaining minerals from those cells, such as the fruit, frequently have difficulties obtaining adequate amounts. Additionally this year, in light or no crop blocks calcium levels may be lower.
The last mineral nutrient that PA orchardists need to monitor on a regular basis is boron. Boron is classified as a micronutrient because it is only required in small amounts. Boron, among other things, helps stimulate pollen tube growth down the style during the flowering period. A deficiency of boron can manifest itself by a lack of fruit set. In pears this is often described as "blossom blast". Unlike most micronutrients the range between sufficiency and toxicity is very close so over application can be a serious problem. Most boron needs can be adequately achieved through foliar sprays.
Other mineral nutrients that have on occasion been seen to be deficient in Pennsylvania orchards include magnesium and zinc. Magnesium is usually a localized deficiency to certain areas of Pennsylvania. Zinc deficiency does not seem to show a preference for certain areas and has only recently been seen to be deficient as we use different types of fungicides.
How Do I Know What Minerals I Need?
While some problems can be determined by observation, the best method is to take a leaf sample and have a foliar analysis performed. Samples should be collected from about the middle of July through the middle of August. Choose leaves from the current season's growth from the middle of the shoot. Select shoots that are about shoulder height. The best sample consists of one variety on the same rootstock. Mixing varieties and/or rootstocks does not produce a consistent result. Rotate through your orchard units so that you collect a sample about every three years. A good number of leaves to collect is 50 to 60 with not more than two leaves from any one tree. Try to avoid trees from any known soil anomalies, such as eroded hilltops or overly vegetative trees due to rich soils. Collect the leaves in a paper bag; do not collect in plastic bags as they may develop mold. Be sure to fill out the accompanying information sheet as completely as possible.
Penn State offers both leaf and soil analysis from their Agricultural Analytical Services Lab. The complete analysis includes results for N, P, K, Ca, Mg, Mn, Fe, Cu, B, and Zn. Recommendations accompany the report that is sent to you. For quicker results the reports can be emailed to you. I review all the results from the Lab and provide interpretation. If you include your email address the reports can be sent electronically to further speed up the process. They arrive as a pdf document that you can either file electronically or print out and file as a paper copy.