Soil Fertility Summary Emphasizes the Need for Soil Testing

Published: April 9, 2014

The MU Soil Test have been flooded with soil samples from early March. Spring is the time when normally the labs receives heavy loads of soil samples for testing. Even though fall sampling is ideal for farmers as it gives the starting point to plan for next year’s nutrient management plan, we have many who wait until spring to test their soils. The 2013 growing season was dominated by anomalous weather conditions for much of the spring and summer, and made it another challenging year for Missouri farmers. The cool, wet weather during April delayed spring tillage and planting opportunities across the state resulting in pushing behind the harvest dates. The severe winter weather conditions during the winter made it difficult to sample the fields until early March.

The soil fertility summary provides a valuable index of the soil fertility status of Missouri farmland and identifies broad soil fertility trends in the state. The trends in soil fertility status summary in the state for 2013 emphasizes the importance of soil testing (Fig.1 and Table 1). Out of the total of 17373 field crops samples tested by the MU soil testing labs in the state during 2013, about 26% tested very low to low in soil pHs (less than 5.3) indicating lime should be applied for economically viable crop production. Another 37% of the samples received tested medium in soil pHs (5.4 to 6.0) is likely to need lime to avoid profit loss. For example the desired soil pHs range for alfalfa and row crops is between 6.1- 6.5. The lower soil pHs will hinder alfalfa establishment and nodulation. The statewide trend in soil P indicated 42% of the samples tested low to very low, and P fertilizer is essential to avoid profit loss by crops. Another 24% of the P tests were medium (23 to 45 lbs of P/ac) indicating P fertilizer is required for economic crop production. The desired soil P levels for row crops, small grains, and alfalfa are 45 lbs/ac and for forages are 40 lbs/ac. The majority of soils (39%) in the state tested medium in soil K (111to 220 lbs/ac) and 16% tested low to very low (less than 110 lbs/ac) and indicating K fertilizer will be required to avoid profit loss by crops. Fertilizer response to high and very high P and K testing soils are unlikely. However, may need maintenance requirements at the high levels depending on the soil test K levels.  In Missouri the soil organic matter (OM) tests are used to estimate N availability in soil. The N credit from soil OM varies depending on soil texture. A general rule of thumb is every 1% of soil OM in the soil will release about 20 lbs of N/ac for crop. Majority of the soils tested (47%) had medium levels of soil OM (2 to 2.9%). (Table 1).

Fig. 1: Statewide Soil Test Summary of Soil pH, Bray 1 Phosphorus (P1), and Potassium (K) Distribution of Samples. Received by the MU Soil Testing Labs for Field Crops, 2013.

VL, LO, ME, HI, VH refers to very low, low, medium, high and very high ratings as given above.

Table 1: Statewide Soil Fertility Status Summary in Missouri Based on Samples Received by the MU Soil Testing Labs in 2013

pHs : Very low  less than 4.5; low  4.5 to 5.3; medium  5.4 – 6.0; high  6.1 – 7.5; very high greater than 7.5
P1: Very low less than 14 lb./ac; low 14 – 22 lb/ac;  medium 23 – 45 lb/ac; high 46-70 lb/ac; very high greater than 70 lb/ac
K:  Very low less than 65 lb/ac; low 65 – 110 lb/ac;  medium 111 – 220 lb/ac; high 221- 330 lb/ac; very high greater than 330 lb/ac
OM: very low less than 1; low 1-1.9:%; medium 2.0 % to 2.9%; high 3.0 – 4.0 %; very high greater than 4%

If you are going to apply nutrients in spring you need to know how much to put on. Without soil testing, nutrient applications are a guess, and there is no room for guessing in today’s atmosphere of narrow margins due to varying fertilizer prices, and public concern of the environmental pollution. Testing soils reduce the risks involved with applying nutrients. What kind of fertilizer do you need to achieve your yield goals?  Well, a good place to start would be the MU soil and plant testing lab.

Soil testing is a farmer’s best guide to the wise and efficient use of fertilizer and soil amendments. A soil test is like taking an inventory of the nutrients available to plants, which are too high, too low or just right. While plant growth and prior yields may offer clues to nutrient availability, a farmer won’t precisely know until they test their soil. Although soil-testing kits are available in garden centers, laboratory testing is more reliable, and the results from laboratories are accompanied with interpretations and recommendations.

