Editor’s Note: Patrick Brown is a pomologist and professor of plant nutrition at the University of California (UC)-Davis. Also making major contributions to this report, a full version of which is posted on the Almond Board of California’s website, were: Sebastian Saa Silva, Saiful Muhammad, Blake Sanden, and Emilio Laca (UC-Davis and UC Cooperative Extension, Kern County).
Efficient and profitable nitrogen (N) application demands that N be applied at the right rate, with the right timing, and in the right location, so that productivity is maximized and the potential for N loss to the environment is minimized. The goal of N management is to apply adequate but not excessive amounts of N. You cannot enhance orchard productivity by providing N in greater amounts than is demanded by the crop.
With proper management, optimal productivity and minimal N loss can be achieved simultaneously. To help growers achieve the goal of efficient and profitable nitrogen application, a new method of tissue testing and yield-driven fertilization has been developed. The following approaches are based on four years of research at multiple sites and were validated in additional trials in 2012.
For mature almonds (more than seven years old), nut yield in the current year is the primary determinant of N demand. The amount of N that will be removed from the orchard for a given yield ranges from 50 to 75 pounds (lbs.) N per 1,000 pounds of kernel yield, depending on the N status of the tree. In four years of experimentation at multiple sites, the ideal N removal rate averaged 68 lbs. N per 1,000 lbs. of kernel yield. This removal rate corresponds to maximal yield and optimal use of N resources, and coincides with a whole-fruit N% of 1.8%. (Note: This conversion stated as kernel pounds also factors in the N removed with shells and hulls to equal the “total fruit” N removal).
Higher fruit N removal rates (more than 68 lbs. N per 1,000 lbs. kernel) occur when trees have received N in excess of demand. The amount of N required for vegetative growth in a yielding tree is small in contrast to that required by the fruit, and averages 20 to 40 lbs. per acre per year in orchards with 70% or greater orchard light interception.
The amount of N required (from fertilizer or other amendments) is determined by crop size (yield x 68 lbs. N per 1,000 lbs. kernel yield) less N supplied from water and other N sources including manures, composts, nitrogen fixing cover crops, etc. Previous N applications in excess of crop N removal can also enhance soil and tree N reserves, thereby reducing current fertilizer N demand.
Nitrogen in irrigation water is an excellent source and free N “fertilizer” and should be included in your total annual N budget. The supply of N (lbs. per acre) from water is calculated by multiplying nitrate concentration in water (ppm) x acre feet irrigation applied x 0.61. If the N concentration in irrigation water is reported as Nitrate-N, then the supply of N (lbs. per acre) from water is calculated by multiplying Nitrate-N concentration in water (ppm) x acre feet irrigation applied x 2.73.
Our ability to estimate the contribution of soil N supply to orchard N demand is limited due to the extensive rooting depths of tree crops and complexities in determining the rate of N availability. A general guideline suggests that if soil nitrate exceeds 10 to 15 ppm, then N fertilization can be significantly reduced. Leaf tissue analysis in April provides information on the general availability of soil N and tree reserve N, and can be used to adjust in-season fertilization.
In the N management approach proposed here, growers establish a preseason N fertilization plan (rate and in-season distribution), based upon predicted yields and N contributions from water and other sources. April tissue sampling and early-season yield estimation are then used to optimize the annual N fertilization plan by adjusting the May through July and/or fruit maturity/postharvest fertilization rates accordingly.
In years of lower-than-expected yield with adequate April tissue N analysis, a reduction in mid-season N fertilization is suggested, while higher-than-expected yields might require an increase in N applications. The goal of this approach is to ensure N fertilization rates are more closely matched to individual orchard productivity in the current year.
Efficient fertilization and N management require that crop nitrogen demand is satisfied, and N is applied coincident with root uptake. The dynamics of N accumulation in annual tree structures (leaves and fruit) and perennial tree structures (roots, trunk, and branches) were determined in a series of experiments conducted in high-yielding orchards throughout California from 2008 to 2012. The pattern and rate of N uptake from the soil can be derived from analysis of N accumulation in fruits (hulls, shells, and kernels) and leaves, and N depletion and accumulation in perennial organs (trunk, branches, and woody roots).
In the period from dormancy (January) through early leaf-out, the tree depends almost entirely upon N that is remobilized from perennial organs, and essentially no N uptake occurs from the soil. Following flowering, during the period of leaf and fruit expansion, uptake from the soil commences while remobilization of N from perennial tissues continues. During the period from full leaf expansion until early hull split, tree N demand is satisfied entirely by soil N uptake.
Following fruit maturity (hull split), tree N demand and root uptake decline rapidly, and stop completely as soon as leaves commence senescence. While fruit is developing, the rate of soil N uptake (lbs. per acre per day) is directly determined by the yield of the tree. The demand for N to supply new tree growth in a mature orchard is small in comparison to the demand of the fruit, and current estimates suggest it does not exceed 40 lbs. per acre per year in a mature orchard. Nitrogen in flowers, leaves, and perennial storage N is predominantly provided from internal and soil N recycling and, hence, does not contribute to annual fertilizer N demand.
