Tree Fruit Soil Fertility and Plant Nutrition in Cropping Orchards in Central Washington
Written by: Bernardita Sallato, Tianna DuPont, David Granatstein, WSU Extension. October, 2018.
Note: Draft publication under review.
Healthy trees need healthy roots to take up water and nutrients; healthy soils to provide water, nutrients, and protection from plant pathogens; sufficient water; and exemplary horticulture. When considering tree nutrition, it is essential to start with regular soil and tissue diagnostics. These tools are often more valuable when used together, when coupled with observations of the tree, and when results are monitored over time for trends.
There are many ways to create a tree fertility program. Here we will describe a strategy with five steps: 1) Know the common nutritional needs for your area; 2) Apply maintenance nutrient sprays; 3) Maintain soil fertility at optimum levels; 4) Fertilize trees based on nutrient demand minus supply; 5) Monitor nutrient management program.
Step 1: Know the Common Nutrition Needs for your Area and How to Best Determine Needs
It is important to be familiar with the type of soil in your region to predict potential deficiencies. In Central Washington, where most Washington orchards occur, the soils are generally relatively young (less than 12,000 years old). These young soils are rich in minerals which become available over time providing phosphorus, potassium, and micronutrients. However, they are also often extremely well drained which leads to leaching of highly soluble nutrients like boron and nitrogen.
The following list briefly describes regional characteristics for major and minor nutrients and how to best determine needs in Central Washington.
pH is often high in Central Washington due to calcareous soils. Use soil analysis to determine levels.
Phosphorus (P) is generally not a problem in Central Washington orchards. Soil mineralization often provides sufficient levels. P-Olsen testing for P availability in Central Washington is accurate to determine phosphorus status.
Potassium (K) deficiency is common in East Coast and Midwestern orchards but is infrequent in Washington orchards. It can appear in older orchards where long-term use of very high-quality irrigation water, coupled with removal of potassium in the fruit crop, has depleted soil K (e.g., the Entiat River valley and similar eastern Cascade mountain valley settings). It can also become apparent on very sandy soils, which are naturally low in potassium, where large amounts of irrigation or evaporative cooling water are used. The relatively small rooting volume of dwarfing rootstocks, particularly in drip-irrigated orchards, also may lead to lower potassium availability than historically documented. Use soil tests, visual examination, leaf analyses, and site history to determine status.
Calcium (Ca) is generally high in many soils in Central Washington since it is a component of caliche or lime (CaCO3) in soils. Depending on its source, irrigation water can also contain appreciable amounts of calcium. However, deficiencies can be observed in sandy and acid soils due to leaching. Soil testing is a useful tool to determine soil calcium levels. Because calcium deficiency disorders (e.g. bitter pit in apples, cork sport in pears) are related to within-tree partitioning imbalances, calcium spray applications are frequently utilized even when soil calcium levels are adequate. Vigor, cropload, and irrigation should be optimized to avoid Ca deficiency disorders.
Magnesium (Mg) deficiency is infrequent in Washington orchards. It can be induced by over-application of lime. An excessive amount of Ca or K in soils may compete with magnesium for root uptake. Use dolomite (calcium-magnesium carbonate) or magnesium calcite (calcite with about 10% magnesium) if Mg is deficient in the soils. Visual symptoms of deficiency are distinctive and tissue analysis can indicate if plant magnesium is low.
Sulfur (S) deficiency has been detected in some orchards in eastern Cascade mountain valleys irrigated with very pure water (e.g., Manson and Wenatchee River valleys). Visual symptoms of sulfur deficiency are similar to nitrogen deficiency. Use soil and leaf tissue testing to assess sulfur needs. Occasional use of sulfur-based fertilizer (Gypsum, ammonium sulfate, zinc sulfate, etc.) will prevent or correct deficiencies.
