There are several ways in which orchardists can develop a more sustainable approach to nitrogen fertility. Fortunately, tree fruit production requires relatively modest inputs of nitrogen. Drs. Gerry Neilsen and Denise Neilsen at the Pacific Agri-Food Research Centre in Summerland, British Columbia, Canada, cite nitrogen removal from orchards in fruit and dead leaves as follows:
—End of year 1 8.4 pounds per acre
—End of year 3 30.9 pounds per acre
—End of year 6 35.8 pounds per acre
Full bearing leaves and prunings can be retained and the nitrogen recycled in the orchard; only 27 pounds per acre are actually leaving in the fruit each year, that need to be replaced.
Of course, nitrogen is accumulated in the growing trees as the orchard is established, starting at 6 pounds per acre in the nursery trees themselves, rising to 24 pounds per acre at the end of the planting year, and to 58 pounds per acre at the end of the fourth year in a dwarfing, high-density orchard.
The trees store a considerable amount of nitrogen in the trunk, branches, and roots—enough to supply as much as 50 percent of the growing season needs in mature trees. And trees are less than 50 percent efficient in recovering nitrogen applied to the soil.
When thinking about a sustainable nitrogen strategy for orchards, there are several options that can be considered.
Reduce losses and increase efficiency
• Use irrigation management that prevents leaching.
• Apply fertilizer in spring and late summer/early fall for maximum tree uptake.
• Base nitrogen rates on leaf nitrogen, tree vigor, residual soil nitrate.
• Fertigate with nitrogen.
These practices are widely known and used in many orchards.
Retain and recycle nitrogen
• Retain and shred prunings and leaves.
• Mow and blow alley grass into the tree row. This practice has been used successfully to move nitrogen from the drive alley and deliver it for root uptake in the tree row.
• Plant a winter cover crop in tree row. Late-season cover crops are not typically used in orchards, but could be explored. For example, an early September planting of oats in the tree row could scavenge available nitrogen into the fall, and the oats would be killed by the winter temperatures, then releasing nitrogen in early spring once soil temperatures reach 45°F. Some growers now achieve this by allowing a moderate amount of weed growth in late summer and fall.
Use renewable nitrogen sources
• Plant legumes in the orchard floor. Washington State University trials in Wenatchee have demonstrated the ability to provide 53 pounds per acre from mowed legume biomass in the tree row with one mowing, potentially releasing half of this nitrogen over a three- to four-week period.
• Integrate livestock.
• Import organic wastes (manures, crop residue, biosolids, etc.).
• Use nitrogen extracted and concentrated from organic wastes. A nutrient extraction process, developed at Washington State University to separately concentrate nitrogen and phosphorus from anaerobic digestion effluent, is in pilot testing and could create a renewable nitrogen supply in the state.
• Use nitrogen fertilizer made from bio-methane. The methane from a digester could also be the feedstock for renewable nitrogen using the Haber-Bosch process.
• Consider new processes. A new ammonia plant is proposed in Quincy, Washington. Several microbial products are on the market that claim to enhance soil nitrogen fixation by the bacteria they contain, but their efficacy is not yet determined.
While synthetic nitrogen fertilizer will be widely used for the foreseeable future, alternatives are in development or already available which represent a more sustainable option. Continued research and development are needed to refine their use, and understand their costs and potential benefits.