A soil test provides information on the basic fertility of soil. It is the starting point for determining how much and which fertilizers to use on a crop. Without a soil test, everyone is really just guessing at what the soil fertility level really is.
How to Collect Soil Samples
Proper collection of a representative soil sample is important for accuracy and analysis of test results.
Follow these steps to obtain a good sample:
- Decide if your field can be treated as one sample or needs to be broken down into separate smaller samples. If you believe the soil type, previous crop and fertilizer treatments are consistent across the field, treat it as one sample. If soil type and topography change across the field, different crops have been planted on different parts of the field, or there are problem spots, break the field down into smaller units to sample.
- Using a soil probe, dig vertically to a depth of 4 inches for established fields such as brome, alfalfa or a no-till field. Fields that are worked up should have samples taken from 6 inches. Avoid sampling in old fence rows, dead furrows, low spots, feeding areas, and other areas that might give unusual results. If information is desired on these unusual areas, obtain a separate sample from the area.
- Take at least 10 – 15 samples from the field and mix the samples together in a clean container to create a representative sample. The more sub samples you take, the more assured you'll be that soil test results are representative of your field. Bring approximately two cups of the mixed soil to the Douglas County Extension Office in a paper bag.
- Samples should be dry. You can let samples air dry, but do not use heat to dry your samples.
Bring or mail the soil sample(s) to:
Douglas County K-State Research and Extension,
2110 Harper St.
Lawrence, KS 66046
Office hours: Monday – Friday, 8:00 a.m. – 5:00 p.m.
We will need the following information:
- Name, complete mailing address, e-mail and phone number
- Sample depth
- Previous crop and its yield
- Intended crop and a yield goal (indicate if this is a new seeding for pastures)
- If pasture, classify as brome, fescue or native/prairie.
The sample will be sent to the Soil Testing Laboratory at Kansas State University for analysis. Their results will be analyzed by our Agriculture agent and a report will be mailed to you. The average time required to complete the analysis is 2 – 3 weeks. During the spring season, this turn-around time may take longer due to the increase in samples volume. Please take this into consideration in your planning.
Soil Test Interpretations & Fertilizer Recommendations
K-State Soil Testing Lab
Many different analyses for soil samples are conducted at the KSU Soil Testing Lab. The most common analyses requested include tests for pH, phosphorus (P), potassium (K), nitrate (NO3), and organic matter. However, other analyses are performed on a daily basis including: calcium (Ca), magnesium (Mg), sodium (Na), cation exchange capacity (CEC), iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), ammonium (NH4), sulfate (SO4), chloride (Cl), aluminum (Al), texture, soluble salts, and salt alkali.
Samples that come into the lab are dried in an oven overnight and ground with a pulverizing-type grinder the next morning. The bulk of analysis will be completed after grinding or the following day. Once the data is complete and has been reviewed, sample information sheets are printed and mailed to the producer. Samples are kept on storage shelves for a time so samples may be available for additional analysis at the producer’s request.
The various tests available from the soil testing laboratory are described below. Read the description of each test and mark on the information sheet which tests you desire on your samples.
This test is recommended where crops grow normally, but you desire to know the amount of lime, if any, and the kind and amount of fertilizer needed for optimum plant growth. The test includes determinations for pH, lime requirement on those samples with a pH of 6.4 or less, available phosphorus and exchangeable potassium. On a majority of the soils in Kansas, this test will be adequate. The results of the general fertility test may be supplemented by one or more of the tests listed below.
This test is recommended on areas that are suspected of having appreciable residual inorganic nitrogen (nitrates). Continuous heavy applications of commercial nitrogen fertilizer and/or heavy rates of manure may result in residual available nitrogen, especially where yields have not been proportional to the nitrogen applied. Under summer fallow conditions, appreciable quantities also may accumulate. Because nitrates are water soluble, movement of the nitrates down in the profile will occur when moisture penetrates into the soil.
Thus, sampling for the available nitrogen test should include subsoil samples to depths deeper than the tillage layer. A surface soil sample (0 to 6 inches) plus a subsoil sample (6 to 24 inches) should be taken. The samples must be air dried as soon as possible to stop microbial activity. This means spreading the sample on a clean sheet of paper or plastic to dry before sending the sample to the laboratory. Caution: Be sure the sample is not contaminated by fertilizer dust, manure, salt, etc. Samples should be dried within 24 hours.
This test is recommended on areas with high yield potential for corn or soybeans, which by erosion, terracing, or leveling for irrigation have had the topsoil removed. Sandy soils, low in organic matter, under high yield conditions also should be checked for zinc. Wheat, alfalfa, grain sorghum, and pastures are not likely to respond to zinc so this test is not recommended for these crops.
This test is recommended on the calcareous soils of the western part of the state to determine the likelihood of iron chlorosis on grain sorghum, soybeans, and corn. The test also can be beneficial for selection of shrubs, etc. around homes.
KCl Extractable Aluminum
Extremely acid soils (pH of 5 or less) may contain appreciable KCl extractable aluminum, which is very toxic to plant roots. This test is recommended for use where extremely low pH's are found and lime is not immediately available for spreading. The test results also are helpful in diagnosing problems of poor plant growth.
The majority of the sulfur in the soil is in the organic fraction and is microbially mineralized to the sulfate form for plant utilization. For interpretation of the sulfate-sulfur test, soil organic matter and texture also need to be known. Therefore, the organic matter test needs to be requested and texture reported on the information sheet. Sulfur deficiency is most likely to be found on low organic matter, very sandy soils. Samples should be to a depth of 24 inches.
Available Profile Chlorides
Wheat has been shown to respond to chloride application. Because the chloride ion is quite soluble and, therefore, mobile, soil samples to a depth of 24 inches is recommended. Chloride research with crops other than wheat is in progress, but no interpretation of profile chloride results are available for any crops other than wheat.
Organic matter in the soil is the storehouse of most of the nitrogen, sulfur, and several of the micronutrients. Organic matter results can be very meaningful to agronomists in better understanding the soil conditions on your farm. Organic matter levels can be useful in determining rates of herbicides. This test is not recommended on all samples, but only in cases where the additional information is desired.