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Agronomyag.purdue.edu/AgryAY-368-WSoil Sampling GuidelinesAuthor: Jason P. Ackerson,Assistant Professorof AgronomySoil testing is an integral part of a soilfertility management program. Effectivesoil testing provides information onthe fertility status of soils within a fieldthat can be used for making fertilizeror lime application recommendations,monitoring changes in soil fertility overtime and even identifying and targetinglow fertility soils within larger fields.Informative soil sampling can improveon-farm nutrient efficiency, leadingto increased return on investmentfor fertilizer and lime application anddecreased risk of off-site nutrientmovement.Regardless of the goal, reliable soiltesting starts with proper soil sampling.In this article, we will outline the basicprinciples of agronomic soil sampling,covering the basic principles of soilsample collection and providingguidelines for establishing an effectivesoil sampling and testing program.Sample Collectionand HandlingProper soil sample collection relies onthree principles:1. Organization: having an orderlysystem for soil sample collectionand handling simplifies samplecollection and minimizes the chanceof human errors such as mislabelingor misplacing soil samples.2. Consistency: collecting each samplein a uniform manner between yearsand within the course of a samplingevent will greatly improve the qualityand reliability of your results. Thismeans taking samples in the samemanner for each sample.3. Simplicity: following simpleprocedures will help ensure samplecollection is consistent and easilyorganized.
AY-368-W Soil Sampling Guidelines4. Insert the soil probe to the desired depth. (See Table1 for details on sample depths.) Take care to ensurethe probe is inserted vertically into the soil and nottilted to the side. Remove the probe and transfer thesoil core from the probe into a bucket (Fig. 1).Samples and subsamplesSoils can be highly variable, even over short distances.Because of this variability, it is often insufficient tocollect soil at just one location. Instead, it is preferableto collect so-called composite samples. Compositesamples are a mixture of individual samples, orsubsamples, generally collected from multiple locationsand mixed together to form a single compositesample. By combining multiple subsamples into asingle composite sample, we can minimize the effectsof soil variability by averaging the soil properties overlarger areas. Composite samples are less sensitive tounusually high or low soil test values that might occurdue to concentrated fertilizer applications (e.g. bandedapplications) or natural soil variation.Sample collectionBefore collecting soil samples, you will need to gathercertain materials and tools: A soil probe A clean plastic bucket A trowelFigure 1. Collecting a soil sample with a soil probe. Insert the probevertically into the soil and remove the soil core. P ermanent markers S ample bags. Many soil testing laboratories willprovide wax-lined sample bags. In lieu of laboratoryprovided bags, consider using paper bags or zip-topbags.5. Move to a new location and repeat Steps 3 and 4.The distance between locations where you collectsubsamples will vary depending on the samplingstrategies you are employing. (See the Samplinglocations and strategies section for more info.) Asa general guideline, the larger the area of land youare sampling, the more distance you need betweensampling locations. As a rough guideline, samplinglocations should be separated by a minimum of20-30 feet. If employing a zone-based or grid-basedsoil sampling program, it is often worthwhile toselect the location of soil samples prior to arriving inthe field for sampling. These preidentified points canbe loaded onto a GPS-enabled device and the GPScan be used to direct you to the sampling location. C lipboard and paper or field notebook G PS-enabled smartphone or handheld gps unit(optional)To collect a composite sample use the followingprocedure:1. Before arriving to the field, determine the numberand approximate location of soil samples. (Seethe Sampling locations and strategies section fordetails.)2. Once the appropriate materials have been collected,travel to the first sampling location. If you’d like,you may record the location with a GPS or GPSapplication on your smartphone. This informationcan be useful later for tracking where samples havebeen collected. You may find it helpful to returnto the same sampling locations in subsequentsampling events.6. Continue this process of sample collection at newlocations until you have collected a sufficientnumber of samples. Typically, a composite sampleshould be comprised of between 10 and 20subsamples. The more subsamples you add into acomposite, the more reliable a sample becomes.3. At the sample location, remove any crop residuefrom the soil surface.2
AY-368-W Soil Sampling Guidelines7. Using the trowel, thoroughly mix the soil in thebucket until you have a homogeneous mixture.Soil testing laboratoriesSoil testing is available for a nominal fee throughseveral specialized laboratories. Each laboratory willhave specific instructions for how to ship and labelsamples. To ensure the laboratory provides accurateand timely results, be careful to follow any laboratoryspecific instructions. For details on laboratory-specificinstructions, contact your chosen soil testing service.8. Place 1-2 cups of the mixture into a sample bag.Using permanent marker, label the bag with aunique name. Names should contain identifiersto the field and sample number. For example,“Smith-Field1-1” is a good label that identifies thefarm (Smith Farm), field (Field 1) and the samplenumber (1).While most commercial laboratories provide quality,reliable testing services, there can be differences