Sampling Soil for Fertility and Nematodes

Sampling for Soil Fertility

High-yielding soybeans require large amounts of plant nutrients that are supplied by the soil. A significant portion of the soil’s macronutrients are removed in the harvested seed (see below table), while some amount may be lost to leaching and erosion.

Soil sampling and testing are the most important steps for successful determination of the amount of soil nutrients that have been depleted by a crop, and subsequently, what amount should be added to attain a desired level for the expected yield of the following soybean crop. Results from proper conduct of these two components, i.e. sampling and testing, will serve as the most reliable indicator of how much of a particular nutrient is available to the crop.

An article in Missouri Ruralist titled “Why fall is the best time for soil sampling” by Mindy Ward (quoting Missouri Agronomy Specialist Travis Harper) provides five reasons that fall vs. spring soil sampling may be best to manage for a following spring-planted crop. They are:

•   Fall vs. spring weather conditions are usually more favorable for collecting the required number of soil samples from a given area or field.

•   Applying lime in the fall according to soil test results will allow time for pH correction before spring planting.

•   Fertilizer that is required according to soil test results can be purchased in the fall when prices may be lower.

•   Turnaround time for results from a soil testing lab likely will be quicker for samples submitted in the fall.

•   The availability of some fertilizer elements, e.g. phosphorus and potassium, for next year’s crop will be enhanced with fall applications.

The steps required for proper soil sampling and sample handling, along with proper sampling protocol, are provided by MSU Extension here. The resulting soil test report should include pH, lime requirement, and cation exchange capacity (CEC–an indicator of soil texture), plus extractable levels of phosphorus, potassium, calcium, magnesium, sulfur, and zinc. Organic matter content may also be reported.

Samples that are submitted to the Mississippi State University Extension Soil Testing Laboratory should include a completed MSU Extension Form 76. Regrettably, this form only allows the producer to enter a crop code number for either “Soybeans” or “Soybeans/Small grain rotation”; i.e., there is no category where the producer can request results for either a low- or high-yielding soybean environment. Costs for various soil tests provided by the MSU Soil Testing Lab can by found here.

Nutrient Removal by Soybeans

Today’s high yielding soybeans in the Midsouth are removing large amounts of nutrients from the soil. Data in the below table show the estimated amounts of nutrients that are removed from the soil by a soybean crop that yields 30 bu/acre (low yield environment) and 70 bu/acre (high yield environment). These values can be used to supplement soil test results to ensure that fertilizer additions at least replace the estimated amounts of nutrients that are removed with each crop harvest. Notice that a considerable amount of K and several of the micronutrients that are taken up by the crop remain in the field since a large percentage is contained in the stover which is returned to the field during harvest. As expected, P and K exhibit the largest removal from soil.

Sampling for Nematodes

Soybean producers in the Midsouth must contend with nematode pests, several species of which may inhabit a single field. Significant yield losses caused by soybean cyst nematode (SCN), southern root-knot nematode (RKN), and reniform nematode (RN) can occur in Mississippi soybeans.

The change in cropping systems in Mississippi in recent years has led to increased concerns about nematode infestations of soybeans. The effect of these changes are:

•   Increased acreage of corn that may be rotated with soybeans has led to heightened concern about soybeans being infested with RKN.

•   Growing soybeans on sites once devoted to cotton has led to heightened concern about soybeans being infested with RN.

Because of these cropping system changes, the need to sample for nematodes has become even more important because of the added risk of infestations from RKN and RN, as well as SCN. Consider the below points.

•   Properly collected and evaluated soil samples are the best tool for detecting the presence and species of nematodes in the soil. Proper sampling protocol for nematodes is provided by MSU Extension.

•   To assess potential damage from nematodes in soybean fields, growers must determine which nematode or nematodes are present to make appropriate nematode management decisions.

•   Accurate identification of the nematode species and population levels present in a field requires that soil samples be collected and sent to a diagnostic lab for evaluation.

•   Properly analyzed samples will indicate where control practices are not needed, and conversely will indicate where control practices are needed to protect yield potential.

•   Predictive sampling (sampling to determine if nematode problems are likely to affect a future crop) should be done when population densities are high to decrease the risk of not detecting the presence of a damaging species. Thus, the best time to sample is generally near or just after harvest. Sampling in the fall will allow enough time for analysis so that results can be used as a guide for variety selection or choosing an alternative crop for the next growing season.

Mississippi soybean producers may submit soil samples for nematode analysis to the Mississippi State University Extension Plant Pathology Lab. Instructions for sample submission and associated costs are contained on the laboratory site and in the links below.

•   If test results indicate that the above nematode species are not present in a field, care should be taken to prevent their introduction since nematodes can be moved from field to field by soil that is transported on field equipment.

•   If test results indicate the presence of nematodes, the goal is to use management practices that will keep the nematode population as low as possible since they are very difficult to eliminate.

•   Soil texture affects movement of SCN in the soil and also may affect its reproduction and development. Basically, major damage to soybean by SCN infestation occurs when the crop is grown on medium- and coarse-textured soils. Apparently, damaging populations of SCN are not sustainable in soils series classified as clay.

•   RKN tends to be associated with sandy soils on sites that have previously been devoted to cotton production in the Midsouth, where the combination of root damage and the reduced water-holding capacity of the soil can result in wilting of infected plants during the heat of the day.

•   Determination of the density and race or type of SCN present in individual fields is required to prevent losses and determine management and control practices to apply. Determination of the race or type is especially important because the different SCN resistance sources convey differing levels of resistance against the varied races or types.

•   Sampling for nematodes should be considered as important as sampling for soil fertility. This is especially true if there is no history of nematode sampling on either old or new soybean production sites. Once documentation of the absence or presence of nematodes is established for given fields, then management options outlined in the White Paper here can be adopted.

Sampling Information Resources

Crouse, K. Soil Testing for the Farmer, MSU Extension Service Information Sheet 346, July 2016.

Oldham, L. Introduction to Soil Testing, Chapter 4 in Nutrient Management Guidelines for Agronomic Crops Grown in Mississippi, MSU Extension Service Publication 2647, Feb. 2012.

Donald, P., Stetina, S., and Heatherly, L. Managing Nematode Pests in Midsouth Soybeans, May 2017.

Allen, T. Soybean Cyst Nematode, MSU Extension Service Publication 1293, Jan. 2012.

Nematode Sample Submission Form, MSU Extension Service, June 8, 2017

Nematode Sample Testing Fees, MSU Extension Service, Aug. 15, 2017.

Composed by Larry G. Heatherly, Sep. 2018, larryheatherly@bellsouth.net