Soil testing can provide answers to many of the questions related to pH, salts and plant nutrient availability necessary for the successful growth of plants. Turfgrass, trees and shrubs and flowers and vegetables have different nutrient needs. Information on each of these categories The way the soil sample is collected and cared for, as well as the soil testing laboratory selected can make a difference in the accuracy of the results.

A soil sample must represent a uniform area. Differences in texture (sand, silt, clay), color, slope, degree of erosion, drainage, past management practices (fertilization, crop rotation, etc.) should be taken into account when collecting the sample. At the same time, waterways, terraces, fence rows and unusual spots should not be included in the samples collected for a neighboring field or garden site.. Those areas, however, could be tested separately if a crop is to be planted in those areas.

Collect small quantities of soil from several locations, mix them together thoroughly, spread out to air dry and send or take the soil testing laboratory about 1 pint of this composite mixture. This sample can represent several hundred acres or a small garden plot in the back yard as long as the soil in the area being tested is uniform as mentioned above. Areas in which plants do poorly are best tested separately. If areas appear to be different soil types (i.e. clay vs. sand) or one area appears saltier than another, each area should be tested separately.

Samples can be collected with a soil probe, soil auger, shovel or trowel. The key is to use a clean, rust-free tool to avoid the contamination of the sample with iron. Collect the sample from the soil surface to the depth desired, typically to the plow depth (usually 8 to 12 inches). A clean plastic pail is a good container to place and mix the soil samples in. Avoid using galvanized or brass containers or the sample will be contaminated with zinc. The resultant soil analysis could indicate a sufficiency of these contaminates when a deficiency actually exists.

Many soil testing facilities provide bags to place the soil in. A heavy duty plastic bag that holds a pint of soil can be used if bags are not supplied. The composite sample should be air-dried before being placed in the bag. Spread the soil out in a thin layer on butcher paper, plastic or another clean dry surface. Do not use newspaper as chemicals from the paper and ink can change the test results. If wet soil is placed in an airtight container, chemical changes result that will affect the final test results.

Soil testing facilities use different extraction techniques when testing for plant nutrients. These different methods can provide different results and recommendations. The extraction method used must be considered when making fertility recommendations.

Studies of fertilizer recommendations indicate some commercial soil testing services consistently recommend the use of far more fertilizer than needed. Consequently it is important that the soil testing facility selected participate in a proficiency testing (Quality Assurance) program. This eliminates the guess work of which laboratory to chose by ensuring that the laboratory provides consistently accurate results.

While participation in a QA program is voluntary, such a program is essential for demonstrating long-term performance of accuracy and precision to the lab's clientele. The various QA programs provide participating labs with a rating or percentage of `errors' allowing the laboratory to continue to refine it's diagnostic techniques and accuracy.

The amount of time necessary to process a sample and return the results differs considerably. Some laboratories can provide a 24 hour turn-around while others may take six weeks or more depending on the amount you are willing to pay and the back-log of samples the laboratory has to process. Check with the laboratory if time is a concern.

When selecting a soil testing facility:

1. Choose a laboratory that is a participant in a Quality Assurance program - the laboratory should have a certificate of participation
2. Ask to see the results of the most current evaluation provided by the QA administrator
3. Don't shop around - select a laboratory that meets your needs and stick with that lab.

• Western States Agricultural Laboratory Proficiency Testing Program - Contact: Janice Kotuby-Amacher - Utah State University - (801) 7907-2217

Some soil testing facilities only provide the results of the analysis. Other labs also will make specific recommendations based on the test results for the crop to be grown. Recommendations will be different for various field crops (i.e. alfalfa, field corn, etc.), vegetable crops, turf, flower gardens, tree and small fruits and woody ornamentals. Some laboratories are able to provide recommendations based on the cultivar of the crop and the yield you expect. Alfalfa is a prime example of this ability to provide yield recommendations based on cultivar. Recommendations also will be different based on whether the crop has yet to be planted or is established. For example, the recommendations will be different for an established lawn as opposed to a site being prepared for planting to grass. It is therefore important that you indicate the type of crop and whether it is an established planting or still in the planning process.

If recommendations are not provided by the laboratory talk to your county or state Extension for recommendations. Keep in mind that if the laboratory used is not in a Quality Assurance program, the Extension agent or specialist may not be able to provide accurate recommendations.

Once the soil test recommendations have been made, follow them. Applying more plant nutrients than are recommended can create nutrient imbalances that may adversely affect the plants being grown. If the recommendations are designed to correct a saline or sodic soil problem, and the recommendations are not followed, the problem likely will not be corrected.

for cool-season turfgrass
for vegetables and flowers
for trees and shrubs

to the Soils of Western Colorado Home Page

WebMaster
Dr. Curtis E. Swift, Area Extension Agent, Horticulture
Colorado State Extension
2775 US Hwy 50, Grand Junction, CO. 81503
voice: 970-244-1834
fax: 970-244-1700