Boron (B) deficiency will eventually occur in most Central Washington orchards if maintenance boron applications are not applied. Boron deficiency is particularly a problem in pears which need relatively high levels of boron for adequate fruit set. Because boron is very mobile in sandy soils, soil testing often is not accurate where boron levels can be quickly depleted due to the application of irrigation or evaporative cooling water. Fruit trees are normally able to absorb sufficient soil-applied fertilizer boron even if the added boron is soon lost due to leaching. Soil testing can be used to determine toxicity in all soils and deficiency in medium- to fine-textured soils. Leaf tissue boron levels usually give a good indication of tree boron status.
Zinc (Zn) deficiencies are common in Washington orchards. Visual symptoms of zinc deficiency (blind wood and little-leaf) are distinctive. Tissue test levels of zinc can lead to inaccurate interpretation due to contamination by zinc spray residues. In high pH soils, zinc is poorly related to tree zinc status. Regular use of zinc maintenance sprays is recommended in high pH soils.
Copper (Cu) deficiencies are rare but can occur in Washington orchards that have unusually high organic matter levels, such as those planted on former cattle feedlots. Excessive Cu in soils can lead to antagonisms with other metals (Fe, Zn, Mn). Tissue test levels of Cu can lead to inaccurate interpretation due to contamination by leaf residues from pesticides containing Cu.
Manganese (Mn) deficiencies appear to be uncommon in Washington orchards but this may relate to a lack of study. If it does appear, it is most likely to show up in calcareous soils and its visual symptoms may be masked by iron deficiency symptoms. Acid soils or anaerobic conditions can lead to Mn toxicities due to an increased solubility and availability for plant uptake. Tissue testing can be a useful indicator of tree manganese status.
Iron (Fe) deficiencies are common in Central Washington and often are linked to over-watering, perched water tables, and/or calcareous soils. Its presence and severity may vary from year-to-year, depending on variations in rainfall and irrigation water management practices. Soil and leaf tissue test are difficult to interpret and not definitive. Deficiency symptoms, soil physical properties, and soil pH are generally the best indicators of tree Fe status.
Sodium (Na) in high amount can lead to increased salinity in soils. High salinity can affect root growth, water uptake, and plant cell death, while soil sodic content affects soil aggregation and stability. High sodium has been detected in areas irrigated with water from the Columbia Basin Irrigation Project. It has also been seen in areas near Ellensburg and Pasco where the irrigation water is derived from deep aquifers in basalt bedrock. Soil Exchangeable Sodium Percentage (ESP), which relates its content with the Cation exchangeable capacity (CEC), and water analyses are good indicators of high sodium. Water management, orchard drainage, and gypsum amendments can be utilized to reduce Na toxicity effects.
Step 2: Apply Maintenance Nutrient Sprays to meet Regional Needs
Boron. Small annual foliar application is recommended. Optimal timing is during bloom for most tree fruit, and any time of year is appropriate for apples. Keep adequate levels in the soil for root growth. Excess of boron can lead to severe toxicity and application rate should be carefully calculated. Fertigation can also lead to uneven distribution and toxicity (around leaks/junctions) and is not recommended. See the WSU Crop Protection Guide application recommendations.
Zinc. Annual foliar applications are recommended in calcareous or high pH soils. Preferred timing is late dormant (stone fruit), silver-tip (apples and pears), and postharvest (all tree fruits except for apricot). Zinc sprays should be avoided during the growing season unless deficiency symptoms occur. Zinc sulfates are common but can cause tissue damage when temperatures are greater than 85 ºF after the application. Zinc sulfate is also not compatible with dormant oil or lime sulfur. Zinc chelates are also available and are less likely to cause russet. See the WSU Crop Protection Guide application recommendations.
Calcium. In order to reduce calcium-related disorders such as bitter pit in apples and cork-spot in pears, calcium foliar sprays and postharvest treatments are generally utilized in tree fruit. See the WSU Crop Protection Guide application recommendations.
