Pistachio Nitrogen Nutrition: What do we know and how do we manage?
Douglas Amaral, Orchard Advisor, Kings and Tulare Counties, UC ANR
Patrick Brown, Professor, UC Davis
More than ever before, California pistachio growers have a wide selection of nitrogen (N) management tools at their fingertips today. Given that N fertilizer mismanagement have increased significantly and the Irrigated Lands Regulatory Program (ILRP) have implemented new regulations to prevent agricultural runoff from impairing surface waters, it only makes sense to use available tools to improve Nitrogen Use Efficiency (NUE).
Importance of Nitrogen Nutrition
Nitrogen is essential for a wide range of processes in plant production. It is the major element found in the chlorophyll, which is needed for photosynthesis, the most important of all the plant processes – converting the sun’s energy into carbohydrates. It is also needed for the formation of amino acids (the building blocks of proteins) as well as DNA and RNA, which is the genetic material that allows cells (and eventually, plants) to regulate plant growth and development.
Understanding Tree Demand
Nitrogen fertilizers are the most important chemical input in modern agriculture. Among all essential nutrients for higher plants, N is required in the largest quantity. It is estimated that over 100 million tons of N fertilizers are applied globally. However, due to excessive use of N in California, less than half of the applied N is utilized by the crops while the rest is lost to air and water by run-off, leaching, denitrification and volatilization.
In order to reduce costs, maximize yield, and to minimize losses to the environment, matching tree N demand with N supply is essential. Therefore, when making fertilization decisions, growers need to consider the different supplies of N, as well as the demand for N due to tree growth and fruit production requirements. An understanding of all different N (re)sources (fertilizer, compost, cover crop, manure, irrigation water) and how N can be “lost” (run-off, leaching, etc.) in the orchard will help growers better make management decisions.
Figure 1. The nitrogen cycle: a balancing act with the goal of applying adequate N to supply tree demand for growth and fruit production, and to avoid nitrogen losses to the environment. Adapted from: ABC N-BMP 2020 guidance.
As previously stated, N is essential for a wide range of processes in tree production such as growth of all annual (flowers, leaves, and kernel) and perennial organs (branches, trunk, and roots). However, the primary factor impacting N demand in pistachio trees 10 years and older is fruit yield. The industry’s understanding of the pattern and rate of nitrogen uptake from the soil is derived from a series of experiments conducted in high yielding orchards throughout California from 2008-2011, it is estimated that with every 1,000 lbs. of pistachio fruits produced (CPC yield), 28 lbs. of N are exported from the field and an additional amount of N is required to satisfy the trees’ annual growth demand (25 lbs. of N per acre per year). Thus, potential yield estimation early in the season can have a significant impact on N management decisions.
Improving Nitrogen Management
In order to achieve maximum productivity and to reduce N losses to the environment, a close attention to the four R’s – Right Rate, Right Time, Right Place, and Right (re)Sources – is needed in all orchard fertility programs. Remember, you need to TRAP the N in the soil: TIME nitrogen application according to tree uptake patterns, identify/understand available nitrogen RESOURCES (fertilizers, composts, manure, cover crops, irrigation water), apply the AMOUNT needed according to tree demand, and PLACE N to the tree’s active root zone. Following the 4 R’s principle can be viewed as a balancing act with the goal of applying adequate N to supply tree demand for growth and fruit production.
Fertilizer applications can be optimized and considerable N losses can be reduced if N applications are matched with the actual tree N requirement. Ideally, N fertilizers should be applied at a rate and timing that are synchronized with the demand curve by using very frequent or even continuous fertigation as illustrated in figure 2.
Figure 2. Nitrogen in pistachio trees during 'on' and 'off' years. Before leaf-out, little N was taken up and the trees relied on N remobilized from perennial tissue. In 'on' years, approximately 30% of N was taken up during spring flush (mid-March to late May) and 70% during nut fill. N uptake was negligible between harvest and leaf senescence. In 'off' years, the proportion of N taken up during spring flush was slightly higher. Source: California Crop Fertilization Guidelines: N Pistachio (cdfa.ca.gov). **Note that the nitrogen (N) amount shown on Y axis is for entire orchard N content, not N recommended rate per acre.
In the period from dormancy through early leaf-out, the tree depends almost fully on N that is remobilized from perennial organs, and practically no N uptake occurs from the soil. After flowering, during the period of leaf and fruit expansion, uptake from the soil starts while remobilization of N from perennial tissues continues. From full leaf expansion until early hull split, tree N demand is satisfied completely by soil N uptake. Following fruit maturity (hull split), tree N demand and root uptake decline quickly, and stop completely as soon as leaves begin senescence. While fruit is developing, the rate of soil N uptake is directly determined by the yield of the tree. Even though the demand for N to support new tree growth is smaller than the demand for fruit development, the fertilization program should not only replace the nutrients removed in the fruits, but also supply the nutrients for annual growth of the trees as well. Nitrogen in flowers, leaves and perennial storage N is predominantly provided from internal and soil N recycling and, consequently, does not contribute to annual fertilizer N demand.
Table 1. Nitrogen application timing.
*Post-harvest application should only occur if trees are healthy and N application was too low.
