Patrick Brown FINAL REPORT R96.09

Development of a N-Fertilizer Recommendation Model to Improve N-Use Efficiency and to Alleviate Nitrate Pollution to Ground Water from Almond Orchards

Final Report - August 1996

Principal Investigator:
Patrick Brown, Associate Professor
Pomology Department
University of California
One Shields Avenue
Davis, CA 95616
(530) 752-0929, phbrown@ucdavis.edu

Other Investigators:
Steven A. Weinbaum, Professor, Pomology Department, UCD
Qinglong Zhang, Postdoc, Pomology Department, UCD

Cooperators:
Lonnie Hendricks, Farm Advisor, Merced County
Anne Marie Ridgley, Community Alliance for Family Farmers

Location of project: San Joaquin County

Commodities: Almonds

Funding:
FY 1996-97: $10,000
FY 1997-98: $10,000
FY 1998-99: $10,000

Overfertilization in almond orchards is attributed to the lack of a reliable tool for measuring tree N status, tree N demand and soil N availability. The purpose of this project was to provide better tools for distinguishing between fertilizer applications that are essential and those that are excessive, and provide growers guidance on when and how much N fertilizer should be applied to obtain optimum yield while minimizing the potential pollution to the environment.

Previous research had suggested the potential value of leaf NO₃-N as an indicator of N status in almond. Contrary to earlier results, studies conducted here demonstrated that nitrate analysis is an unreliable indicator of tree N status due to large variations in tree nitrate concentrations over time and a strong interdependence on plant water status.

The second aim of this research was to determine the seasonal patterns of N demand in mature almond. To this end, sequential whole tree excavations were conducted at 1/21, 3/20, 5/20 at the Delta college orchard, Manteca, CA and there will be one excavation at harvest and in the spring of the following year. Weight of individual trees excavated ranges from 570 kg to 799 kg dry, with corresponding total N content of 4 to 6 kg N. The highest proportion of total N was present in root and root stock in January and March. Fruit and canopy had the largest proportion of total N in May. Nitrate and total soluble N represented only a small proportion of total N presented in the whole tree.

We have completed analysis of seasonal N uptake dynamics and total yearly N demand. This information has now been integrated into a user friendly interactive computer program that is available for distribution. In summary, the determination of N fluxes in almond demonstrates that the majority of N uptake and demand occurs from late February through early September and that the primary demand for N is for nut fill and nut development. N demands can therefore be predicted by estimating yield and can be applied during the periods of greatest N uptake from the soil which occurs during nut development.

By timing N applications with periods of greatest demand, and matching N application rates with crop load we provide growers with a tool that will encourage maximum efficiency of use of N fertilizers. Maximum efficiency of use will result in a minimization of N loss from the orchard system. The computer program is available at www.ucdavis.edu/grants/Reports/brown/nmodel.html.

Specific Results

Objective 1 and 2: Contrary to earlier indications, analysis of tree nitrate cannot be used as a reliable indicator of tree N status. This objective was completed.

Objective 3 and 4: These objectives were fully realized with the primary product being the computer program that is now available. Additional results are in the process of being analyzed and summarized.

Work Completed

The research proposed here aims to develop a decision making process for making N fertilizer recommendations for mature almond orchards. This objective is based on the critical concept that N fertilization is most efficient (N losses are minimized) when N applications are coordinated with tree N demand. Maximizing N-use efficiency is the only viable method of preventing ground water NO₃-N pollution from almond production. The project will define fertilizer requirement by determining tree N status and by estimating the total N required for the production of total biomass at specific stages considering all nonfertilizer credits in the system. Fertilizer recommendations after growth commence will be fine tuned based on tissue analysis at specific growth stages. The recommendation of this project will be presented in a simple balance spreadsheet form that will list all potential N inputs and the amount of fertilizer N needed at specific growth stages for optimum almond production. Integration of this information along with site specific information into a computer program will enable it to be easily used by pest control advisors (PCAs), fertilizer representatives, county extension personnel and growers. This research is expected to provide information for making judicious fertilizer applications that will help improve the N-use efficiency and thus alleviate ground water NO₃-N pollution. Ultimately, the model can be developed into a complex management model for multiple nutrients under diverse management system. Successfully implemented, this program will represent one of the most significant contributions of research to sustainable Almond production and environmental quality.

For objectives 1 and 2, We proposed to validate our critical level of nitrate concentration by two integrative approaches: 1) a field survey that includes 8 to 10 orchards with different fertilities and N management levels; 2) an experimental approach that will be superimposed upon the existing plots in two mature and one 6 year old orchard currently growing with differential N application (1992-1996) ( 0, 125, 250, 500 lb N/acre/yr). The nitrate-N concentration, % N, tree and nut yield parameters will be determined at various stages. Multiple regression and covariance analysis will be used to characterize changes in leaf nitrate and to relate those to total N and yield components. The critical level of leaf nitrate concentration at each sampling stage associated with optimum yield will be established. The potential use of immature fruit to determine tree N status will be explored and compared to leaf nitrate levels. Nitrate level in leaves and immature fruits at early growth stages will be tested by using a portable nitrate meter (HORIBA Inc.) and Merck color indicator strips and their use as a new criterion for detecting N excess will be determined. The ease with which fruit can be sampled in conjunction with their uniformity and the ease which they can be classified by size and the obtainability of fruit sap in immature fruits make them the best candidate for an "on-site" nitrate test. No such test has ever been done using immature fruits in nut crops, and it is unknown whether almond immature fruit N reflects whole tree N status.