Step 3: Maintain Optimal Soil Fertility Levels
Most nutrients are absorbed primarily through the root system. Soil availability of essential nutrients is the most efficient way to ensure fruit tree growth, with some exceptions (above). Soil analyses can measure soil nutrient levels and be compared to levels where research has shown sufficient availability, deficiencies or toxicities. A soil test will indicate if the elements evaluated are present in adequate, deficient or excessive amounts (Marschner 2002) but will not indicate how much nutrient the plant is absorbing, or if there are other biotic or abiotic factors affecting the orchard. See Appendix A: Soil Sampling in Western Orchards.
Soil tests are excellent tools to determine the levels of nutrients with low to moderate mobility in the soil. In Central Washington orchard soil testing is most accurate for:
Electrical Conductivity (EC) an indicator of salinity
Copper (for toxicity)
In Central Washington, additional interpretation is needed for:
Boron. In sandy soils with high boron mobility, complement with foliar analyses. Soil testing can be used to determine toxicity in all soils and may be useful for interpreting the boron status of medium- to fine-textured soils.
Sulfur. In sandy soils with high sulfate mobility, complement with foliar analyses
Metallic micronutrients (Zinc, Iron, Manganese, Copper). High levels of volcanic ash in some Central Washington soils and alkaline soils can tie up metals and make them unavailable to trees. Complement with foliar analyses or visual symptoms.
Nitrate (NO3) and ammonium (NH4) are the two primary forms of available nitrogen for plants. Both elements can be measured in the soil. These forms correspond to the inorganic nitrogen and they are continually changing. Due to the mobility and variability, their analysis in the soil will only represent the specific moment when the sample was collected and does not correlate well with the soil supply. A better indicator for nitrogen is the organic matter (OM) present in the root zone.
Use soil fertility analysis to ensure that nutrient levels are maintained in adequate ranges (Table 1). Foliar recommendations generally assume that soil contains adequate amounts of nutrients and a healthy root system. When utilizing soil tests, make sure you are comparing the same method and units. The following table provides levels considered optimum for tree fruit and can be used as a guideline to ensure an adequate nutrient supply. Soil testing alone should not be used to determine the nutritional status of the orchard.
See Interpreting soil tests for Washington orchards for additional information.
Table 1 Recommended soil test levels and testing methods for tree fruit.
6.0 – 7.5
1:2.5 / 1:1
E.C 1:2.5 or 1:1
1:2.5 / 1:1
Nitrate _N (NO3_N)d
15 – 40
150 – 250
0.4 – 0.6
4.1 – 20
0.5 – 2.5
10 – 40
1.0 – 1.5
CaCl 0,01 mol/L
9 – 20
0.6 – 1.0
0.6 – 1.0
1 – 5
0.11 – 0.20
Note: mg/kg = ppm. ppm= meq/100g*MW/charge*10. a Methods: Plant, Soil and Water Reference Methods for the Western Region. 2005. Gavlak, R.G., D.A. Horneck, and R.O. Miller. http://www.naptprogram.org b The method has a detection limit of 2.0 mg kg-1 (dry basis) and is generally reproducible to within ±15%. Better to look at in tissue analysis. c Soil analysis for Fe, does not correlate well with Fe availability. d Soil testing is not very reliable in sandy soil due to mobility.
Nutrient availability in the soil is greatly controlled by the cation exchange capacity of soils (CEC), type of clay present and soil mineral content. Within the cation pool (Ca2+, Mg2+, Na+ and K+), when one is excessive in relation to the other, it can lead to cation antagonism. Excessive fertilization, over liming (CaCO3), or sodic soils can promote antagonism and affect nutrient absorption. More common antagonisms are between K and Ca, and Mg and Ca (Marschner 2002). Ratios can be misinterpreted since the same ratios can be expected with adequate, excess or low levels; however, in general, Ca>Mg>K should be maintained.
Step 4: Fertilize trees based on tree demand
The fertilization dose can be calculated as the demand of the tree minus the supply from the soil, water or other sources (Equation 1).
