Soil and Tissue Fertility Testing: Get the Most out of Your Orchard Crops
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.
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.
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.
• Avoid anomalous areas within the cropped area.
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:
INTERPRETING SOIL AND TISSUE RESULTS
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 (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.
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.
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