Time of sampling
Soil samples to determine the availability of potassium, phosphorus, micronutrients and salt content can be taken any time of the year. However, fall sampling is usually preferred as it allows for enough time to adjust the fertilization program for the following year.
Taking soil samples every 3-5 years is usually adequate. In recently planted orchards, annual sampling or twice per year may be done until the soil fertility program is established.
To monitor available nutrients over the years, samples should always be taken during the same season, but preferably in fall.
For soil nitrate analyses, samples should be taken in spring/early summer before the period of high nitrogen uptake by the trees. Samples also to be taken every year, as the nitrate content in the soil is very variable.
Samples need to be taken before fertilizer is applied.
Sampling procedure
Divide each field into blocks based on soil survey data, slope, or cropping history.
Ideally, a field is divided into blocks of 2-5 acres and a composite sample of five cores from each block is taken. When larger blocks are sampled, 15 to 30 cores should be taken from each block for a composite sample.
Cores are taken from the entire area of the field or management area in a W-shaped sampling pattern or by walking a zigzag course around or through the area as shown in Figure.
Figure: View of optimal sampling location under orchard trees. Soil samples are taken within the wetting zone halfway between the trunk and the drip line.
The sample is taken halfway between the trunk and the drip line and within the wetting zone of the sprinkler/emitter.
Sample by foot increments to a depth of 2 to 4 feet or deeper if restrictive layers may be encountered in the subsoil. When diagnosing a problem, deeper cores may be recommended.
Mix the cores thoroughly; remove large stones, pieces or roots and other foreign material.
Sample handling
Collect the samples in a clean plastic bucket. Samples are best taken with a soil probe or auger.
Very wet samples should be air-dried before packaging. Do not dry the samples in an oven or at abnormally high temperature.
Put about one quart of soil in a clean bag and label it clearly. Follow the instructions of the laboratory that will do the analysis.
To receive accurate fertilizer recommendations, the sample information sheet needs to be filled out carefully.
Include the information sheet within the package submitted to the test lab.
Below are Soil analysis Lab results
Understanding and Applying Information from a Soil Test: pH and Saturation Percentage
Soil testing helps understand the orchard soil environment and how to prevent or correct nutrient deficiencies, toxicities, or conditions that affect the availability of water to the trees in a cost effective manner. Soil testing is not a substitute for plant tissue testing, rather it is complementary.
The saturation percentage (SP) equals the weight of water required to saturate the pore space divided by the weight of the dry soil. Saturation percentage is useful for characterizing soil texture. Very sandy soils have SP values of less than 20 percent; sandy loam to loam soils have SP values between 20 and 35 percent; and silt loam, clay loam and clay soils have SP values from 35 to over 50 percent.
The pH of a soil measures hydrogen ion concentration (activity) and is sometimes referred to as soil reaction. Soil pH is closely related to bicarbonate concentration and can influence the availability of nutrients. Soil pH below 5.5 may result in calcium (Ca), magnesium (Mg), phosphorus (P), or molybdenum deficiency and perhaps excesses of manganese (Mn), iron (Fe), or aluminum (Al). Soil pH above 7.5 will begin to immobilize Mn, Fe, zinc (Zn), and copper (Cu) and deficiencies are more likely to occur when the soil pH is above 8.4.
Nitrogen
Nitrogen occurs in soils as organic and inorganic forms and soil testing may be performed to measure levels of either. Nitrate nitrogen (NO3-N) is most commonly measured in standard soil tests because it is the primary form of nitrogen available to trees and, therefore, an indicator of nitrogen soil fertility.
Table 1 provides guidelines for evaluating NO3-N soil fertility levels.
Table 1. Guidelines for interpreting nitrate nitrogen (NO3-N) levels in soil test results.
Fertility Level | ppm | lbs/acre1 |
Low | <10 | <36 |
Medium | 10-20 | 36-72 |
High | 20-30 | 72-108 |
Excessive | >30 | >108 |
Some laboratories report NO3-N as lbs/ac rather than as a concentration (ppm). A soil bulk density is assumed in this calculation so the NO3-N fertility levels should be considered an estimate rather than an absolute level.
Phosphorus
Soil tests are performed to determine the concentrations of plant available phosphorus in soil. The Bray P1 Test is used for neutral and acid soils (pH 7.0 and lower) and the Olsen sodium bicarbonate test is used primarily for alkaline soils (pH>7.0) but can be used on soils with pH >6.5. Table 2 provides guidelines for evaluating phosphorus soil fertility.
Table 2. Guidelines for interpreting phosphorus (PO4) levels in soil test results.
Fertility Level | Bray P1 method
PO4 Concentration (ppm) |
Olsen method
PO4 Concentration (ppm) |
Low | <20 | <10 |
Medium | 20-40 | 10-20 |
High | 40-100 | 20-40 |
Excessive | >100 | >40 |
Potassium
Potassium undergoes exchange reactions with other cations in the soil such as calcium, magnesium, sodium, and hydrogen and this affects the plant available potassium. Table 3 provides guidelines to interpret potassium soil test results.
Table 3. Guidelines for interpreting potassium (K) soil test results using the ammonium acetate method.