inmethodology and results between laboratories. Forthis reason, it is often desirable to use the same soiltesting laboratory every year. This will ensure that anyobserved change in soil-test results from year to yearare attributable to true changes in soil fertility statusand not due to deviations in testing practices betweendifferent laboratories.Table 1. Sample depth guidelinesTillage SystemConservation tillage (Lessthan 50% of crop residueincorporation)Sample depthTake separate samples: 0-4 inches for pH/limingrecommendations 0-8 inches for fertility analysisConventional tillage0-8 inches for pH/liming recommendations(Greater than 50% of crop and fertility analysisresidue incorporation)Sampling locations and strategiesDetermining where to take soil samples dependslargely on the management strategy you employon your farm. These management strategies canbe broken down into two types: whole-field andspatially explicit (Fig. 2). In a whole-field managementthe field is managed as one unit. When fertilizer isapplied in a whole-field approach, one fertilizer rate isapplied uniformly across the entire field. Whole-fieldmanagement is simple to implement and does notrequire any special equipment or data handling.Sample HandlingAfter collecting a composite sample, it is importantto properly store samples to prevent contamination.Typically, most laboratories prefer to prepare samplesin their lab. This means that you can often sendsamples directly to the laboratory without doingany processing yourself. Some laboratories requiresamples to be submitted in specific sample bags orcontainers. Check with your chosen laboratory forspecific information on its requirements for handlingand packaging samples.In spatially explicit management, the field is brokeninto smaller sections, and each section is managedindividually. Spatially explicit management can identifyareas of the field with specific fertilizer of liming needsand provides a map of a field’s nutrient and limingrequirements. Spatially explicit management is anessential part of precision agriculture. In spatiallyexplicit management, variable-rate technology canbe used to alter fertilizer and lime applications so thateach zone receives a targeted, zone-specific fertilizer orlime application.If you are not sending samples directly to thelaboratory, consider storing samples in the refrigeratoror freezer to minimize the chance of mold forming inthe sample bag.If the soil is excessively wet or you cannot storesamples in a refrigerator/freezer, allow the samplesto air dry slightly by spreading the soil in a thin layeron a flat surface like a table. You can put down somepaper such as used newspaper to protect the surfacefrom getting dirty. Never dry a sample in an oven ormicrowave; excessive heat can damage the sample andalter laboratory results.3
AY-368-W Soil Sampling GuidelinesFigure 2. Whole-field management (Fig. 2a) and spatially explicit management (Fig. 2b).Ultimately, the decision between whole-field andspatially explicit management will depend on thespecific goals and constraints of each operation.Whole-field management is simple to implementbut risks over- or under-fertilizing some areas of thefield. Under application of fertilizer and lime can leadto poor yield while overapplication leads to wastedmaterial and can cause environmental damage.Conversely, spatially explicit management can bemore difficult to implement but the spatial informationcan be used to minimize the risk of over- and underapplication of fertilizer and lime.Soil sampling for whole-field managementFigure 3. An example of zig-zag sampling pattern for whole fieldsampling. Subsamples are collected by traveling in a zig-zag patterncollecting subsamples at each locations indicated by black dots.Background lines represent the various soil types in the field. Ideally, zigzag sampling samples each soil type equally.The goal of soil sampling for whole-field managementis to get a sample representative of the typical soil inthe field. To do this, subsamples are distributed acrosslarge areas to ensure the entire field is represented. Toachieve this, collect composite samples in a zig-zagpattern (Fig. 3). Each composite sample should consistof 10 to 20 subsamples spread evenly across a field.Collect at least one composite per 20 acres.Soil sampling for spatiallyexplicit managementThere are two main methods for soil sampling inspatially explicit management — zone-based samplingand grid sampling. With each method, soil samplesare collected from predefined areas in a field. Bycorrelating the soil test results with the area of the4
AY-368-W Soil Sampling Guidelinesfield where samples were collected, you can generatea map of soil fertility patterns in the field. With bothmethods, the smaller the predefined region (i.e., zoneor grid cell), the more detailed the resulting map. Eachmethod is described in more detail below.sampling has several disadvantages when comparedto zone-based sampling.The first disadvantage of grid sampling is an increasedrisk of bias. For example, if a majority of grid sampleslie on a particular soil type, the grid sample mayunderrepresent the true variability in a field (Fig. 5).Zone-based samplingIn zone-based strategies, the goal is to collect samplesthat represent the average soil within each zone. Zonesare typically developed based on unique soil types orfrom patterns in yield maps. The key aspect of eachzone is that zones represent areas of homogeneousor uniform soil conditions. Typically, the more variablethe soil, the smaller the zones need to be in order toaccurately map soil fertility patterns. In most cases,zones are between 2 and 10 acres in size.For zone sampling, collect a composite sample fromeach zone (Fig 4). Larger zones require more samplesthan smaller zones. As a rule of