Without a soil analysis, it’s nearly impossible to tell what your soil needs to help your crop grow. A laboratory soil analysis, or a soil test, provides information on the capacity of your soil to supply adequate nutrients. This helps you select the correct mix of fertilizer and liming materials, which can help you to develop and maintain your soil and increase crop production.
The following recommendations are based on fertilizer experiments, soil surveys, and results obtained from on-farm trials.
A soil sample can help:
For annual crops, such as vegetables, test soils when you first cultivate a field or change crops or rotations. If you plant successive crops in a single season, you don’t need to test before each planting.
For perennial crops, such as orchards, tree plantations, alfalfa, grass seed, and permanent pasture, the most important time to test the soil is before planting so necessary nutrients can be incorporated into the soil. If you plan to compare soil test results with the results of a leaf analysis, take samples in August. For more information about leaf analysis, refer to Leaf Analysis of Nutrient Disorders in Tree Fruit and Small Fruits (FS 118), https://catalog.extension.oregonstate.edu/fs118.
In high rainfall areas of western Oregon, soils are likely to be acidic and require periodic liming. Testing these soils in the late summer or fall allows time for these amendments to react with the soil before the following growing season.
Soils should be analyzed often enough to recognize potential nutrient management issues before they adversely impact plant growth. In general, test every 2 to 3 years for annual crops, pastures, and legumes, and test every 3 to 5 years for Christmas trees, fruit and nut trees, berries, and grapes. Take samples at the same time of year so results are comparable from year to year.
The area from which to collect a soil sample may depend on the soil type, topography, crops grown, management history, or all of the above. For example, the farm in Figure 1 has three separate sampling areas: A (orchard), B (pasture), and C (vegetable row crops). In this example, a separate soil sample should be collected from each of the three areas. The same concept applies to smaller acreages; for example, a lawn and a vegetable garden should each be sampled separately.
If you are using raised beds, such as for vegetable crops, take your samples in the beds instead of the areas between the beds where there are minimal roots.
Avoid sampling in small areas where you know that conditions are different from the rest of the field (for example, former manure piles, fertilizer bands, or fence lines). You often can spot these places by looking for plants growing especially well or particularly poorly.
Each sample should consist of subsamples taken from 15 to 20 locations within the sampling area (Figure 2).
Use a soil probe (Figure 3) for ease and consistency of sampling. If a soil probe is not readily available, a shovel will work.
To use a shovel, begin by pushing the blade into the soil at an angle to the desired depth (see “Take the soil sample to the correct depth”). You can make a mark on the shovel with a piece of tape as a guide for consistent sampling depth. Next, tilt the shovel back to remove the blade full of soil, being careful to keep the soil intact. At this point, one option is to use your hands or a trowel to remove excess soil from the shovel to arrive at a subsample with approximately equal amounts of soil across all depths. Another option is to use a trowel to remove a thin slice from the face left behind in the soil by the shovel. (Figure 4)
Sample the part of the soil where the plant roots will grow. For most annual and perennial crops, sample from the surface down to about 6 inches (Figure 5) or to the depth of tillage.
For perennial crops such as pastures and orchards or other soils that have limited or no tillage, refer to Evaluating Soil Nutrients and pH by Depth (EM 9014), https://catalog.extension.oregonstate.edu/em9014, for more information about collecting your soil sample. Soils with limited or no tillage can experience significant pH changes in the top 2 inches resulting from the addition of nitrogen fertilizers and lime. However, those changes may be obscured when samples are collected to a depth greater than 2 inches.
Collect samples at the same depth. For example, if you take initial samples at a 6-inch depth, keep that same depth for all future samples to get a more accurate comparison.
Place all of the soil subsamples from a single sampling area in a clean container and mix thoroughly (Figure 6). Do not worry about breaking the sample up into tiny particles. Labs have soil grinders to further mix the sample.
Once you have researched and selected a laboratory, plan to use the same lab for future tests to keep sample analysis consistent and detect changes in soil nutrients. Also, plan to take your soil sample at the same time of year, same depth, and same approximate field location.
Once you have received the analysis results for your soil, use the following tools to make decisions:
You can also consult your local OSU Extension Service agent.
Visit the OSU Extension Catalog at http://extension.oregonstate.edu/catalog/ to find these publications:
Analytical Laboratories Serving Oregon (EM 8677), https://catalog.extension.oregonstate.edu/em8677
Applying Lime to Raise Soil pH for Crop Production (Western Oregon) (EM 9057), https://catalog.extension.oregonstate.edu/em9057
Christmas Tree Nutrient Management Guide (EM 8856), https://catalog.extension.oregonstate.edu/em8856
Eastern Oregon Liming Guide (EM 9060), https://catalog.extension.oregonstate.edu/em9060
Evaluating Soil Nutrients and pH by Depth (EM 9014), https://catalog.extension.oregonstate.edu/em9014
Fertilizing with Manure (PNW 533), https://catalog.extension.oregonstate.edu/pnw533
Fertilizing Your Garden: Vegetables, Fruits, and Ornamentals (EC 1503), https://catalog.extension.oregonstate.edu/ec1503
Leaf Analysis of Nutrient Disorders in Tree Fruit and Small Fruits (FS 118), https://catalog.extension.oregonstate.edu/fs118
Monitoring Soil Nutrients Using a Management Unit Approach (PNW 570), https://catalog.extension.oregonstate.edu/pnw570
Soil Fertility in Organic Systems: A Guide for Gardeners and Small Acreage Farmers (PNW 646), https://catalog.extension.oregonstate.edu/pnw646
Soil Test Interpretation Guide (EC 1478), https://catalog.extension.oregonstate.edu/ec1478
Soil Management for Small Farms (EB 1895). Washington State University Extension, https://s3.wp.wsu.edu/uploads/sites/411/2014/12/Paper_SoilManageSmallFarms.pdf