Soil fertility fluctuates throughout the growing season each year.

The quantity and availability of mineral nutrients are altered by the addition of fertilizers, manure, and lime in addition to leaching and de-nitrification losses. Furthermore, large quantities of mineral nutrients are removed from soils as a result of plant growth and development, and by the harvesting of crops. The soil test will determine the current fertility status. It also provides the necessary information needed to maintain the optimum fertility year after year.

The soil test takes the guesswork out of fertilization and is extremely cost effective. It not only eliminates the waste of money spent on unnecessary fertilizers, but also eliminates over-usage of fertilizers, hence helping to protect the environment.

Soil samples can be taken in the spring or fall for established sites.

Although fall and early spring are typical times to test soil, one can really do it any time the soil is not frozen, but avoid sampling after recent fertilizer or lime applications. For new sites, soil samples can be taken anytime when the soil is workable. However, fall is a preferred time to take soil tests if one wants to avoid the spring rush. Fall soil testing will allow you ample time to apply lime to raise the soil pH.

As clearly evident from the statewide soil fertility status summary, soil testing is highly recommended for field crops. The cost of soil testing is minor in comparison to the cost of seed and plants and labor. Correcting a problem before planting is much simpler and cheaper than afterwards. Routine fertilizer or lime applications can result in excessive soil nutrient levels or deleterious soil pH. For example many fertilizers tend to have lower soil pH, and after several years of fertilization the pH may drop below desirable. 

The test results are only as good as the sample taken. It is extremely important to provide a representative sample to the testing lab so that a reliable test and recommendations can be made for the entire area. This can be accomplished by submitting a composite sample. Take 15 random samples in a zigzag pattern at plow depth, mix well, and submit a sub-sample from it to the lab. We recommend that you divide your field and submit one sample for each 40 acres.

Testing your soil for nutrients and pH is important to provide balanced application of nutrients, while avoiding over application. At University of Missouri Soil Testing Laboratory we offer a regular fertility test that includes measurements of pH, lime requirement, organic matter, available phosphorus, potassium, calcium, magnesium, and cation exchange capacity. Soil pH greatly influences plant nutrient availability. Adjusting pH often corrects the nutrient problem for most plants. The optimum pH for most plants is between 6.0 and 7.0. The lime requirement measurement indicates the amount of amendment (usually lime) necessary to correct a pH problem. Organic matter has several roles in the soil; generally the more organic matter the better. Nitrogen recommendations are based on the organic matter level. Phosphorus, potassium, calcium, and magnesium are all essential plant nutrients. The cation exchange capacity (CEC) value is a measure of the soil’s ability to hold nutrients.

Test costs vary according to the number of nutrients tested. The University of Missouri Soil Testing Laboratory charges $10.00 (when submitting direct to the lab) for a regular fertility test. Several other specific analyses are available. These include but are not limited to soil analysis for sulfur, micro-nutrients (Zinc, Iron, Copper, Manganese, Boron), salt content (electrical conductivity), heavy metal analysis, and soil texture. Test reports provide interpretation and nutrient recommendations. The turnaround time for a soil test is 24 hours. Customers have to add mailing time to get the reports by regular mail services.

You can contact your Regional Agronomy/Horticulture/Natural Resources Specialist or local County Extension Office to obtain Sample Information Forms and sample boxes, and can submit samples through their offices. These Regional Specialists at your local Extension Offices can be a source of information for interpreting and personalizing your soil test reports and recommendations. Samples can be also submitted directly to the University of Missouri Soil Testing labs at 23 Mumford Hall, Columbia, MO 65211 (Tel: 573-882-0623). Customers can drop the sample off at their County University Extension offices or in person at the MU Soil Testing Lab located at 23 Mumford Hall, University of Missouri, Columbia or at the Delta Soil Testing Lab located the Delta Research Center at Portageville or mail them in. Every sample submitted should have a sample information form duly filled. Samples submitted directly to the lab should be accompanied by a check written in favor of MU Soil Testing for the amount due.

The lab maintains a comprehensive web site at http://soilplantlab.missouri.edu/soil/. The site includes information on how to collect soil and plant samples, and how and where to submit samples. The web site provides a list of services provided by the lab, costs of tests, sample information forms, location of the lab and other relevant information. The lab also provides web access of soil test results with a specifically assigned password to clients upon request. We also have the option for electronic mailing of data if required.