Equation 1. Dose = (Demand – Supply)
For nitrogen, due to its dynamic nature in the soil-plant system, we also need to consider that not everything we apply is absorbed by the plant so we adjust the dose with an efficiency by dividing the calculated amount by the efficiency estimated for the block.
Nutrient demand corresponds to the amount of nutrients that the tree needs for growth and fruit production, including the allocation to fruits, leaf, shoots, roots, permanent wood, etc. By quantifying these nutrient allocations, it has been possible to determine the total demand for different tree fruit crops. For example, Cheng and Raba (2009) calculated nutrient requirement as a function of fruit yield (bushels/acre) based on the nutrient requirement obtained in their study, assuming that the ratio between net accumulation of each nutrient in the whole tree from budbreak to fruit harvest and its amount in fruit remains constant across the range of fruit yield (Table 2).
Table 2. Nutrient requirements as a function of fruit yield for dwarf trees Gala/M9. (Cheng and Raba 2009).
Fruit yield (b/a)
Fruit yield (b/a)
In apples and other tree fruit species, the fruit and new shoots account for more than 90% of total nitrogen demand (Cheng and Raba 2009; Silva and Rodriguez 1995). For P and K, the partitioning is mostly to the fruit and secondarily to the leaf, representing more than 75% of the nutrient demand (Silva and Rodriguez 1995). Assuming leaves and pruning material are recycled in the orchard, fruit extraction generally correlates well with nutrient demand in established orchards. The following table summarizes nutrient demand reported by different authors, for different tree fruit, based on fruit extraction and tree growth in well-equilibrated orchards (Table 3).
Table 3. Tree fruit demands for primary macronutrients per ton of fruit harvested.
lbs N/ US ton
lbs P/ US ton
lbs K/ US ton
1.9 – 2.2
3.0 – 3.9
Cheng and Raba 2009;
Palmer and Dryden 2006;
Silva and Rodriguez 1995.
Silva and Rodríguez 1995
8.3 – 11
Silva and Rodríguez 1995;
Weinbaum et al. 1992.
8.8 – 12
Silva and Rodríguez 1995;
Nielsen et al. 2007;
Fallahi et al. 1993.
4.5 – 12
Silva and Rodríguez1995;
Weinbaum et al. 1992.
1.3 – 2.7
Silva and Rodríguez 1995.
Note: Use the lower end of the range in mature orchards with small amounts of pruning and where pruning material is recycled. Consider higher ends of the range in weak sites, limited areas and when leaf nitrogen shows deficiency.
For P and K, due to their recycling process and mobility, the dose can be calculated directly with the demand (Dose = Demand) when levels in the soil are adequate. See Example 1.1.
For micronutrients (Cu, Zn, S, Mn, Fe and B) the requirements are very small (grams per acre) and normally foliar fertilization is best when leaf analysis or visual symptoms indicate (note: annually for Zn and B). However, it is important to keep adequate levels in the soil as some nutrients are required in the root zone for growth development and signaling.
Apple production of 80 bins/acre = 37 tons/acre
N: 70 to 80 lbs/acre
P: 16 lbs/acre
K: 110 to 143 lbs/acre
Cover crops and weeds will also have nutrient demands.
Once the demand is estimated, the next step is to determine how much the soil or other sources can supply.
Soil mineralization, nutrients in irrigation water, and soil amendments will deliver nutrients to the soil-plant system and should be accounted for and subtracted from the demand.
Phosphorus. Phosphorus in soils is generally bound to soil particles. The weathering of rocks causes the release of phosphate ions which are distributed in soils and water. Soil tests have the goal of estimating the amount of P from both the solution and active pools that the plant can use. Each type of test measures a different fraction of the phosphorus in soils and can only estimate supply when calibrated with regional and crop specific fertility trials. Due to the lack of regional trials in tree fruit the method here relies on ‘replacement’ where once soil levels are brought up to the optimum range fertilizer is applied at the rates needed by plants to maintain soil and plant nutrient levels.
