what kind of field conditions would it be more favorable to grid sample

As various aspects of precision agriculture are implemented in Nebraska, some of the most frequent questions asked by producers, fertilizer dealers, and ingather consultants relate to soil sampling. Should I soil sample this field on a grid? What filigree spacing should I utilise? How often should I sample? Tin I use a yield map to tell where to soil sample? All of these are good questions, but often nosotros do not have definitive answers. Site-specific management enquiry conducted in recent years in Nebraska, yet, provides some direction on how to implement a soil sampling program for precision agriculture.

Basic Sampling Principles

Historically, the objectives of soil sampling have been to determine the average food status of a field and to provide some mensurate of nutrient variability in a field. Soil sampling for precision agriculture has these same objectives with some modifications. Instead of a field, producers are interested in areas within fields. They also are interested in relating trends in soil fertilizer levels to other field properties that are predictable or easily measured. Knowledge of factors influencing soil food levels including soil blazon, topography, cropping history, manure application, fertilizer application and leveling for irrigation will help the producer make up one's mind the nigh effective sampling approach. The basic principles of soil sampling all the same apply to precision sampling. An adequate number of samples should be collected to accurately characterize nutrient levels. The samples should be collected to the proper depth for non-mobile and mobile nutrients. Samples should be handled and stored to minimize contamination and degradation.

Grid Sampling

When variable-rate fertilizer application was beginning adept 8-x years ago, application maps were about frequently derived from grid soil samples collected at average densities of i sample for every three to 4 acres. In enquiry studies conducted in Nebraska, fields have been grid sampled at much higher densities (up to 42 samples per acre) to judge the true spatial variability of a number of soil nutrient levels. Sampling at high densities allows the evaluation of lower sampling densities on nutrient maps. In some cases, fewer samples tin issue in inaccurate maps.

Consider grid sampling if:

• Previous direction — such as confined livestock, heavy manure awarding, or aggressive leveling for irrigation — has significantly altered the soil nutrient level.
• Small fields with different cropping histories have been merged into one.
• An authentic base map of soil organic matter is desired.

Consider directed sampling if:

• Yield maps, remotely sensed images, or other sources of spatial data are bachelor and bear witness consistency from one layer to some other.
• You have feel farming the field that you feel would provide direction on where to delineate management zones.
• There is limited or no history of livestock or manure influence on the field.

The following figure shows how a tenfold range in sampling density at a inquiry site in Lincoln Canton resulted insignificantly unlike patterns. In this instance, the coarser sampling filigree missed a systematic pattern in soil nitrate, probably related to livestock fencing. The average recommended nitrogen rate for the field at the higher grid density was 148 lb N/acre. The average recommended nitrogen charge per unit was162 lb Northward/acre at the lower grid density; 45 percentage of the field received a different nitrogen recommendation with the coarser grid. The coarse grid was denser than about commercial grid sampling practiced by fertilizer dealers and crop consultants.

In other situations, accurate maps can be generated at much lower sampling densities. At a site in Buffalo County, a filigree density of fourteen samples per acre was compared to a density of 1 sample per three.seven acres. The coarse grid is similar to that used commercially. In this case, the nitrogen rate maps were not greatly different — 17.6 percent of the field received a different nitrogen recommendation with the coarser filigree, and the average nitrogen rate was the aforementioned for both grids — 158 lb N/acre.

The optimum grid density depends on the site, and to some extent what nutrient is being assessed — soil organic matter, nitrate, phosphorus, zinc, etc. It helps to know the spatial variability of the field in order to know the optimum grid density —which, after all, is the reason for grid sampling. This also raises the basic question of why we would cull to grid soil sample. Is there a better way to obtain the desired information?

Directed Sampling

Directed soil sampling is in many means simply an extension of how soil samples were frequently collected in the past. For case, if a field contains significant areas of more than one soil series, the University of Nebraska–Lincoln recommendation was to collect samples from each soil series. Too, if parts of the field had different preceding crops, different fertilization histories, eroded areas, or an old farmstead location, these areas were to be sampled separately. In these situations, the producer is using his cognition of spatial factors to straight where samples are taken to make up one's mind if they have dissimilar fertilizer needs. The new tools of yield maps, aerial photographs, and remotely sensed images simply provide more than information near variability in the field and where soil sampling can aid interpret variability.

Recommendations

Producers interested in soil sampling for precision agronomics should first consider how they will use data from soil sampling. Some variable-rate fertilizer application equipment is controlled by software based on grid samples. In these situations, the field will demand to be filigree sampled or there must exist some mode of generating grid data from directed samples. Check with your custom applicator to insure that the information you collect will be compatible with the variable charge per unit equipment requirements.

Grid Sampling

Density. A well-done nutrient map derived from a grid sample can be a valuable resource for many years. Consequently, the density should be adequate to provide conviction in the accuracy of the maps developed from the data. We suggest analyzing one sample per acre, which is composited from five cores collected in a tight radius about the sample point (Figure three). This density will result in a map that will be good for many years —10 to 20 years for soil organic thing and cation exchange capacity; five to ten years for pH; and four to five years for phosphorus, potassium, and zinc. On fields in which variability is expected to be low, a sampling density of ii to ii-and-half acres per sample may be acceptable. Filigree sampling at densities coarser than one sample for every 2.v acres is not recommended if the goal is to develop a resources of nutrient maps that can be used with confidence over several years.

