New Nitrogen Management Protocol Helps Almond Growers Comply with Mounting Regulations

The Central Valley Regional Water Quality Control Board approved in December new Waste Discharge Requirements under the Irrigated Lands Regulatory Program for the East San Joaquin Water Quality Coalition. Similar requirements will be imposed for all Central Valley coalitions by the end of 2013. The revisions include protection of groundwater, largely from nitrates, as well as additional surface water efforts.

The recently approved Waste Discharge Requirement for the East San Joaquin Coalition, which is expected to be the model for other coalitions as they are approved later this year, requires that all irrigated farms complete a Farm Evaluation Plan by 2017. Farm sites determined to be in “vulnerable areas” must also submit certified nutrient management plans annually. The newly developed nitrogen (N) fertilization protocol should help almond growers as they work to integrate these new regulations into their farming operations.

The new nitrogen management protocol is the result of Almond Board of California–funded ongoing research initiated in 2008 and led by UC Davis plant scientist Patrick Brown. It provides a strategy for growers to apply N at the right rate, at the right time and in the right place so that productivity is maximized and the potential for N loss to the environment, particularly through nitrate leaching, is minimized. To help growers achieve these goals in mature orchards, yield-driven fertilization is combined with new methods for early-season leaf sampling and analysis to make in-season adjustments.

The updated protocols for in-season nitrogen and nutrient management are now available on the Almond Board website.

Right Rate

The right rate means that the amount of nitrogen correlates to the needs of the tree and the developing crop. Kernel yield in the current year is the primary factor driving N demand in mature almond trees, although some N is needed to grow new shoots and spurs for future crops. Studies have shown the amount of nitrogen removed at harvest ranges from 50 to 75 pounds N for every 1,000 pounds of kernel yield. The new protocol suggests using an average of 68 pounds N per 1,000 kernel pounds yield.

A key to developing a nitrogen management plan is to consider N availability through all inputs in the system, including N status in the soil, N in the irrigation water, and the amount of N applied or provided to the orchard (e.g., applied through fertigation, application of dry or foliar fertilizers, organic matter sources and N provided by legume cover crops). The new protocol provides guidance to account for these various inputs.

While it is difficult to estimate available nitrogen in the soil, the new early-season leaf sampling protocols have been developed to estimate N adequacy from leaves sampled in April. If the new April leaf N sampling protocols indicate N adequacy and there is an average-to-light crop, for the remaining season, N fertilizer can be reduced from the standard program. This approach holds great promise to reduce the potential for nitrogen loss from the soil while maintaining productivity.

The protocol also takes into account nitrogen use efficiency (NUE). Guidance and calculations in the protocol are based upon 70% NUE. Four years of trial work show that efficiencies can be higher for N if it is properly timed and applied through fertigation.

Right Timing

Efficient fertilization and N management is best accomplished by timing spoon-fed applications concurrent with demand and root uptake. Current best practice suggests 80% of nutrients should be applied during active tree growth between full leaf expansion in late spring to mid-June. The protocol suggests applications be split during this period a minimum of at least two times. The following sample program includes even more applications:

  • 20% late spring based on leaf expansion (February – March)
  • 30% April – May
  • 30% June

If indicated by tissue analysis and yield levels, the last 20% of N can be applied early postharvest while trees are still healthy. Applications of N postharvest to trees that are nearing senescence or that have adequate tissue N and modest yields should be avoided. Ultimately, the need for this application can be determined based on final yield and leaf sampling/analysis guidelines provided in the protocol.

Right Place

Nitrogen in the soil moves easily with water. Fertigation, or delivering fertilizer through micro-irrigation systems, targets fertilizer to active roots. As long as irrigation is managed to deliver only needed water, fertigation is a highly efficient method of fertilization. If portions of a particular orchard differ significantly in soil characteristics or productivity, it may be necessary to subdivide the irrigation system to provide site-specific water and fertilizer demands, or apply a portion of the annual nitrogen demand in a site-specific ground or foliar application.

Growers using flood or solid-set sprinklers should apply N fertilizer in the herbicide strips along the tree row, not as a general broadcast application, as there are more almond tree roots in the tree rows than out in the middles. Under these circumstances, methods to stabilize N in the soil should be considered, such as incorporation of fertilizers into soil, use of coated fertilizers or urease and/or nitrification inhibitors, and enhancement of soil organic matter.

In-Season Adjustments and Early-Season Leaf Sampling/Analysis

This research has developed early-season leaf sampling and analysis guidelines as an additional tool to manage fertility programs and make in-season adjustments. The primary opportunity for in-season rate adjustment is after late April to early May, when reasonable yield estimates can be made and April leaf samples have been analyzed. Taking into consideration the visual yield estimate and April leaf-sampling results, growers can re-evaluate the necessity of June or postharvest fertilization.

The early-season sample is taken approximately 43 days (plus or minus six days) after full bloom, typically around mid-April. The protocol gives specific instructions on sampling, what the lab analysis should include, how to interpret the results, and making adjustments for the remainder of the season. If the April N leaf concentration is adequate and the yield is less than initially planned, then subsequent in-season fertilization can be reduced or eliminated.