It is important to note that all fertilization decisions will be influenced by local environment and must be adjusted accordingly. For example, in regions with rainfall that may persist well into leaf-out, application to the soil may be problematic. Also, in areas with substantial rainfall growers must adopt practices that minimize the amount of N that resides in the soil at that time. Pre- and post-harvest foliar applications of N, as a substitute for soil applications, could be used to provide N to trees if yield and tissue sampling indicates a need.
Spring Leaf Sampling: Useful Tree Status Monitoring Tool
Determination of the N concentration in leaves from the current season growth can also be useful in estimating tree N status and annual leaf testing is recommended. This is an important tool to measure the adequacy of the fertilization program and to diagnose nutrient deficiencies and toxicities. However, the current practice of sampling leaves in July is too late to allow for current-season adjustment of fertilization practice. As a result, a validated model for predicting July nitrogen levels from tissue samples taken earlier in the season has been developed and can effectively predict summer tissue values. For example, a 3% N tissue analysis 40 days after full bloom would predict that you would have 2.6% in July, a level sufficient to begin kernel filling. The model allows growers to assess the future N status of their orchards from late April and May tissue samples and can be found at: http://fruitsandnuts.ucdavis.edu/files/208409.xls.
Table 2.Nitrogen Concentrations in Summer Leaf Samples.
Critical Values (CV) are minimum concentrations for adequate tree growth and yield. Suggested Range also refers to the concentration for optimal growth. **Growth and yield is not improved with July tissue levels above 2.5 percent.
Recommended Sampling Criteria
Leaf sampling is valuable and provides information on the current status of the tree and the effectiveness of grower’s management decisions. Sampling can be conducted in mid-May to help predict seasonal nitrogen demand, while July sampling can be used to monitor plant N status and make end-of-year adjustments.
For each orchard/block or sub-block that you wish to have individual information on, do the following:
· Collect sub-terminal leaves from non-fruiting branches.
· Collect leaves from 18–20 trees per orchard.
· In each tree collect 10 leaves around the canopy from well exposed branches located between 5-7 feet from the ground.
· Combine all leaves in a single bag for submission to a reputable laboratory. Each sampled tree must be at least 25-30 yards apart. A minimum of 180 leaves per sample bag is required.
Figure 3. Following the sampling protocol guidelines is important to get around the challenges of leaf sampling canopy variability (A). Each sampled tree must be at least 25-30 yards apart (B) and only sub-terminal leaves from non-fruiting branches should be collected (C).
Nitrogen Management Plan
It is important to note that leaf sampling alone does not provide sufficient information to make fertilizer recommendations. The recommended approach to N fertilization scheduling consists of the following steps and should be repeated for each orchard block:
· Conduct a pre-season (January) estimate of expected yield, based on historic yield trends for each orchard, last year’s yield, winter chilling, and spring flowering weather. Grower judgement and observation remain critical.
· Estimate annual inputs of N in irrigation water, manures, composts, fertilizers, etc.
· Calculate preliminary fertilization rates and timings, and make first application of fertilizer in early- to mid-spring (mid-May).
· Collect and analyze May leaf samples according to preceding instructions.
· Conduct in-season yield estimation (April – May).
· Adjust fertilization strategy for remainder of year to reflect May leaf and yield estimates.
Too Much of a Good Thing
While it is always important to ensure sufficient N supply for optimum growth and yield, it is equally important that we do not assume that more is better. Nitrogen is the nutrient most often abused in terms of this ‘more-on’ approach. Adding high levels of N to the soil early in the season will not result in greater plant uptake unless the tree is deficient. In addition, excess nitrogen can cause several problems. When nitrogen is overapplied, it negatively affects the uptake of other nutrients and can also cause excessive growth rather than production.
Nitrogen management plays an important role in determining your bottom line so ensure you manage this nutrient well in order to keep tree performance, production and fruit quality up as well as costs down. Some approaches that can be adopted to minimize risks and optimize N use include: applying N to meet tree demand and according to tree uptake patterns. For this an efficient irrigation system is essential – fertilizers must be present in the root system where they are most likely to be used by the tree.
In summary, an efficient management of N requires that every orchard that differs in age, soil, environment or productivity should be sampled and managed independently. Every individual orchard must have a specific N management plan, there is no “one-size-fits-all” approach.
Useful Tools to Help Growers Improve Their Nitrogen Management Plan
One tool is Nitrogen and Potassium Prediction Model for Pistachio for interpreting spring leaf analysis. Here, growers can learn about pistachio early-season sampling and in-season nitrogen applications to maximize productivity and minimize loss.
Another recommended tool is California Fertilization Guidelines: Pistachio. On the tool’s website, growers will find an interactive display that provides different sets of information and recommendations for nitrogen, phosphorus and potassium, such as general deficiency symptoms, soil testing, leaf analysis and soil and foliar applications of each.
One last recommended tool is the Pistachio Prediction Model (PPM), which uses nutrient analysis of spring leaf samples to predict late summer nutrient concentrations of nitrogen (N) and potassium (K).
Two presentations, Managing Pistachio Nutrition: Macronutrients and Pistachio Micronutrient Management, are also available online to help growers understand pistachio nutrition.
**Note that these tools and guidelines have been developed only for ‘Kerman’, but are also considered good practices for other pistachio cultivars. The same tools and guidelines are currently being validated for the new cultivars ‘Golden Hills’ and ‘Lost Hills’ and will be released to the public in the next 2-3 years.