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Soil sampling and testing – doing it right

Soil sampling in late fall after soil temperature has dropped to 5 to 7 C is often the most practical time. Photo by Ross McKenzie.

Fertilizer is one of the most costly inputs on the farm. Yet fertilizer is essential to ensure optimum economic crop production to keep the farm sustainable. Farmers should utilize soil sampling and testing to determine plant available soil nutrient levels to optimize fertilizer inputs. Taking soil samples correctly and ensuring the lab is using the right analyses are critical steps to develop fertilizer plans.

When to take soil samples?
Ideally, annually cropped fields for spring seeding should be sampled in the spring just before seeding for most accurate results. But realistically, springtime is often too short to complete soil sampling, analysis and develop fertilizer plans. Therefore, sampling in late fall after soil temperature has dropped to 5 to 7 C is often the most practical time. But keep in mind, soil nutrient levels may fluctuate during late fall, winter and early spring, particularly if soils are moist with warmer than normal conditions.

Fields for fall-seeded crops should be sampled a few weeks before seeding. Forage fields for pasture or hay can usually be sampled after Oct. 1. Problem soil areas can be sampled anytime. I usually do not recommend sampling frozen soils because of the difficulty in obtaining representative depth samples.

How to sample fields correctly?
The first step is how and where to take samples in the field. Soil samples must be representative of the field or portion of a field. Soil variability is a major concern when deciding how to undertake representative soil sampling. Don’t necessarily depend on your fertilizer dealer or agronomist to do this.

I strongly recommend you spend time with the person taking soil samples on your farm to ensure sampling is done in appropriate areas in your fields and to make sure enough sampling sites are taken. Checking to make sure the sampling is done correctly can be time well spent. Then, you know where and how the samples were taken.

There are a number of ways field soil sampling can be done. Three more common sampling methods are:

  1. Random sampling of a whole field: Take representative soil samples throughout the entire field, making sure to avoid unusual areas. This method works best in fields with relatively uniform soil and topography.
  2. Benchmark soil sampling: Select a representative area of a field. Soil sample the same location in each field, each year. The sampling area should be one to two acres and be representative of the majority of the field. If the field is variable in soil or topography, two or more benchmark locations may be needed to represent different areas within the field.
  3. Sampling soil/crop management zones: A field is mapped into uniquely different soil sampling zones based on soil characteristics, topography, management history and/or crop yield potential. Then representative soil samples are taken within each management zone. This method can work well in fields with variable soil or topography. Each management zone can be randomly sampled or benchmark sampled. You may have to work with a very knowledgeable agronomist to carefully prepare a soil/crop management zone map for each field with soil or topography variability.

When selecting benchmark areas or soil/crop management zones, use observable features such as changes in soil colour, crop growth differences and landscape/topography to identify where different soil types occur. A good time to identify different soil areas is by observing crop development. Look for differences in crop establishment, vigour, colour and growth. Also, make use of crop yield maps, aerial photos, topographic maps, soil salinity maps and/or satellite imagery information, if available, to assist with defining soil/crop management zones.

What number and depth increments to soil sample?
It is very important to take a minimum of 20 soil cores for each field, soil/crop management zone or benchmark area. This is critical to have a good representation. Typically, each soil sample sent to a soil testing lab weighs about 2 lbs. One acre of land, 6 inches deep, weighs about 2,000,000 lbs. If an 80 acre field is soil sampled to 6 inches, a 2 lb soil sample must be representative of 160,000,000 lb of soil. The soil sample would represent about 0.0000013 per cent of the field. This is an extremely small representation of the total field; therefore, it is very important that an adequate number of soil cores be taken. A common mistake is only taking four or five soil cores from a field or management zone, which is not enough and will often result in less reliable analytical results.

Ideally, separate each soil core into depth intervals of 0 to 6, 6 to 12 and 12 to 24 inches (0 to15, 15 to 30 and 30 to 60 cm), and place the three sampling depths into three clean plastic pails. Do this at 20 representative coring sites in the field, benchmark location or soil/crop management zone. Sampling three depths will give a good picture of the amounts of each nutrient and where the nutrients are located in the soil profile.