Potassium in soils is in three forms: mineral, slowly available, and solution potassium. Mineral potassium contained in feldspars and micas are only available very slowly over time as the minerals weather. Slowly available potassium is K trapped between the layers of clay minerals which can serve as a reservoir for readily available K. Potassium dissolved in soil water plus that held in the cation exchange capacity of soils (held to clay and organic matter particles) is the potassium routinely measured in soil tests. Again, different soil tests measure a different fraction of the potassium in soils and must be calibrated with fertility/yield trials to estimate fertilizer needs. Due to the lack of regional trials in tree fruit the method here relies on ‘replacement’ where once soil levels are brought up to the optimum range fertilizer is applied at the rates needed by plants to maintain soil and plant nutrient levels.
Nitrogen. The organic matter of the soil will deliver nitrogen throughout the process of mineralization and its level in the soil is a good indicator of nitrogen availability. Organic matter of the soil can supply an average of 20 pounds of available N per year per percent organic matter. Soils in central Washington generally have low levels of OM (1.0 – 2.0%) which may supply only nominal levels of nitrogen. If your soils accumulate organic matter above 1% consider accounting for nitrogen derived from mineralized from organic matter. Keep in mind mineralization rates vary by moisture, temperature and biological activity and are only estimates. It is important to recognize that some soil tests report organic carbon and others organic matter. Organic matter is a calculated value based on the total organic carbon measured in the soil by combustion (Equation 2).
Equation 2. Organic matter (%) = Total organic carbon (%) x 1.72.
Nitrogen from compost and manure applications will also be available slowly over time. Depending on soil temperatures, soil moisture and the C:N of the material, five to thirty percent of the nitrogen in a compost or manure application is available per year. The first and second year after application mineralization rates should be considered after which time the nitrogen will be tied up in organic matter and OM levels can be used to calculate availability. Compost and manure also supply large amounts of phosphorus, potassium and other nutrients, but these levels will be accounted for in any soil tests taken AFTER application. (For more details see Compost Considerations).
Irrigation water considerations. Irrigation water is the largest input of material into central Washington orchards each season. Water contains varying amounts of different minerals, depending on the source (river water, tailwater, well water). Even when present in water at low concentration, large amounts of minerals (some of which are important plant nutrients) can be added over the course of a season or multiple years. Accounting for the addition of nutrients in irrigation water can lead to better fertility analysis and management. Test irrigation water to know what you are adding. Tests at different times of the season may be needed if you suspect water quality may change, or if you switch to different sources. Since soil nutrition is based on soluble forms of nutrients, the quality of the irrigation water has the potential to influence nutrient availability. In areas with elevated nitrates in groundwater or canal irrigation water, the nitrogen addition from well water can be important and lead to a reduction in applied fertilizer N. Irrigation water with high levels of certain compounds such as carbonates may tie up desirable nutrients like phosphorus, and can affect soil pH (Grattan, 2002; Ayers and Westcot, 1985).
Irrigation water quality in Central Washington varies greatly depending on the source. A standard laboratory analysis can give the amount of Nitrogen as NO3-N per liter or parts per million (ppm). For example, if you have 5 mg/L of NO3-N (5 ppm) (Maximum Contaminant Level in water is 10 mg/L), and you irrigate with 35 inches/acre/year, the amount of N supplied in the season is 40 lbs/acre (Example 1.2).
Apple production of 80 bins/acre = 37 tons/acre.
Soil OM = 1.0% = 20 lbs N
Water supply = 35 inches/acre/season (1 inch/acre ≈ 103,000 L/acre)
Water analyses = NO3_N = 1 ppm or mg/L (1 mg/L of water = 0.23 lbs/inch of water)
Then 1 ppm = (1 ppm * 0.23 * 35 inches) = 8 lbs of NO3_N/acre/season
Total supply = 28 lbs of N
Nitrogen demand: N: 75 lbs/acre. 28 lbs of N = 47 lbs of N/acre needed.
