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¶ Introduction
Planting an orchard is a large investment. It is important to assess the soil properties before the orchard is established. Assessment costs are much lower than the costs from planting an orchard at an unsuitable site.
The pre-plant assessment should:
- Determine soil properties such as soil pH, soil texture, nutrient availability, or salinity.
- Identify problem areas due to physical barriers that might affect root growth or drainage.
- Identify soil variability in soil properties to develop nutrient management plans.
¶ Sampling Before Planting
Divide each field into blocks based on soil characteristics, slope, or cropping history. Even when a field appears to be uniform, it is worth dividing it into several blocks. Each block should be sampled and analyzed separately.
It may be necessary to divide the field into blocks of 2 to 5 acres each if the field is highly variable. Collect a composite sample of five subsamples (cores) from each block. When the blocks are larger than 20 to 30 acres, collect 15 to 30 cores from each block for a composite sample. Collect soil cores a zig-zag pattern from a 0 to 8inch depth increment.
If possible physical limitations are suspected, deeper samples for observation are suggested. Laboratory analysis of these deeper samples may or may not be required, depending on the situation. A soil probe can be used to collect samples by 12-inch increments to a depth of 3 to 5 feet (or deeper) if restrictive layers may be encountered in the subsoil.
If a soil probe is not available, a backhoe can be used to dig a series of pits to identify textural changes and layers that restrict root growth and drainage. This information helps to identify whether the site is suitable for orchard planting or to determine the best method to modify the soil.
¶ Sampling in Established Orchards
Samples should be collected annually in recently planted orchards. Samples for soil nitrate analysis should be taken in spring or early summer before the period of high nitrogen uptake by the trees. Samples for nitrate analysis should also be taken every year because the nitrate content in the soil is very variable. After the soil fertility program for phosphorus, potassium and other nutrients have been established, the soil sampling can be rotated among the blocks so that each block is sampled every three to four years.
¶ Figure 1. Sampling in Established Orchards Diagram
Collect 15 to 20 cores from each block for a composite sample. Collect one core from each tree. Collect samples from half-way between the tree trunk and the drip line. Collect samples from within the wetting zone of the sprinkler or emitter if the orchard is irrigated.
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.
When diagnosing a problem, it may be necessary to sample by 12-inch increments to a depth of 24 inches. In some cases, it may be necessary to collect samples deeper than 24 inches to identify the problem.
Collect the cores in a clean plastic bucket. Galvanized or rubber buckets may contaminate samples with zinc. When all the cores for an area are taken, mix them thoroughly. Place about 1 quart of sample in the sampling bag and label it clearly, then ship to the laboratory with the necessary sampling information
¶ References
Adapted from Geisseler & Horwat. 2013. “Soil Sampling in Orchards”. http://apps.cdfa.ca.gov/frep/docs/Soil_Sampling_Orchards.pdf