Some agronomists and dealers prefer to only sample one or two depths making the sampling process simple and faster. If only the 0 to 6 and 6 to12 inch depths are sampled, you have no idea of the amount of nitrogen (N) or sulphur (S) that may be present in the subsoil, and this information is important to develop accurate N and S fertilizer recommendations. If the 0 to 6 inch and 6 to 24 inch depths are sampled, the 6 to 24 inch depth is 18 inches of soil. This is a considerable amount of soil depth to sample. Nutrient levels are determined in parts per million which are multiplied by 6 to estimate lb/ac, which may over or under estimate soil nutrient levels. Eighteen inch depth samples can be difficult or misleading to interpret.

Thoroughly mix each composite sample and layout the soil samples to completely air dry to stop nitrate and sulphate changes. To air dry, spread a thin layer of soil onto clean paper, plastic sheets or place into clean, shallow plastic or aluminum trays. Dry the samples at room temperature in a clean room (no cats or other animals to prevent contamination). Do not use artificial heat to dry samples. If samples are sent directly to the lab in a moist condition, they must be shipped in coolers and kept below 5 C and arrive at the lab the next day for drying. If the samples take two or more days to arrive at the lab, nutrient levels may have changed. When moist soil samples are in sealed bags at room temperature, soil microbes can rapidly alter the levels of plant available N, phosphorus (P) and S, causing incorrect estimates of soil nutrient levels.

What analysis is required on each sample?
The important plant available macronutrients to test a soil sample for are nitrate-nitrogen (NO3-N), phosphate-phosphorus (PO4-P), potassium (K+), and sulphate-sulphur (SO4-2-S). Determine plant available N, P, K and S in the 0 to 6 and 6 to 12 inch depths and test for N and S in the 12 to 24 inch depth. Normally, there is no need to test for plant available calcium (Ca+2) or magnesium (Mg+2) as these nutrients are very rarely deficient in Western Canada.

It is a good idea to check the soil micronutrients copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), boron (B) and chloride (Cl). Testing for micronutrients every year is only necessary if one or more micronutrients are in the marginal or low range; otherwise testing every three or four years is adequate. It is important to realize the tests for B and Cl are not that reliable. Often soil analysis levels may appear low in B or Cl, but crops will not respond to added fertilizer.

Determining soil organic matter, pH (a measure of soil acidity/alkalinity) and electrical conductivity (E.C. – a measure of salinity) are useful to monitor soil chemical properties of your fields. Some agronomists may recommend determining Cation Exchange Capacity (CEC) and determining base cation saturation ratios. Research has shown this is not a useful determination for making fertilizer recommendations for most soils or crops in Western Canada.

Finally, make sure the soil testing lab uses the correct soil test methods. For Alberta farmers, all soil test P calibration has been with the Modified Kelowna method, since 1990. For Saskatchewan and Manitoba farmers, all soil test P calibration has been with the Olsen method (also referred to as the Bicarb method). Most other soil test P methods, such as the Bray method, have never been calibrated to Western Canada soils. Therefore, it is my opinion that other methods that have not been calibrated for western Canadian soils should not be used.

Soil testing labs determine nutrient levels in parts per million (ppm). Most labs will convert the macronutrient ppm levels to pounds per acre (lb/ac), but not the micronutrient levels. See the summary box to learn how a lab converts ppm to lb/ac.

During the coming months, watch for Ross’s articles in Top Crop Manager that will touch on soil test interpretation of N, P, K, S and micronutrient soil levels, and how to develop fertilizer recommendations to assist with your fertilizer planning for next spring.

How soil test labs convert ppm to lbs/ac
Laboratories make the assumption that:

  • One acre of soil 6 inches deep weighs about 2,000,000 lb; therefore multiply ppm X 2 = lb/ac
  • One acre of soil 12 inches deep weighs about 4,000,000 lb; therefore multiply ppm X 4 = lb/ac
  • Once acre of soil 18 inches deep weighs about 6,000,000 lb; therefore multiply ppm X 6 = lb/ac

Examples:
If Nitrate-N is 10 ppm in a 0 to 6 inch depth sample then:
10 ppm X 2 = 20 lb/ac of N.

If Nitrate-N is 10 ppm in a 12 to 24 inch depth sample then:
10 ppm X 4 = 40 lb/ac of N.

If Nitrate-N is 10 ppm in a 6 to 24 inch depth sample then:
10 ppm X 6 = 60 lb/ac of N.

Remember – great care is needed to take samples at the correct depths to accurately estimate soil nutrient levels.

 


September 8, 2015
By Ross H. McKenzie

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