The efficiency corresponds to the percentage of nutrient the plant uses in relation to the total amount applied. Nitrogen can be lost due to denitrification, leaching, volatilization, and immobilization (Sato and Morgan 2008). The magnitude of these losses will vary. For example, in deep sandy soils with excessive drainage, leaching could account for the bigger proportion of losses, while in a heavy soil with poor drainage and excess of water, denitrification could become the main way of losing nitrogen.
For example, in western orchards, Nielsen and Nielsen (2002) reported low nitrogen use efficiency, of less than 30%, in a newly planted high-density apple orchard (less than 6 years old) on Malling 9 rootstock.
The efficiency is influenced by many different factors and the percentage can vary under different orchard conditions. In addition to the irrigation and soil type, the method of application, source, timing of application, weeds or cover crops demand and competition, root development and many other factors will determine nitrogen efficiency. Good irrigation water management is essential to good nitrogen management. Table 5 presents a general estimation of efficiencies under different conditions.
Generally, we only consider efficiency for nitrogen. Other major and minor nutrients are held within the cation exchange capacity of soils and not easily lost.
Table 5. Estimated nitrogen efficiency for different soil conditions.
Estimated Nitrogen Efficiency
Fertigation, loamy soils, good drainage, healthy roots. Max efficiency.
Loamy soils, good drainage, well irrigated.
Sandy soils, over irrigated soils.
Sandy soils, coarse soils, excessive drainage, compaction, sick trees, excess nutrient status or, other limitations.
Apple production of 80 bins/acre = 37 tons/acre.
Nitrogen demand: N: 75 lbs/acre. – 28 lbs of N (from OM and water) = 47 lbs of N/acre
Fertigation – Well irrigated – Loamy soil
Assume 70% efficiency
Dose = 47 lbs of N/ acre x 70/100
Final Dose = 67 lbs/acre
Sandy soil with excess drainage. Long hours of irrigation per week.
Assume 40% efficiency
Dose = 47 lbs of N/ acre x 40/100
Final Dose = 118 lbs/acre
Step 5: Monitoring the fertilization program
Once we have developed the fertilization program, nutrient absorption should be verified and monitored on an annual basis. Plant tissue analysis is used to directly measure the amount of nutrients in the trees. For established perennial crops, it is a good indicator of nutrient status (Marshner 2002). For correct interpretation of results, it is important to follow the recommended sampling procedure. For most species, results have been calibrated for samples of leaf that have recently reach maturity, which normally correspond to leaf in the middle of the season growth, between mid-July to mid-August. At this time, the elements are more stable while maintaining good predictability.
For crops with longer growing periods like apples and pears, foliar nutrient analysis can alert growers of nutrient deficiencies or toxicities before they are visible, when corrective action can still be made and before yield or fruit quality are reduced.
Leaf analysis can also be used as a tool to diagnose problems outside of the standard recommended period of sampling by collecting samples from abnormal trees and comparing them with healthy ones. In this case is important to collect leaves of equivalent branches and age.
Tissue Analysis Levels
The following table provides guidance for leaf analysis interpretation. As indicated above, tree leaf nutritional status changes over the course of the year and thus leaf analysis interpretation levels will vary if sampling is not in the more stable mid-July through August period.
In Central Washington orchards tissue testing is most useful to determine status of nitrogen, potassium, calcium, copper and zinc.
Remember that leaf samples can be contaminated by fungicides (containing copper, zinc, etc), foliar nutrient applications (ie calcium) and nutrient containing dust. Samples should not be taken within 15 days of an application containing nutritional elements and caution should be taken in interpretations.
After reviewing a large amount of data in tree fruit species with different cultivars/rootstocks and under different environmental conditions (Shear and Faust 1980; Righetti et al. 1990; Silva and Rodriguez 1995; Reuter and Robinson 1997; Marscher 2002; Cheng and Raba 2009; Palmer and Dryden 2006), the range of recommended levels within a crop in full production, did not differ significantly. The summarized recommended values from different sources are shown in Table 6. Determining the adequate level under local conditions can be accomplished with an integrated analysis utilizing the described tools.
