For fruit trees, which remain rooted in one place, proper nutrition is crucial, and this responsibility falls to the grower. Among essential nutrients, nitrogen stands out as vital for producing amino acids, proteins, and nucleic acids. Stone fruit trees, in particular, need a consistent annual supply of nitrogen to support healthy growth and abundant fruit production.
Nitrogen absorption in fruit trees primarily occurs through their fine roots, taking in ammonium or nitrate. Several factors influence this uptake, including soil type, pH levels, temperature, moisture content, and the time of year. For trees bearing fruit, fertilization should aim to strike a balance, promoting sufficient leaf and overall tree growth without negatively impacting the quantity and quality of the fruit.
Maintaining a healthy leaf canopy after harvest is also important. This allows nitrogen in the leaves to be relocated back into the tree before leaves fall. This stored nitrogen acts as a reserve, supporting blooming and the initial development of new leaves in the following spring, before the roots begin absorbing nitrogen from the soil.
Finding the Nitrogen Sweet Spot
Insufficient nitrogen reserves carried over from the previous fall can impede canopy development in the spring. If a nitrogen deficiency persists throughout the growing season, it leads to stunted leaf and shoot growth, ultimately reducing yield. Premature leaf drop may also occur in the fall. Visually, nitrogen-deficient leaves might appear chlorotic, exhibiting a yellowish hue often accompanied by reddish tints. Twigs will be underdeveloped, appearing spindly with a reddish bark. While the fruit might develop good red color, it tends to be small and lack flavor.
Conversely, excessive nitrogen can lead to an overly lush shoot canopy, larger and denser than necessary. This dense foliage reduces sunlight penetration and increases humidity within the canopy, hindering the effectiveness of foliar-applied pesticides. The negative outcomes include increased susceptibility to diseases, reduced fruit size and color, and delayed ripening. Inadequate sunlight reaching the inner and lower parts of the canopy can also inhibit flower bud initiation, potentially impacting the next year’s crop negatively.
Generally, over-applying nitrogen is not only ineffective but also wasteful. Beyond the drawbacks already mentioned, excessive vegetative growth increases pruning labor costs. Nitrogen that the tree doesn’t absorb can also contribute to environmental pollution through runoff or leaching into groundwater.
Latest Research on Nitrogen Application
Research has indicated that applying a large dose of nitrogen in a single spring application can be inefficient. This is partly because the tree’s nitrogen demand from the soil is low during spring, as it primarily relies on nitrogen reserves stored within. Heavy spring rains can leach nitrogen away from the root zone, and cooler soil temperatures can limit root uptake.
Studies suggest that annual nitrogen application rates can be reduced by up to 50% without negatively affecting tree growth and fruit production. The key to better nitrogen utilization is to apply smaller amounts of nitrogen multiple times during the growing season, coinciding with periods of active growth and root absorption. This can be achieved by broadcasting granular fertilizer or, more efficiently, using fertigation systems with water-soluble nitrogen fertilizers in low-pressure microirrigation.
The crucial question is determining the optimal timing for these applications. Ongoing research at Clemson University, involving root physiologist Dr. Christina Wells, utilizes minirhizotrons (underground root observation tubes) to monitor peach root growth throughout the season. The aim is to align fertilizer applications with periods of root “flushing,” correlating these with above-ground phenological stages like full bloom, shuck split, pit hardening, final swell, and postharvest. The long-term objective is to precisely time fertilizer applications to match the tree’s periods of greatest nitrogen need.
Furthermore, research in California and Italy has shown that postharvest nitrogen fertilization using foliar urea applications in the fall effectively supplements tree reserves while minimizing environmental impact. However, this method may not be suitable for colder northern climates as it could interfere with hardening off and increase the risk of winter injury. For further information, refer to resources on fertility management practices.
By understanding why nitrogen is important and how to manage its application effectively, fruit tree growers can optimize tree health, fruit yield, and minimize environmental impact.
