Utah Pests News Spring 2008

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Soil and Tissue Fertility Testing: Get the Most out of Your Orchard Crops 

Authors Grant Cardon, Extension Soils Specialist, and Brent Black, Extension Fruit Specialist, are in the College of Agriculture at Utah State University.

Good decisions begin with a strong base of information—any other way is simply trial and error.  Years of experience working with a particular soil and crop may get you by in any given year, but many conditions resulting in long-term productivity or crop longevity problems can be avoided with regular nutrient monitoring and adjustment of fertility management practices.

Soil fertility testing is recommended no less frequently than every other year in perennial crops like orchards.  The results allow one to monitor changes in soil fertility level, pH, organic matter content and other important soil conditions that affect not only annual production, but also root system health, winter survival and spring-time recovery of the plants, and soil physical conditions (aeration, compaction, etc.).

For orchard crops, in-season tissue testing is also an option.  Tissue testing allows the grower to monitor levels of nutrients at important stages of plant growth and fruit development.  This allows for mid-season correction of deficiencies that may occur as a result of differences in environmental conditions, fruit load, age of the trees in certain areas of the orchard, or other in-season factors.

The USU Analytical Laboratory (USUAL) offers a wide range of testing services designed to address the routine and not-so-routine needs for information.  A complete list of services for plant, soil and irrigation water testing, along with on-line forms for submitting samples, can be obtained at: www.usual.usu.edu.

SOIL SAMPLING

One of the most important aspects of soil testing is proper soil sampling.  The basic concept behind a good soil sample is to form an adequate “physical” average representing the area being tested through proper sample compositing.

Figure 1. Zigzag, random sample pattern.

Compositing soil samples involves taking soil from multiple, representative locations within the cropped area, mixing the soil together, and then collecting a sub-sample for testing.  Done properly, sample compositing results in a physical average of the soil conditions within the cropped area using a single soil sample.  A well-composited soil sample can adequately represent up to 25 acres.  Here are a few tips for making sure that the composite sample is representative of the area in question.

• Take soil from the proper depth.
For orchards, the most active root zone occurs in the top two feet of soil. In general, soil samples should include soil along the length of that two-foot zone, including surface, one-foot, and two-foot depth.  Taking soil only from the surface will not properly reflect the soil conditions experienced by the plant. In all cases, use a sampling tool that is least invasive of the root zone (causes the least amount of disruption to the roots).  Sampling probes can generally be checked out from your local USU Extension office.

• Choose an adequate number of sample locations in a zigzag, random pattern.
The number of separate soil samples included in each composite sample should be no less than 5-10 for small areas (less than ¼ acre), 10-15 for mid-sized areas (up to 5 acres), and 20-30 for large areas (no more than 25 acres).  A zigzag, random pattern covering the whole cropped area should be used when selecting a sample location (see Figure 1).  In orchards, most of the sub-samples should be taken from the outside edge of the drip line of the trees, inward toward the trunks. 

 • Clean sampling equipment between samples.
Some effort should be made to reduce the potential for cross-contamination between soil samples that can result from soil being left in or on the sampling equipment.  Brush or scrape off any soil residue between sampling locations.

• Collect a uniform volume of soil from each sample location.
This is most easily accomplished using a tube-type sample probe (see Figure 2) or by trimming samples taken with a shovel or spade (see Figure 3).

           
  Figures 2 and 3. Tube-type soil sample probe (left); sample collection using a shovel (right).

• Avoid anomalous areas within the cropped area.
Low spots, areas of different soil texture, or other anomalies in the condition of small areas within the crop should not be included in the composite sample. If these areas are large enough that they warrant separate management, then sample these separately as an area of their own, and submit a composite sample representative of the anomaly.

TISSUE TESTING

Much like soil sampling above, the most important characteristic of a good tissue sample is that it be representative of the general condition of the area in question.  To determine a nutrient problem, sampling should occur when the first symptoms are expressed.  To determine general nutrient sufficiency at given stages of growth, sampling should occur two to three weeks in advance of that stage to allow for sample processing and timely correction of fertility conditions if necessary.