Table 6. Target nutritional ranges used to interpret leaf analysis values for tree fruite.
1.7 – 2.5
2.00 – 3.03
2.7 – 3.5
2.4 – 3.3
0.15 – 0.3
0.10 – 0.27
0.1 – 0.30
0.1 – 0.3
1.2 – 1.9
1.20 – 3.3
1.2 – 3.0
2.0 – 3.5
1.5 – 2.0
1.20 – 2.37
1.0 – 2.5
1.10 – 4.00
0.25 – 0.35
0.30 – 0.77
0.25 – 0.50
0.25 – 0.80
0.01 – 0-10
0.20 – 0.40
0.2 – 0.4
0.20 – 0.40
5 – 12
6 – 20
0 – 16
4 – 16
4 – 16
15 – 200
20 – 60
12 – 50
20 – 50
25 – 150
17 – 160
20 – 200
20 – 160
60 – 120
57 – 250
120 – 200
60 – 250
20 – 60
20 – 60
17 – 60
20 – 80
20 – 70
Adapted from: aShear and Faust 1980; bReuter and Robinson 1997; cRiguetti et al. 1990; Silva and Rodriguez 1995. dPenn State University. eBased on July-August sampling of mature leaves.
If soils levels are adequate and the leaf analysis is below the standards then:
Verify possible absorption problems as: root development, lack or excess of water, biotic and abiotic limitations.
Correct the fertilization dose by adjusting the efficiency, demand (higher value of the demand range).
In some cases, foliar applications can be considered depending on the element and limiting factor.
Low levels of nutrients in the leaf does not always mean that we have low content in the soil and that we need to apply greater amounts. Understanding the scopes, limitations, and utility of the soils and tissue analyses will secure a correct interpretation of these tools.
Soil sampling procedure recommendations can vary slightly from lab to lab. It is important to follow the sampling procedures used to create the recommendations the lab provides to you as they assume specific sampling depths and placement which will affect the results. The following are general soil sampling considerations for tree fruit.
Designate a management unit: Generally, the management units you soil sample are orchard blocks of the same variety and age of trees which you already manage separately on your farm. However, sometimes an orchard block may contain several distinct soil types or tree vigor levels that you may want to sample separately and consequently manage fertility separately.
Take a representative sample: A statistically representative sample would need more than 100 subsamples per acre. As this is not practical, 15 to 20 subsamples per 2 to 5-acre management unit is recommended. Take samples randomly throughout the block (see figure 1). Mix subsamples thoroughly in a clean bucket and remove a representative subsample to send to the lab.
Sample depth and placement: Sample the top 6 to 8 inches in the drip line (area of wetting) (generally the edge of the weed strip). Try to concentrate sampling where the majority of tree feeder roots will be located which will vary by tree age and training system. pH can quickly stratify in soil and it may be useful to sample at two to three depths.
Timing: Soil samples can be taken at any time of year. However, it is good to sample at the same time every year so that you can see trends. Fall tests allow you to have time to make a plan for the following year. However, significant overwinter loss of available N is likely, thus a fall test for N may overestimate what is there for the tree the next season. Spring tests are a starting point for the season and still allow time for action based on test results. Avoid taking samples in one year before fertilizer or compost added, then the next time after a nutrient addition.
Table 7. Steps for collecting a soil sample for shipping.*
Collect 15 to 20 samples from each block at a depth of 8 inches to form a composite sample.
Mix the sample thoroughly and collect a subsample by taking a scoop of soil from different areas of the sample (See lab directions for amount of soil approx. 1 pint.)
Allow the sample to air dry before placing in the bag for shipment. The bag should be labeled with enough information so that it can be easily matched with the tissue sample information for the block.
*From Tissue and Soil Sampling Guide for Perennial Fruit Crops, University of Maryland.
Send to a certified laboratory who conducts the tests appropriate for your region.