thumb, collect twosubsamples per acre in a zone. Regardless of zonesize, a minimum of five subsamples per zone shouldbe collected. Subsamples should be located randomlywithin a zone.Figure 4. An example of zone-based soil sampling. In this example,different management zones are represented by different colors.Subsamples are collected and composited for each zone. Subsamples arecollected for the orange-colored zone at each location denoted by blackdots. Each of these samples will be mixed into a single composite sample.The real value of zone-based sampling is that zonebased data can be used to create a map of soilfertility and pH within a field. This map can showareas of particularly high or low fertility and can beused to develop precision, variable-rate fertilizer andlime applications. Because zone-based samples areultimately used to generate a map, it is especiallyimportant that samples be labeled with the correctzone and that the location of each sample or zone isrecorded. This will ensure that laboratory results fromeach soil sample can be correctly correlated with theappropriate location in the field.Grid samplingMany producers and farm service provides implementsoil sampling on a grid-based system. In a grid system,soil samples are taken at locations spaced in a uniformpattern. Typical grid sizes range from 1 to 5 acres, withsmaller grids (i.e., 2.5 acres or less) providing the bestresults. For a one-acre grid cell, collect at least fivesubsamples. For grid cells between 2.5 and 5 acres,collect between 8 and 10 subsamples.Figure 5. An example of grid-based soil sampling with sampling points(dots) located on an evenly distributed grid. This system can lead to bias.In this example, the orange zone is sampled in 30% of the grid points(white points) but the orange zone only represents 22% of the total field.In this case, the resulting soil sample is highly biased to the soil conditionsrepresented by the orange zone.Grids are attractive because they are easy toimplement and provide improved spatial informationover whole-field soil sampling. However, grid-samplingdoes not utilize or account for underlying variation inmanagement zones or soil types. Subsequently, grid5
AY-368-W Soil Sampling GuidelinesGrid sampling can also be inefficient. Because gridsdo not target areas of known variation in a field, moresamples are needed to cover the complete rangeof soil types and variability. While grid sampling ispreferred to whole-field sampling, it is inferior to themore informative zone-based sampling.Sample timingAreas to avoid Schedule soil sampling and testing prior toapplication of fertilizers or limeRegardless of soil sampling strategy, when collectingsamples in the field or identifying soil-samplinglocations, there are several areas you may wish toavoid. These problem areas (Table 2) are areas inthe field where, due to some disturbance of naturalvariation, the soil may not be representative of therest of the soil in a field or zone. If your soil samplingpoint lies within or near one of these problem areas,consider relocating the sample point to a new location.If problem areas occupy substantial portions of a field(i.e. larger than 5-10 acres) consider sampling eachproblem area as a unique zone.Table 2. Areas to avoid when collecting soil samplesExamples of Problem Areas End-rows or areas of heavy equipment traffic Wet spots or depressions Highly eroded areas Locations of former farmsteads or animal enclosures Locations within 100 ft. of roads Areas where lime is stored before applicationThe most convenient time to collect soil samples iswhen there is no standing crop in the field (i.e. in thespring prior to planting or in the fall after harvest).Both sample collection times can be useful for a givenmanagement program. When selecting a samplecollection time consider the following points: Collect samples early enough to provide thelaboratory sufficient time to return the data Keep sample collection timing consistent to avoidyear-to-year variability. For example, if implementinga spring sampling one year, do not adopt fallsampling for the next year of sampling.In most soils, try to sample fields once every four years.This should provide sufficient data on changes in soilfertility. An exception to this guideline is for sandy soilsor highly managed soils (e.g., irrigated fields, silage orhayfields, etc.). In these circumstances, soil fertility canchange rapidly and these fields should be sampledonce every two years.ConclusionsSoil sampling and testing can be highly informative.Information from a well-conducted soil-samplingevent can be useful in monitoring changes in soilfertility, developing fertilizer recommendations, andimproving on-farm nutrient efficiency. Regardlessof the particular soil-sampling program you employ,soil sampling using a consistent, well-conducted, andorganized approach will lead to the most usable andinformative soil test results.purdue.edu/extensionIt is the policy of the Purdue University Cooperative Extension Service that all persons have equal opportunity and access to its educational programs, services, activities, and facilities without regard to race, religion, color, sex,age, national origin or ancestry, marital status, parental status, sexual orientation, disability or status as a veteran. Purdue University is an Affirmative Action institution. This material may be available in alternative formats.Find out more atTHE EDUCATION STOREedustore.purdue.eduNovember 2018
For zone sampling, collect a composite sample from each zone (Fig 4). Larger zones require more samples than smaller zones. As a rule of thumb, collect two subsamples per acre in a zone. Regardless of zone size, a minimum of five subsamples per zone should be collected. Subsamples should be located randomly within a zone.