To collect and send tissue samples:

1. Collect 40-50 leaves from at least 10 plants.
2. Choose leaves with similar appearance and avoid those that may be discolored due to other factors. Select the most recently expanded leaves first.
3. Rinse leaves, gently dab dry, and allow to air dry for several minutes. Over-drying can alter the eventual analysis so simply ensure that surface water is removed before mailing.
4. Send samples immediately to the lab. Generally, mail services are rapid enough that samples will arrive in proper condition if sent in a paper envelope. Plastic bags or mailers can be used, but can cause condensation and possible molding of the tissue sample during shipping.

INTERPRETING SOIL AND TISSUE RESULTS

Nitrogen

The dynamic cycling of nitrogen (N) and the competition that occurs for this nutrient between all living things in the soil system, require annual management according to the prescribed needs of the plant.  Nitrogen fertility is best managed on field history and tree age rather than on soil or tissue test levels.

However, more is not always better, especially for nitrogen. In orchard crops there is a trade off between vegetative growth and fruit yield.  Too much vegetative growth as a result of excessive nitrogen levels, will reduce fruit set and yield.

In general, typical nitrogen needs for fruit crops are between 0.01 to 0.04 lbs N per tree, per year of age, with a limit of 0.3 lbs N per tree.  Vegetative growth is the primary indicator for adjustment of N application rate.  New growth in younger trees should be between 16 and 24 inches, on older trees it should be 12 to 15 inches.  If the growth is greater than this, adjust the N rate down in subsequent additions and conversely if the rate of growth is too low.

Phosphorus and Potassium

The need for these nutrients is best determined by soil testing.  The level of these nutrients in the soil doesn’t change as rapidly as that of nitrogen, so their management is more effectively monitored using soil testing and periodic tissue sampling for sufficiency.

Phosphorus (P) is critical to root growth and function and the proper cycling of energy in the plant.  Hence its sufficiency at the time of planting a new orchard, or renovation of orchard sections, is important for seedling establishment.  Sufficient P should be applied and incorporated within the root ball area of new trees before planting.  In older plantings, excess P can cause imbalances in the uptake of zinc (Zn) and iron (Fe) and adjustment is best made on soil test levels.  Mid-season adjustment of P levels in soils is generally not practical, so providing adequate levels at the beginning of the season is the best strategy for management.

Potassium (K) is critical in the water relations of plants and in the assimilation and cell-to-cell transfer of other nutrients, particularly calcium (Ca) which is so important for fruit quality, particularly in pome fruits.  Levels of K in Utah soils are regulated by the weathering of clay minerals and are generally sufficient without fertilizer application.  However, on sandy or gravelly soils low in clay content, K deficiencies do occur and will often be expressed by Ca or other micronutrient imbalances in the plant.  In-season adjustment of K nutrition is possible with foliar sprays of potassium chloride or potassium sulfate solutions.

Adequate soil test levels of P and K for fruit trees in Utah are 10 to 30 parts per million (mg per kg of soil) for P, and 75 to 400 ppm (mg per kg soil) for K.  The summer-time tissue sufficiency level for P for most of the fruits grown in Utah is between 0.15 to 0.40 % (on a dry weight basis).  Summer-time tissue sufficiency levels for K vary with crop and are generally about 1.20 to 1.90 % for apple, 1.00 to 3.00 % for cherry, and 1.50 to 2.50 % for peach.

Micronutrients

Sulfur (S) deficiencies in Utah are not very common.  There is generally sufficient sulfate dissolved in irrigation waters to supply sufficient sulfur.  However, deficiencies do occur on sandy and gravelly soils that do not accumulate and store sulfur efficiently.  Sulfur can be added with many of the other nutrients such as K, Zn and Fe which are often applied as the sulfate salts of these nutrient elements.  Under low S test levels (below 8 ppm or mg/kg soil) 25 to 50 lbs S per acre should be applied.  Tissue testing for sulfur is not generally effective.

 

For the micronutrients such as Zn, Fe, Ca and boron (B), soil test levels are not always adequate indicators of sufficiency.  The cycling and release of these nutrients is so heavily affected by in-season soil temperature, moisture and pH conditions that plant tissue testing is often the best indicator of sufficiency.  All of these nutrient elements can be effectively delivered as foliar sprays of zinc and iron sulfate solution, calcium chloride solution, and sodium borate or boric acid solutions.  The summer-time tissue test sufficiency levels for these nutrients are given in Table 1.
 

-Grant Cardon, Extension Soil Specialist 
  -Brent Black, Extension Fruit Specialist