By Sam Wang, Shawna Loper, Mike Ottman, and James Walworth, University of Arizona
Pre-plant soil sampling is critical for profitable crop production. Soil analysis can help decide the pre-plant fertilizer application, whereas plant tissue analysis is appropriate for post-plant N application decisions.
To collect soil samples, use a sampling probe, auger, or small shovel. In a uniform field, a composite sample consisting of at least 20 representative soil cores should be collected from the top six inches of the soil. The more soil cores taken mean the more representative the sample will be for the field.
When the field is not uniform or if the field is larger than 40 acres, divide the field into sections and take soil samples from each section for possible individual management decisions.
Pre-plant soil samples should be collected with adequate time to receive test results to plan your fertilizer program. Typically samples are dried before being submitted to a lab.
After soil samples are collected, spread the soil on clean paper in a clean and dry area to dry for two to three days. Do not dry the samples where the samples may be contaminated (i.e. dust, fertilizer). Do not heat the samples in a kitchen oven because it might alter the soil chemical properties. Be sure to label the sample to identify the field location.
It is a good idea to find out preferred sample handling methods by contacting the soil testing lab. Most laboratories conducting soil and plant analysis in Arizona should return the results in one-to-two weeks.
A full-nutrient analysis includes some soil properties (including pH and electric conductivity), soil macro-nutrients (N, P, K, S, Ca, and Mg), and other plant essential nutrients (including Fe, Mn, B, Mo, Cu, and Zn).
Detailed explanations of soil analysis are included in the publication “Soil Sampling and Analysis.” In most areas in Arizona, growers need to pay attention to nitrogen (N) and phosphorus (P) particularly.
Pre-plant N application is not necessary if the soil nitrate nitrogen (NO3-N) concentration is higher than 10 parts per million (ppm). Applying pre-plant N fertilizer, while there is enough N in the soil for early growth, can result in N leaching out of the root profile.
When soil NO3-N is less than 5 ppm, 50 to 75 pounds (lb.)/acre of preplant N is recommended. When non-legume residue is incorporated before planting, N could be tied-up by soil microorganisms. In this case, add 15 lb. N/acre per ton of residue - up to an additional 50 lb. N/acre.
P fertilizer is not needed if the sodium bicarbonate extractable P (Olsen P) level in the soil is greater than 13 ppm. When soil P is less than 7 ppm, 50 to 100 lb/A of pre-plant P2O5 is recommended. When soil P level is higher than 13 ppm, the grain yield response to P fertilizer is unlikely.
Pre-plant soil testing is especially important since most P fertilizers should be applied before planting and incorporated into the soils.
Deficiencies of other nutrients in small grains have not been documented in Arizona. There is plenty of K, S, Ca, and Mg in most soils and/or irrigation water in Arizona. Application of these nutrients and micro-nutrients should be based on soil or plant tissue analysis results and be economically justified.