Fertility Level | Extractable K (ppm) |
Very Low | < 75 |
Low | 75 -150 |
Medium | 150 – 250 |
High | 250 -800 |
Very High | > 800 |
Orchards growing on soils with extractable potassium concentrations less than 150 ppm in the root zone are most likely to respond to potassium fertilization. Combining soil and plant tissue testing is preferred to monitor trends in potassium nutrition and guide management.
Calcium and Magnesium
Water soluble cations are determined from the saturated paste extract soil test procedure while the exchangeable cations are determined with the ammonium acetate procedure.
Table 1 provides ranges of exchangeable Ca and Mg levels that may be observed in soils.
Table 1. Common ranges in exchangeable calcium (Ca) and magnesium (Mg) in soils in California expressed in three different soil test reporting units.
Element | meq/100 g soil1 | ppm (mg/kg)2 |
Calcium (Ca) | 5 – 50 | 1000 – 10,000 |
Magnesium (Mg) | 2 – 30 | 240 – 3600 |
1The units of meq/100 g soil imply the determination of exchangeable cations with the ammonium acetate procedure. The units for saturated paste extract determinations are expressed as meq/L. 2The units of meq/100 g soil are converted to ppm with the use of the equivalent weight of the cation.
Sulfur
Trees absorb sulfur from soil in the water soluble, inorganic form of sulfate (SO42-). Sulfur (S) containing soil amendments are more often applied to lower the pH of orchard soils above 8.0. Sulfur nutrient deficiency is uncommon in orchard crops of California. Still, S fertility levels are evaluated and reported in soil tests.
Micronutrients
Micronutrients are essential to almonds and other nut crops, yet are required in much smaller amounts than macronutrients such as nitrogen (N), phosphorus (P) and potassium (K) or secondary nutrients such as calcium (Ca), magnesium (Mg), or sulfur (S). The eight micronutrients are boron (B), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), and zinc (Zn). They fulfill important roles in the plant. For instance, zinc is needed for plant cell expansion and it influences pollen development, flower bud differentiation, and fruit set while boron is a building block for the plant cell wall and strongly influences pollen tube germination and growth. Flower abortion in almond and walnut has occasionally been associated with boron deficiency. Nickel has recently been determined to be an essential nutrient and there are no known deficiencies in California.
Zinc, iron and manganese deficiencies commonly found in the San Joaquin Valley. Zinc deficiency is most common in almond and other nut crops. Other micronutrient deficiencies that are occasionally seen in almond include B, Fe, and Mn. Copper (Cu), Mo, and Ni deficiencies have not been documented in almonds; however, Cu deficiency is common in pistachios.
Five of the micronutrients (Cu, Fe, Mn, Ni, and Zn) largely exist in the soil as positively charged metal cations bound as minerals or adsorbed to the surfaces of colloids or soil particles. Soil pH greater than 7.5 has the major influence of reducing the tree availability of Zn, Fe, and Mn in the soil and to a slightly lesser extent Cu.
The micronutrients Cl and Mo generally exist in soil as negatively charged anions. Boron generally exists in soils as a non-charged acid in acidic soils and as an anion in alkaline soils. Chlorine and B have a much higher likelihood of leaching than do the positively charged metal micronutrients. Leaching is more likely to occur in sandy soils, particularly with rainfall or low salt irrigation water. Molybdenum (Mo) exists in minerals and is strongly adsorbed to soils so it does not leach as readily as Cl and B. Molybdenum is different from most of the micronutrient cations in that it increases in plant availability as the soil pH increases.
Table 2 outlines low, medium, and high soil fertility levels for B, Cu, Fe, Mn, and Zn where the crop is not expected to respond, possibly respond, or be highly responsive to micronutrient additions. The micronutrients have different ranges in fertility levels and anticipated responses.
Table 2. Guidelines for interpreting micronutrient levels measured with soil fertility tests on samples taken in the top 6 inches of soil*.
Level of Expected
Crop Response |
Boron (B)
(Hot Water Extract) |
Copper (Cu)
(DTPA Extract) |
Iron (Fe)
(DTPA Extract) |
Manganese (Mn)
(DTPA Extract) |
Zinc (Zn)
(DTPA Extract) |
Soil test level (ppm) | |||||
Highly Responsive | 0.0-0.5 | 0.0-0.8 | 0.0-5.0 | 0.0-2.0 | 0.0-0.7 |
Probably Responsive | 0.5-1.2 | 0.8-1.2 | 5.0-15.0 | 2.0-10.0 | 0.7-1.5 |
Not Responsive | > 1.2 | >1.2 | >15.0 | >10.0 | >1.5 |
Almonds and other nut crops are perennials that can store and translocate nutrients and the root systems tend to be more extensive and make acquiring representative soil samples challenging. If a micronutrient deficiency is in question, comparative soil samples from good and poor areas can give added confidence in diagnosing the problem with soil testing.
Foliar applications of micronutrients may be more economical and efficient for managing deficiencies. Since soil pH, particularly when above 7.5 has such a major role in determining the availability of the micronutrients Zn, Fe and Mn and to a lesser extent Cu, banded acidification of soils with materials such as sulfuric acid and elemental sulfur may be a desirable long term solutions in some orchards. Conversely, liming acidic soils with pH below 5.5 may be viable if leaf tissue and soil sample analysis confirm a problem with micronutrients.
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