Sampling Design and Depth. An first filigree pattern is recommended equally shown in the figure beneath. This will provide more information at a lower cost than a regular grid pattern. Individual cores should be collected in a radius of viii-10 feet of the grid bespeak, to a depth of 8 inches. The grid point should represent the central position of a composited sample. Collect samples within the 8-10 pes radius randomly, in order to avoid systematic patterns such as starter or preplant bands. Bear a general fertility analysis on the samples, including soil organic matter, pH, phosphorus, potassium and other nutrients of interest.

Frequency. As already mentioned, a nutrient map derived from a grid-sampled field tin can last a long time. If the variable rate application of fertilizer or lime occurs, this will have the potential to alter nutrient levels or soil pH over time. Soil phosphorus levels will not change drastically with single variable rate applications. We suggest that grid samples be collected every five years for phosphorus. Lime awarding according to recommendations should amend soil pH for viii-10 years. Even if the variable rate lime application has occurred co-ordinate to a grid-sampled map of pH, it should non be necessary to filigree sample for soil pH for eight-10years after application.

Residual Nitrate Sampling. Filigree sampling for nitrate-N is non recommended considering annual fluctuations in nitrate levels would require annual grid sampling, which is not cost effective for well-nigh crops with current fertilizer prices. Instead, rest nitrate sampling (to a depth of 3 feet) should be done on a directed sampling ground.

Directed Sampling

Consider Multiple Information Layers. Patterns which show consistency from 1 data layer to another, such equally multiple years of yield maps, or a yield map and an aerial photo, are more than likely related to soils than other sources of variability. In many cases, a soil series map or topography map tin can be a good base upon which to overlay yield maps and other sources of spatial information. Your experience gained from tillage, tillage, harvest and field scouting tin can also serve as effective data layers.

Minimize Subdivision. After deriving information from multiple data layers, including your experience, subdivide the field into management zones. Wait for general categories when subdividing and avoid creating many subdivisions. By and large, 4 to six subdivisions should be adequate. Excessive subdivision may create small areas which are non really manageable. Management zones need not exist face-to-face. Samples collected from more than i surface area of a field may fall into the same range of yield, soil color, etc. and thus the aforementioned zone.

Soil Fertility Isn't Everything. As you await for consistent patterns in fields, call up that soil fertility will not be the only cistron influencing patterns in yield maps, remotely sensed images, and other sources of spatial data. Soil factors other than fertility, such as compaction, pinnacle-soil depth, and texture will influence patterns. Other sources of stress, such as disease, weeds and insects may significantly influence yield and other patterns. Consider scouting fields for these factors during the growing season co-ordinate to categories derived from spatial data.

Accurately Sample Each Zone. Soil samples should be collected from each zone according to current recommendations (NebGuide G91-m, Guidelines for Soil Sampling). For general fertility recommendations, collect 10-fifteen cores to a depth of 8 inches from within the zone, and so blended samples into ane to ship to the lab for analysis. Samples can exist georeferenced with a GPS receiver for repeatability if desired. This will allow y'all to collect samples in the future from basically the aforementioned locations, even though you are compositing the cores for assay.

Residual Nitrate Sampling. Collect half-dozen to 8 cores to a depth of three feet for residual nitrate from each zone, compositing the samples into one to transport to the lab for nitrate analysis. For convenience, consider collecting a deep sample for residual nitrate at every other location that you collect surface samples, particularly if georeferencing sample locations.

Choosing a Method

Both grid and directed soil sampling are valid options for precision soil sampling — each has advantages and disadvantages. Unless the grid is dense plenty, grid sampling may miss patterns and boundaries that are axiomatic from looking at soil surveys or yield maps. Grid sampling is very expensive — both to collect and to analyze the samples. Directed sampling uses other sources of spatial information to make informed decisions on where to sample, nonetheless, in that location may be patterns in soil fertility which are not detectable except with filigree sampling.  The adjoining figure is an example of such a situation. This is a map of soil phosphorus from a field in Clay County. The pattern of soil phosphorus is strongly influenced past the location of a farmstead with bars livestock in the northern portion of the field at some time in the past — 40 or more years agone. Without knowing the farmstead's location to direct sampling, a directed sampling approach would not be likely to detect this expanse of loftier soil phosphorus. Other sources of spatial information (the county soil survey, yield map, aerial photograph) give no indication of high soil phosphorus or the by presence of a farmstead. This field also is an case of the benefits of precision sampling over traditional sampling methods. The averageBray-1 P examination is 15.1 ppm — just slightly over the disquisitional level of 15 ppm at which phosphorus fertilization is recommended past the Academy of Nebraska for corn. Traditional sampling procedures might suggest that this field does not need phosphorus fertilizer; however, precision sampling shows that near of the field actually tests well beneath fifteen ppm and phosphorus fertilization should significantly increment yield

For additional informaton, meet UNL Extension Round Soil Sampling for Precision Agriculture - EC 154 (157 KB; 4 pages)

Mention or display of make names is for identification purposes only. No endorsement or criticism is intended for those mentioned or any equivalent products non mentioned.

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Source: https://cropwatch.unl.edu/ssm/soilsampling

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