Kathryn C. Taylor, Former Stone Fruit Horticulturist
When the stone fruit tree moves into its bearing years, it requires a shift in emphasis from exclusive attention to vegetative development for building a tree structure. The new emphasis requires balance between maintaining enough vegetative growth to promote adequate fruiting wood and return bloom for the following season’s fruit crop and managing the current season’s fruit crop. Too much tree vigor during summer months will cause shading of developing fruitwood, limiting its size and the number of flower buds it will produce. If enough shading occurs, the fruiting wood will be moved up the axis of the tree, reducing yield and increasing the cost of managing a taller tree.
Fertilization
Proper fertilization management is central to keeping the tree in proper balance for the bearing years. Bearing trees, trees entering their third leaf, should be capable of producing sufficient fruit to justify a production management program. Generally nitrogen, phosphorus, potassium, calcium and magnesium are required annually. Annual testing is easy. Foliar sampling can be done in July. Simply collect the fourth fully expanded leaf from the tip of shoots at eye level around the periphery of the tree’s canopy, collecting about 20 leaves total. Dry these in a paper bag before sending them in for analysis. Your county extension agent can handle foliar tests for a small fee. Peach sufficiency levels are given in Table 1.
| Table 1. Sufficiency levels for essential elements. |
| Element |
Sufficiency Ranges |
| N (%) |
2.75-3.50 |
| P (%) |
0.12-0.50 |
| K (%) |
1.50-2.50 |
| Ca (%) |
1.25-2.50 |
| Mg (%) |
0.20-0.50 |
| Mn (ppm) |
20-150 |
| Fe (ppm) |
60-400 |
| Al (ppm) |
<400 |
| B (ppm) |
20-100 |
| Cu (ppm) |
5-20 |
| Zn (ppm) |
15-50 |
Nitrogen
More than any other element, nitrogen controls growth and fruiting in plants. Nitrogen management presents the fruit grower with a dilemma. When the nitrogen level is optimum for fruiting, vegetative growth may be inadequate, and vice versa. Nitrogen interacts strongly with pruning and irrigation. For maximum fruit production, manage trees to produce maximum leaf area early in the season. This involves moderate pruning, establishing high nitrogen levels in the early season, early thinning, maintaining adequate soil moisture, and slowing vegetative growth just prior to harvest by depletion of nitrogen. Generally, we recommend you provide 60 pounds of nitrogen per acre annually for trees spaced 20 feet x 20 feet. This rate may change, however, based on a high or low crop yield and cultivar differences in tree vigor.
Research shows that peach tree survival greatly improves when the annual nitrogen fertilization is split, with some nitrogen applied in mid- to late August (after harvest but no later than September 1) and the remainder applied in late winter. Because the post-harvest nitrogen application is used to help maintain healthy foliage in the fall and improve winter hardiness of the tree, the amount applied at this time should be considered food for next season’s crop. So determine the annual quantity of nitrogen applied to a tree as that applied before harvest of next season’s crop — not the total applied during a calendar year.
While calcium nitrate is more expensive, it maintains a better soil chemistry than ammonium nitrate and, in some studies, calcium is shown to improve fruit quality.
Do not over-fertilize third leaf trees with nitrogen. Excessive nitrogen will encourage too much vegetative growth that will compete with developing fruit for the current season and with developing fruitwood for next season. When very vigorous growth shades the following year’s fruitwood, the tree produces fewer reproductive buds, and fruitwood in the lower, shaded areas of the tree is weak. A 30:30 pound split nitrogen recommendation for bearing trees should be altered depending on the vigor and fruit load of trees in the orchard. When trees have very vigorous growth during the season, reduce the rate of the late summer application or leave it off altogether; but if the fruit load is particularly heavy, increase the late summer application rate. Apply the post-harvest application under the dripline on bare ground or inject it through a drip irrigation system so the trees get most of the nitrogen. Apply the “spring” application of nitrogen in mid- to late winter. Proportionally, more fertilizer is needed if calcium nitrate is substituted for ammonium nitrate (i.e., the calcium nitrate requirement is generally two times the ammonium nitrate requirement). Make this application at least six weeks before bloom for early maturing varieties.
Phosphorus
Fruit trees remove relatively little phosphorus from the soil each year. Only about 12 pounds of P2O5 per acre are removed by a heavy fruit crop. Developing trees are estimated to retain about 3 pounds P2O5 per acre (not returned to the soil by leaves and pruning). Therefore, trees require no more than 15 to 20 pounds P2O5 per year to maintain phosphorus once adequate levels (moderate to high) are established in the soil.
Application of P2O5 on alternate years should be a practical approach for most southeastern orchards. Many southeastern soils test “high-plus” for phosphorus. Continued addition of phosphorus to these soils may cause deficiencies in zinc, iron or copper. Foliar copper levels are marginal or low in many orchards where high levels of phosphorus are present. Continued addition of phosphorus to soils where soil test results read moderate or hi
