WOOSTER, Ohio — Soils can be sampled and tested at any time during the year, but for plants to get the most of soil fertility, nutrients, and fertilizer, sampling should be conducted in the fall.
Maurice Watson, an Ohio State University Extension soil specialist, recommends fall sampling for a variety of reasons.
"Fall soil sampling is often recommended so that more time is available to plan a fertilizer program and to apply lime, if needed. Also, if the sample is taken soon after harvest, there is recent knowledge of the crop yield and any problem areas in the field," said Watson, who is based at the Ohio Agricultural Research and Development Center. "Another good reason for fall soil testing is that excessively wet springs prevent sampling that could have been done in the fall. Generally the pH decreases slightly during the growing season. In some soils, potassium levels may tend to be slightly higher in the spring than in the fall due to weathering of minerals over winter that release potassium."
Watson offers the following tips when growers sample their fields:
• Be aware of sources of variation in the field that can affect soil fertility. "There are two groups: natural variation and variation induced by human activity," said Watson. Examples of natural variation include soil erosion and vegetation growing in the soil. Examples of variation caused by human activity include tillage, fertilizer application methods and the form of nutrient source added.
• Choose the best method for obtaining a representative soil sample. Growers should consider one of three types of sampling methods: random sampling (for fields considered uniform in slope, soil type, management history, and fertilization practices); grid sampling (for non-uniform fields where fertility variation may be large); and zone sampling (where specific areas of a field that require sampling may be identified on a subjective on intuitive basis or from crop yield information). "Remember that the sample is a representation of the portion of the field you are interested in and the test results are only as good as the sample taken," said Watson.
• Determine depth of sampling based on production practices. For conventional tillage, sampling up to 8 inches deep is adequate in acquiring a good soil sample. For conservation tillage, sampling depth should be no deeper than 4 inches. "It is often recommended that two samples be taken for a no-till system: one at 4 inches and one at 8 inches," said Watson. "The pH is measured on the shallow sample, and the pH plus plant-available phosphorus and potassium are measured on the deep sample. It is important to determine the pH of the shallow depth in no-till systems because excessive acidity may alter herbicides' effectiveness."
• Keep sampling equipment clean and well maintained. "It is important that the sampling tools be kept clean between each sample taken. Also, the sampling probe should be constructed of stainless steel, especially if the soil is to be tested for micronutrients," said Watson. "It is important to use clean plastic buckets for collecting the soil sample cores to prevent contamination of the soil sample. Avoid sources of contamination such as cigarette or pipe ashes and dirty bench surfaces if the samples are spread out to dry, and be sure to accurately label each sample so they do not become mixed."
Generally, it is sufficient to test the soil once every three years. However, for soils that are intensively cropped or are used for high value crops, it is important to test more frequently. Additional information on soil testing can be found on OSU Extension's Ohioline at: http://ohioline.osu.edu/hyg-fact/1000/1133.html, http://ohioline.osu.edu/b760/b760_7.html, or http://ohioline.osu.edu/hyg-fact/1000/1132.html.