Sampling for Lab Analyses
Water and soil testing provides information applicable to a range of management decisions. However, a test is only reliable if the sample is taken correctly and is properly handled after collection, and submitted with the necessary information. Irrigation water sampling is simpler than soil sampling.
- To collect a sample, rinse with the sampling water, then fill a 4 – 8 oz. plastic container. Fill to the brim to eliminate air, which promotes calcium carbonate precipitation. Same-day submission is preferred—if stored, refrigerate the sample to minimize precipitation.
- Well water: run the pump for 2 – 4 hours to flush well and to draw from the primary water-bearing strata. If groundwater depth varies over time and is declining, use a portable EC meter to monitor total salinity at 4 -8 week intervals, and submit a sample for lab testing when total salinity increases by 20%.
- Surface water: sample flowing water. Submit first sample for lab testing as a reference point and use a portable EC meter to gain an understanding of salinity fluctuations and to determine the frequency of lab testing.
Soil Sampling Equipment
- Soil probe or shovel; a backhoe or soil auger may be used.
- Clean moisture-proof quart-size bags. Use paper bags only if soil is very dry.
- Clean, plastic bucket to mix samples
- If collecting samples for microbiology, nematode and pesticide residue analysis, have a cooloer on hand, as these samples generally need refrigeration
Soil Sampling Procedure
A soil sample should include at least 15 – 20 samples from each 20 – 40 acre block. A normal sample should include cores from a depth of 12" – 18". When diagnosing a problem or developing an orchard, cores from a depth of up to 18", 36", or 64" may be recommended. For uniform fields (or for an average of a field), samples should be taken from the entire field. Non-uniform fields should be sampled by taking a composite sample from areas with the same characteristics. Take the samples from the rootzone (e.g. within the irrigated soil) following these steps:
- Divide acreage into 20-40 acre blocks, based on the following:
- Soil type
- Crop history
- Tree cv., rootstock, age, harvest quality, growth patterns or symptoms
- Irrigation source and know drainage problems
- Walk a zigzag course through the orchard (or orchard site) taking 20 – 30 samples. Stay away from edges. Take samples at one-foot increments to the depth of rooting, which may require three to five samples from the same spot.
- Remove plant residue from sample spot. Collect one quart of soil and put in a clean quart-size bag (NOT plastic).
- If combining soils from multiple sites, composite by mixing in clean plastic bucket and taking one quart per sample.
- Clearly print on each bag using an indelible marker your identification, the site and sample description.
- Place sample bags in cardboard box and seal. Include a completed work request form supplied by your testing lab.
Work Request Forms
Management recommendations included with a lab report may be based on the grower’s responses to questions, in addition to analysis of the provided sample. Details requested can include cropping history, yields, fertilizer use, depth of soil and water table, water source, and irrigation system and practice. Provided information can include any problems that may have a bearing on the crop. Regarding fertilization recommendations, providing complete and accurate information is essential. Contact a soil testing lab of your choice, then:
- Obtain and complete the Work Request form, to submit with your sample.
- Collect samples.
- Submit samples on the collection day. Refrigerate if delayed.
- When submitting material, be very clear about analyses needed. As an example, when requesting nitrogen analysis, specify TN, TKN, NH4-N, NO3 or NO3-N.