Nutrient Management for Recycled Orchards

decomposing wood chip — Side-by-side comparison of similarly-sized wood chips that had been buried (L) and on the soil surface (R). This orchard was recycled Fall 2016 and replanted early 2018. Robert Stretch Ranch (Madera, CA), 3/1/2019

When an orchard is recycled, approximately 30 to 60 tons of wood chips, containing 15 to 30 tons of organic carbon, are returned to each acre of orchard soil. Research has shown that there are benefits to this large volume of carbon: improved soil physical, chemical, and microbiological properties, increased water holding capacity and infiltration, and improved tree water status.

However, the large volume of organic carbon incorporated into an orchard can significantly increase the carbon-to-nitrogen (C:N) ratio in soils. A high C:N ratio can cause soil microbes to immobilize any nitrogen available in the soils in order to balance out their sudden increase in carbon consumption. This can result in a temporary deficit of nitrogen available for trees subsequently planted in the orchard.

Over time, however, as wood chips decompose, they return nutrients to the soil. Organic matter decomposition in soil can provide nitrogen, phosphorus, calcium, sulfur, and other minor nutrients needed for crop nutrition. Phosphorus from organic amendments binds quickly to soil minerals and moves very little from where it is placed. Potassium, calcium, and magnesium are relatively soluble when obtained from soil organic matter. Organic matter can also provide a balanced source of many minor elements needed for tree growth.

How should growers best manage these dynamics in their recycled orchards? With increasing numbers of California growers using whole orchard recycling (WOR), UC researchers are evaluating the impacts of WOR on nitrogen dynamics in subsequent second-generation orchards. Initial results and observations are reported here; additional research in nitrogen and nutrient management following WOR is ongoing, and results will be reported as they become available.

How does whole orchard recycling impact orchard nutrient management?

One-year-old almond trees in demonstration orchard at CSU Fresno — One-year-old almond trees in a recycled orchard at CSU Fresno, 10/30/18

Recommendations for nitrogen application for first-year almond trees may need to be reconsidered for orchards established following WOR. Observations from WOR trials showed the following:

Reduced shoot growth in second-generation orchards was observed in early spring after replanting, likely as a consequence of a greater C:N ratio in the soil.
Trees responded well to the application of additional nitrogen in the first year following WOR, with greater shoot growth, trunk diameters, and leaf tissue nitrogen levels.
WOR trial plots in this study did not require additional nitrogen in the second year to achieve expected tree growth.
A previous study in 2004 showed that, without any nitrogen fertilizer, almond trees growing in wood-chip-amended soils exceeded the growth rate and leaf petiole nutrient levels of trees growing in non-amended soils by the third year after planting. In the context of current and ongoing WOR research, these results suggest a potential for growers who incorporate wood chips into their orchard soils to reduce fertilizer inputs without sacrificing tree health or development.

Additional UC research is ongoing (as of 2019) to better understand aspects of nutrient management for WOR.

Results discussed here were also reported in: Holtz, B. and Culumber, M. (2019) “2019 Nitrogen Considerations: Nitrogen fertilization is important on first-year second generation almond trees following whole orchard recycling.” West Coast Nut, February 2019, pgs. 14-19.

Initial Results: Manteca, San Joaquin County

An initial study carried out in 2017 in a first-year orchard in Manteca applied 11 ounces of N per tree (approximately 100 lbs N per acre). Trees that had initially showed reduced shoot growth following WOR responded well to the additional nitrogen.

Trees did not require additional nitrogen in the second year in order to reach the expected growth rate, suggesting that, in orchards planted following WOR, applications of nitrogen early in the first year affect tree development the most, and late-season and subsequent-year applications are less important.

Initial Results: Private orchard in Parlier

Small almond tree showing healthy growth — Figure 2 Shoot growth improved quickly with nitrogen application

Small almond tree showing stunted growth — Figure 1 Reduced shoot growth on a tree not supplemented with nitrogen during the first year after previous orchard was recycled

A subsequent nitrogen trial at a private orchard in Parlier examined the optimal increase in first-year nitrogen application to rebalance the C:N ratio and promote early shoot growth. This study applied triple 15 granular fertilizer (15-15-15) to each tree by hand at five different treatment rates, each with five replicates.

Nitrogen rates of 0.0, 0.4, 0.6, 0.8, and 1.0 ounce per tree were applied once per month for five months between March and July. After July, each treatment received 0.0, 2.0, 3.0, 4.0, and 5.0 ounces of nitrogen per tree. These applications were in addition to the grower’s fertigation through a drip system at a rate of 1.73 ounces of N per month from April to August (with the exception of May, when a 2.5-ounce application was made). It was expected that the grower-applied nitrogen was not all immediately available to the young trees because only 20% of the emitters, of a double line drip system, were within the vicinity of the small tree roots in the first year growth.

Considerable precipitation in March effectively dissolved the granular nitrogen, and this first application in March appeared to quickly impact shoot growth (Figures 1 and 2).

Analysis of leaves showed that nitrogen applications early in the season had a greater impact on nitrogen tissue content than applications later in the season. Trunk diameter measurements showed that trees receiving up to 4 ounces of nitrogen had the best growth (Figures 3 and 4).

Leaf nitrogen content graph — Figure 3 Nitrogen tissue content of leaves from different monthly nitrogen application rates.

Trunk diameter — Figure 4 Average trunk diameter of trees under different monthly nitrogen application rates.

Previous Research

Wood chips removed from orchard floor for nutrient sampling — Figure 3 Samples of wood chips were analyzed for their nutrient content. Nutrients will not be available immediately in the next generation orchard, but will be release gradually as the woody material decomposes and organic matter builds.

Previous research showed that, even though wood chips initially tied up available nitrogen, as they decomposed they returned more nutrients to soils than would otherwise have been present. Samples of wood chips were analyzed for their nutrient content in 2017, and averaged 0.31 percent nitrogen, 0.20 percent potassium, 0.60 percent calcium, and 50 percent carbon (Figure 3). For example, returning 64 tons of wood chips per acre provides 396 pounds of nitrogen, 768 pounds of calcium, 256 pounds of potassium, and 64,000 pounds of carbon per acre in the wood chips.

Holtz and Caesar-TonThat [1] showed that, even without the addition of nitrogen fertilizers, Nonpareil almond trees grown in containers of soil heavily amended with wood chips caught up to, and then exceeded, the rate of shoot growth of trees in a control treatment (with no wood chip amendments) by the third season.

While these results suggest a potential for trees in WOR plots to meet or exceed the typical non-WOR growth rate, it should be noted that trees in this study were each grown in a container placed in an existing almond orchard, and that the application rate of wood chips far exceeded the amount typically seen in WOR—each container consisted of 2/3 soil from the surrounding orchard (Tujunga loamy sand) and 1/3 wood chips from almond prunings (Figures 5 - 8).

[1] Holtz, B., Caesar-TonThat, T. (2004) “Wood chipping almond brush to reduce air pollution and its effect on soil and petiole nutrients, soil aggregation, water infiltration, and nematode and basidiomycete populations.” In: Lartey, R.T., Caesar, A.J. (Eds.), Emerging Concepts in Plant Health Management, 2004. Research Signpost, Kerala, India.

Additional Considerations and Ongoing Research

Increased application of N for first-year orchard trees may impact emissions of nitrous oxide and other greenhouse gases. Research is ongoing (as of 2019) at Parlier and other sites to better understand nitrogen use efficiency and to monitor nitrous oxide and carbon dioxide emissions, soil carbon and nitrogen dynamics, and soil health indicators in the years following WOR.