Studies were initiated in 9 commercially managed orchards with 12 trees each and two locations with predetermined N application rates. Leaf nitrate analysis were conducted bi-weekly at all sites and related to yield variables and total N content. The results suggested that both leaf total N and leaf nitrate-N are poor predictors of almond meat production. The lack of response of leaf Nitrate-N to different N application rates resulted in no improved criteria to diagnose plant N status, this is illustrated in Table 1. After extensive consideration this objective was abandoned.

For objectives 3 and 4, the first step in this process was to identify orchards in which to conduct the experiment, to conduct extensive tree sampling to choose optimal experimental trees for tree excavation, to supply adequate N to ensure optimal tree growth in 1997 to prepare and schedule all experimental procedures for 1997. Exhaustive measures will be taken to ensure optimum tree choice and to quantify any tree differences. After tree identification, tree N demand and uptake patterns was determined by conducting whole tree excavations and analysis, this involved 25 entire mature trees. Whole tree N demand and seasonality of uptake was assessed in two complementary approaches that have been developed on the basis of our previous experience in Pistachio.

1) At two months before the commencement of each growth stage ¹⁵N enriched N fertilizer was added to the soil near each of five individual replicate trees. ¹⁵N applications were made in uniform high amounts to ensure maximal isotopic enrichment in the soil and plant. Initial plant and soil N status were determined before the experiment. Five single replicate trees were then excavated at each of four growth phases (dormancy, early spring growth, shell hardening, post harvest). Each tree was divided into six components (roots, trunk, perennial shoots, current shoots, leaves, and fruits) and their dry weights was obtained. Representative samples of each component were then obtained and analyzed for total N and label ¹⁵N concentration. The total amount of N and its distribution among six components was determined based on dry weight and N concentration. The tree N uptake during a specific growth stage is calculated by the difference in N content between that stage and previous stage. This was determined for each organ. The relationship between almond nut yield, total biomass and current growth was then established.

2) A new regressive approach was also used to estimate total and seasonal N uptakes. This method involves taking representative samples of each tree component (leaves, fruits, current shoots, perennial woods and trunk and roots ) at monthly periods from the experimental trees describe in 1 above (five replicates). Seasonal changes in N concentration in each component were then determined. Fruit yield and leaf loss were recorded and all leaves were recovered. N content of each organ was obtained from the product of dry weight and its N concentration. The dry weight of current growth (leaves, fruits and current shoots) at specific time ( T ) was estimated from changes in unit dry weight over the season relative to the each excavation (F).

Yields were obtained for these five trees at harvest and the final dry weight of each component was obtained by excavation. The amount of N present in each component at specific stage was calculated from the measured N concentration and the estimated dry weight at that specific time. Summation of N amounts in all components provides the total N content of the tree at time T. The N uptake at specific stages is calculated by the difference method as in approach 1. This procedure utilizes tree excavation to verify and calibrate the results and helps overcome possible variation caused by different individual trees.

The combination of these two approaches was used to accurately reflect the periodicity of N demand and uptake in Almond. The procedure will also provide information on the partitioning of N in different components of the tree at different growth stages, as well as contribution of storage N to the new growth.

For objective 4, all available information was assembled into a computer based N management program in which growers can input local variables and receive a best management recommendation for N fertilization. Evaluation of the model effectiveness in providing recommendation will be tested in almond orchards throughout the state in subsequent years. Since this model will initially use only nonsite specific plant data it will be broadly applicable. The addition of site-specific data when it is available will improve the precision of the recommendations.

We have coordinated our efforts with those of the BIOS program and have integrated cover crops and manure data into the program. The resulting program is available for industry use though continued refinements are still underway.

Potential Benefits

Orchard crops occupy 35% of Californian cropland, yet they account for 60% of fertilizer N usage in California. Heavy application of nitrogen fertilizer in orchards is largely due to growers desire to hedge against possible yield loss due to N deficiency. Fertilizer management advice for almond has depended on generalized recommendations with little regard for local conditions and only occasional use of leaf N analysis. In general, such practices have resulted in over fertilization, low N-use efficiency and thus contributed to the high level of nitrate in groundwater in many California counties, thus raised the concern of the substainability of the entire production system.

An alternative environmental friendly fertilizer management practice requires the development of better tools for determining fertilizer applications that are essential and those that are excessive. Such practice requires a better understanding of seasonal N requirements and the contribution of all non-fertilizer N inputs into the orchard system. In this project we have developed improved N management practices by determining 1) the amount and time of N application based on plant demand. It is our aim to minimize the practice of applying unnecessary N and to increase N use efficiency in almond orchards. This is based on the hypothesis that N leaching loss from orchard crops is minimized by coordinating N fertilizer application time with the period of highest plant N demand. This information is now available as a user friendly program that guides the fertilization process.

Education and Outreach

This information was presented at the 1999 CDFA fertilizer research conference, the 1999 Almond Board conference, the 1999 California Agronomy Society conference and at a variety of local farm advisor meetings. The program was made widely available at each of these events and we are working on a web based-version.

Table 1 Correlation coefficient of tree meat yield and leaf total N and nitrate-N

Colusa County (Carmel and Nonpareil)

Sampling date	Total N	Nitrate-N
4/3	0.49^*	0.32
4/21	0.21	0.65^*
5/22	0.50^*	0.26
7/22	0.60^*	0.30
9/9	0.55^*	0.10

Yolo County (Mission)

Sampling date	Total N	Nitrate-N
4/11	0.37	0.11
4/28	0.23	0.35
5/13	0.49	0.35
5/28	0.75^*	0.30
6/22	0.64^*	0.30
7/22	0.60^*	0.11
9/4	0.65^*	0.10

* correlation coefficient significant at 5 % level.

Development of a N-Fertilizer Recommendation Model to Improve N-Use Efficiency and to Alleviate Nitrate Pollution to Ground Water from Almond Orchards

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