WSU recommends working with certified laboratories that participate in the agricultural North American Proficiency Test. The list of approved laboratories can be obtained at http://naptprogram.org. Certain soil tests have research showing they work better is different types of soils and climates. For example, the Olsen P is standard for Central Washington versus Mehlich and Morgan P tests used in the east. When working with a lab outside of your region make sure to select tests appropriate for your soil type.
Appendix B: Leaf Sampling
Sampling Procedure adapted from Temperate Zone Pomology.
Collect leaf samples during mid-July through August.
A single sample should not represent an area larger than 5 acres (2 hectares).
Include only one cultivar or strain in a sample and preferably only one rootstock type.
Mark or map each plant or area sampled for future resampling.
Select leaves from the periphery of trees at shoulder height or higher from the middle of the current season’s terminal shoots of about average vigor.
Collect 10 leaves per tree from shoots randomly selected from all sides of the tree. Select leaves free of disease or damage (unless diagnosing a trouble spot).
Collect fully expanded leaves that are midway on the current season’s growth (Figure 2).
50 leaves per sample are sufficient unless leaves are small.
Remove leaves with the petiole (use a downward pull).
For trouble spots, take a separate composite sample from five affected trees and five non-affected trees and label bags accordingly.
Contaminated samples (by soil, spray, or other residues that would interfere with analysis) should be cleaned with a nonionic detergent solution and rinsed with distilled/deionized water (not tap water). Wash leaves ASAP and quickly (for one minute or more).
Dry leaves at 80oC or air dry. Samples should not be stored in a location that is moist where mold or damage may occur. Samples can be sent in a paper envelope where they will dry en route in dry climates.
Submit the dried sample to laboratory for analysis along with the following information: orchard name and location, date of sampling, soil type, cultivar, fertilizer practice, and special problems. Make sure the samples are labeled in a fashion that the corresponding location in orchard can be found easily.
Timing: Leaf nutrient levels change over the season. Make sure to sample at the same time that the recommendations were created, i.e., mid-July to August.
Crop loads: Heavy crop loads can lead a dilution of mineral content found in the leaves. Light crop loads tend to be associated with low N and high K.
Appendix C: Other diagnostic tools
SPAD meter: A portable hand-held device that rapidly estimates leaf chlorophyll content, which is correlated with leaf nitrogen. Neilsen et al. (2017, p. 254) have a nice table on SPAD correlations for different apple cultivars. Use for leaves.
Cardy meter: Is a portable nitrate meter (not for in field use, but can be used on farm). Need to extract nitrate from leaf or soil first.
X-ray fluorimeter: A non-destructive instrument that can read nutrient levels in leaves and fruit (e.g. Tracer 5, Bruker Mfg., Kennewick, WA, https://www.bruker.com/). It may be useful on all elements in the periodic table from magnesium to uranium, which excludes nitrogen and boron. It is possible to do repeated measurements on growing fruit over time.
Remote sensing: Various soil and vegetation data can be collected from aerial images (airplane, drone) that relate to soil organic matter, water content, and plant vigor. Other tools such as EC mappers can also help characterize site soil variability and the properties involved.
Sap analysis: Some growers are experimenting with sap analysis (sap “squeezed” from fresh leaves) looking at old and new leaves on the same shoot to estimate more real-time nutrient status in the trees during the growing season. This method has been used extensively in greenhouse vegetables but the data for correlation to tree fruit are generally lacking.
With many thanks to Dr. Frank Peryea, WSU Professor Emeritus, Department of Crop and Soil Sciences and Dr. Joan Davenport, WSU Professor, Department of Crop and Soil Sciences for their substantial help and review of this document.
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Frank Peryea. Think Zinc Tree Fruit Research and Extension. Prepared February 1995.
Frank Peryea.Micronutrients in Pacific Northwest Orchards. Prepared in 2001 for the Oregon Horticultural Association.
Ayers, R.S and D.W Westcot. 1985. Water quality for agriculture. Food and Agricultural Organization (FAO) of the United Nations. FAO Irrigation and Drainage Paper 29.
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