Theoretically, N fertilizer should be applied at a rate and timing that are coincident with the demand curve by using very frequent or even continuous fertigation. Frequent fertigation with smaller amounts of N ensures soil N concentrations are always adequate for plant uptake while reducing the periods of high N concentration that may be subject to leaching loss in subsequent irrigation or rainfall events.
Continuous fertigation, however, is often not practical as most growers do not currently have the facilities, water delivery schedules, or engineering control to implement such a program. Furthermore, continuous fertigation may not be necessary, as the majority of mature orchards have relatively extensive root systems, which combined with low rainfall in season and careful irrigation strategies, means that N can be stored in the soil for subsequent root uptake. Therefore, short-term N excess will not substantially increase the potential for N loss by leaching below the root zone. Thus, less-frequent fertilizations can be used effectively if irrigation and fertigation are well managed.
The challenge of retaining N in the root zone is greatest in orchards grown in light-textured soils, particularly where water moves below the root zone due to rainfall or irrigation management, or under conditions that develop a restricted root distribution. In these situations, it is very important to minimize the amount of residual N in the profile prior to leaching events (water application or rain greater than root zone soil-water holding capacity).
Feed When Hungry
Irrespective of the irrigation or fertigation system available, at least 80% of nutrients should be applied during the active tree growth period commencing in early spring (after leaf-out begins) and continuing through early hull split. A minimum of two fertilization events and, ideally, four or more should be initiated during this period, with the amount of each application proportional to the demand of the crop. An additional 20% of annual fertilization can be provided during the period after hull split through early postharvest while leaves are still healthy. However, this decision should be made based upon the current-year yields, prior N fertilization rates, and July leaf N values.
All decisions on fertilization will be influenced by the 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. Similarly, in areas with substantial rainfall soon postharvest, growers must adopt practices that minimize the amount of N that resides in the soil at that time. Pre- and postharvest 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. However, the implications of immediate pre-harvest foliar N on disease and vegetative regrowth in well-managed trees are not well studied. An online spreadsheet has been developed to provide orchard-specific fertilization guidelines, and a smartphone/tablet application is under development.
Since yield estimates will not be available before mid- to late April, the primary opportunity for in-season fertilization rate adjustment is the period from late April to May through postharvest. An April to May yield estimate coincident with receipt of an April leaf analysis can be used to adjust fertilization rates for the remainder of the year to ensure efficient fertilization strategies.
To optimize the use of N fertilizer in almonds, fertilizers must be delivered and present in the root system when they are most likely to be used by the plant. Nitrogen in the soil moves easily with irrigation water, hence the application of N in a large single dose during times of limited plant growth exposes that N application to movement below the root zone.
Smaller applications applied frequently and timed with periods of plant demand limit the potential for N loss. The uniformity of your irrigation system will define the uniformity of N application.
If portions of a particular orchard differ significantly in soil characteristics or productivity, it may be necessary to subdivide the fertigation system to meet site-specific water and fertilizer demands, or to consider applying a portion of the annual N demand in a site-specific ground or foliar application.
Check Almond Leaves Now
The current practice of sampling leaves in July is too late to allow for current-season adjustment of fertilization practice, and leaf sampling alone does not provide sufficient information to make fertilizer recommendations. An improved method of leaf sampling and fertilization management has been developed that utilizes April leaf sampling and yield estimations to predict N demand and to allow for in-season fertilizer adjustments.
The following leaf-sampling method recognizes that growers generally collect one combined leaf sample per orchard, and is effective in orchards of average variability. If the orchard to be sampled has substantial variability, then the sampling protocol should be repeated in each zone, and N should be managed independently in each of zone. Management of N in each zone can be achieved through separation of fertigation systems or by supplemental soil or foliar fertilization in high-demand areas. Efficient management of N requires that every orchard that differs in age, soil, environment, or productivity should be sampled and managed independently.
To follow the UC-Davis Early-Sampling Protocol, or “UCD-ESP,” for each orchard/block or sub-block that you wish to have individual information on, do the following:
• Sample all the leaves of five to eight non-fruiting, well-exposed spurs per tree at approximately 43 days after full bloom when the majority of leaves on non-fruiting spurs have reached full size. In the majority of California orchards, this corresponds to mid-April. Should sampling at this date not be possible, then please note the date of the sample collection on the sample bag.
• Collect leaves from 18 to 28 trees per orchard. Combine all leaves in a single bag for submission to a reputable laboratory. EACH SAMPLED TREE MUST BE AT LEAST 30 YARDS APART. A minimum of 100 leaves per sample bag is required.
• Send the samples to the lab and ask for a full nutrient analysis (N, P, K, B, Ca, Zn, Cu, Fe, Mg, Mn, S) and application of the UCD-ESP program.
• These techniques have been validated only for the Nonpareil variety in orchards that are at least eight years old. If other cultivars are used, please note which cultivar was sampled on the sample bag. Method development for other cultivars is under way. However, this current approach will result in valuable information for any cultivar, as cultivar-specific nutritional requirements likely do not vary significantly.
• Repeat for all orchards and orchard regions that differ in productivity, age or soil type. Identify your areas of low performance, and collect samples from them independently.
• Label all samples well with collection date, field number, cultivar, and within field location if needed. Please note if foliar fertilizers have been applied.
Editor’s Note: For information on data interpretation and integration, please see the full report on the Almond Board of California’s website.