Soil Sampling

The challenge of crop nutrient management is to balance production and economic optimization with environmental impacts. Successful crop production is dependent upon effective nutrient management that includes identifying nutrient deficiencies and excesses and balancing nutrient inputs and crop removal outputs. Obtaining a good soil, manure, or water sample is imperative for accurate analysis and calculation of crop needs and agronomic application rates. The following guidelines outline proper technique for obtaining samples as well as handling and transport.

Sampling Frequency

Soil tests are used as the basis for nutrient application, and should be available for each of your fields where crops are grown and fertilizer (manure) is applied. At a minimum, each field or management unit should be comprehensively sampled once as a baseline. This will help establish an average for the field and identify any areas of variation that may require different nutrient requirements. After a baseline is achieved, testing may be conducted once every three years thereafter as a minimum. More frequent testing will aid in better agronomic application rates. If manure is going to be applied to your field, an annual fall nitrate test should be taken to a depth of 12 inches on all fields after harvest in the fall.

Nutrient Analysis

Soil samples should be analyzed for: pH, EC, OM, nitrate-N, ammonium-N, P (Bray), and K. Additional tests may be required for various practices. For instance, a Pre Sidedress Nitrate Test (PSNT) shall be performed on corn ground prior to each sidedress application.   

Interpreting Your Results

Once you have your comprehensive and/or fall nitrate soil results, comprehensive manure results, and irrigation nitrate results, it is time to interpret what they all mean. Analyses of your results will aid you in adjusting your nutrient balance and agronomic application rates on an ongoing basis. Creating graphs and will help you track your nutrient balances over time.

How To Take A  Soil Sample

Field sampling and soil testing has become an important tool for assessing soil fertility and arriving at proper manure application rate recommendations. It’s also a valuable management aid for studying soil changes resulting from cropping practices and for diagnosing specific cropping problems. Soil testing provides an index for the nutrient availability in soil and is a critical step in nutrient management planning. Proper soil sampling will provide accurate soil test results and reliable nutrient recommendations. Soil sampling technique, timing of sampling, and type of analysis all need to be considered for accurate results. A good, representative soil sample should be taken for every field that manure is applied to.

Collection and handling of soil samples shall comply with Pacific Northwest Land Grant University (Cooperative Extension) procedures and protocols. See Sampling Guidelines below as well as the Reference Materials for more information on soil testing and interpretation.

When you should sample

Cultivated fields for spring seeding should be sampled after October 1. These fields can also be sampled in the spring, but time may be limited by then. Fields for fall-seeded crops should be sampled one month before seeding. Forage fields for seed, pasture, or hay may be sampled after September 1. Problem soil areas may be sampled anytime. Frozen and waterlogged soils should not be sampled because of the difficulty in obtaining a representative sample.

Where you should sample

Soil variability is a major concern when deciding how to collect a representative soil sample. Soil samples submitted for analysis should be representative of the field or portion of a field. Therefore, by sampling from an area of the field where yield is typically average, soil test results should come back with an average representation of the field. Identifying areas that are representative can be difficult without a first hand knowledge of the field. If the person taking the soil samples does not take the time or have the knowledge required to take a sample in the appropriate location, the results can come back somewhat sporadic.

Sampling Types

Benchmark sampling is recommended for fields with no prior nutrient analysis. A small area (generally about ¼ of an acre) representing the majority of the field is sampled the same number of times as in random sampling (15-20 samples). This is the reference area from which fertilizer recommendations are made. The benchmark site should be marked with a global positioning system (GPS) or other means so that one can return there for subsequent years sampling. Sampling from the same area will reduce sampling variability to create a better picture of year-to-year changes. More than one benchmark is recommended if you cannot identify a dominant production area on your field.

After a benchmark has been established, sampling should be conducted every 3 years. There are a few types of field sampling methods that can be conducted, but random and managed random sampling are the most common. Random soil sampling is the traditional approach that works for uniform fields with little variation. In this case, you would take 15-20 samples located across the entire field. This would provide an average picture of the entire field. Managed random sampling technique samples from areas identified as “average” production areas. This method is recommended if you have spots of variability in production across your field, and want to only represent the typical or average parts of the field. In this case, you would take 15-20 random samples only from the areas of the field that have a “typical” production rate.

Each field must be sampled separately. Size up each field and observe variations in yield and crop growth, texture, color, slope, degree of erosion, drainage, and past treatment. Sizable areas of fields where growth is significantly different from the rest of the field should be sampled separately. Avoid unusual areas such as dead or back furrows, old straw, hay or manure piles, waterways, saline spots, eroded knolls and old fencerows. Dividing a field into management zones based on production rates allows for an understanding of different conditions within a field. This is particularly effective in rolling and variable landscapes. For example, a large depression may be a very productive area, but a separate soil test may indicate it can be optimized with a higher rate of nitrogen than the benchmark is indicating. While most producers do not have variable rate capabilities on manure application equipment, rates can often be easily increased through other adjustments.

How you should sample

First, select 15 to 20 sampling sites representative of the portion of the field to be tested. Second, take each sample in the same exact fashion as the last to ensure there is limited or no human induced variability between samples. A variety of sampling tools are available to collect soil samples. They range from a spade to hydraulic powered coring equipment. Representative soil samples can best be obtained by using a core-sampling tool. Take soil core samples down to at least 12 inches at each of the 15 to 20 sampling sites. For improved nitrogen and sulfur evaluation or where problem soils are encountered, separate samples should be taken from varying depths at the same 15 to 20 sites and analyzed separately.  

Sample Handling and Storage

Preparing samples for laboratory analysis is just as important as collecting the soil sample. Place all sample cores in a clean pail or bag and mix them thoroughly, crushing lumps in the process. Remove a well-mixed, homogeneous sub-sample of that mixture and place it in a clean airtight bag. Write sample identification information (e.g., date, field #, etc.) on the bag. Store the sample in a cool, dry location and send to the laboratory within 12-24 hours of sample attainment. Samples can also be stored in a refrigerator for a couple of days or frozen if sample delivery is delayed. Provide complete information for each soil sample on the sheet supplied by the lab. Where unusual problems exist, these should be noted in detail. Keep a completed field plan of the area represented by each sample for your own records.