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Seven steps to accurate soil testing

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Agronomy Management: Get your money’s worth from your fertilizer by using good soil test practices

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Many Prairie farmers spend $250,000 or more on fertilizer each year. Are you getting your moneys worth? Applying fertilizer nutrient that is not needed is a waste of money. If you don’t apply a needed fertilizer, you’ll see a loss in yield and revenue.


Soil testing can provide an excellent picture of the nutrient levels in each field and can be very useful in fertilizer planning. But only about 10 per cent of Prairie farmers soil test regularly.


You likely do not need to soil test every one of your fields, each year. But testing selected fields each year is a wise decision to assist with your fertilizer planning to ensure you are spending your fertilizer dollars wisely. 


Related: Profit-making N strategies for growing corn

1. Go out with the sampler

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If you don’t normally go out to the field with the person taking your soil samples, you should. Here’s why:


1. Getting out in the field gives you a close up look at your soils. View the depth of top soil and subsoil, view the structure of your soil and develop a sense of the soil variability in each field. 

2. Ensure the right fields are sampled.

3. Ensure soil samples are taken from representative areas of your fields. No one knows your fields better than you!

4. Ensure that the correct depths are sampled and at least 20 soil cores are taken from each field to get a good representation of each field


Riding along with the soil sampler can be a great learning opportunity and ensure the sampling is done correctly.


Related: Putting life back into tired soil

2. Sample in late fall

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Soil sampling just before spring seeding gives the most accurate measurement of soil nutrient status. But spring is often too short and rushed to allow for soil sampling and analysis in time to develop fertilizer plans. 


Fall sampling is often the most realistic. Some fertilizer dealers, labs and agronomists are promoting soil sampling right after combining. They often state there is not enough change in soil nutrient levels over the winter to worry about. In some cases, this is correct, such as when soils are dry between harvest and freeze-up. Or, if combining isn’t completed until early October. 


But, if soil is warm and moist for four to six weeks after harvest, 20 to 40 pounds of nitrogen per acre or more can be released in a fertile soil. Significant release of N and other nutrients will impact nutrient levels and should be taken into consideration to make good fertilizer decisions. Soil sampling right after combining may not always give the best information.


I much prefer sampling in late fall after soil temperature has dropped to 5 to 7 C. Hopefully this will more closely represent conditions in the spring. 


But remember that soil fertility is very dynamic and nutrient levels may change depending on environment conditions. My concern with late fall sampling is that availability of nitrate-nitrogen (NO3N), phosphate-phosphorus (PO4-P), potassium (K+), and sulphate-sulphur (SO4-2-S) can fluctuate from fall to spring, particularly if soils are moist with warmer than normal conditions. Excessively wet conditions in spring can lead to denitrification. Also, nitrate and sulphate are quite mobile in soil and may leach downward under wet soil conditions, particularly in sandy soils. 

Generally, I do not recommend sampling frozen soils because of the difficulty in obtaining representative sampling depths.


Related: When it comes to healthy soil, you want to lump it

3. Know the common sampling methods

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Many prairie fields have undulating to rolling topography resulting in soil variability. This is a major challenge when deciding how to take representative soil samples. 


If you have rolling topography, work with your agronomist to help you decide how to sample each field. 


There are three more common sampling methods: 


1. Random sampling of a whole field. This works best in fields with relatively uniform soil and topography. Take representative soil samples throughout the entire field, but make sure to avoid unusual areas. 


2. Sampling soil/crop management zones. This works best in fields with variable soil and topography. Map uniquely different zones based on soil characteristics, topography, and/or crop yield potential, then take representative soil samples within each management zone. This method works well in fields with variable soil. Each management zone can be randomly sampled or benchmark sampled. Consider working with an agronomist to prepare a soil/crop management zone map for each field.


3. Benchmark soil sampling. Sample a one- to two-acre area that is representative of the majority of the field or soil/crop management zone. Sample the same area each year. When a field is variable in soil or topography, three or more benchmark locations may be needed to represent the different field areas.


When selecting soil/crop management zones use crop yield maps, aerial photos, topographic maps, soil salinity maps and/or satellite imagery information. Also, use your personal field knowledge and observations of crop growth differences (crop establishment, vigour, colour and growth) and landscape/topography of each field to identify where different soil types occur. 


Related: A healthier soil test

4. Sample at least 20 sites

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I suggest taking a minimum of 20 soil sampling sites for each field, management zone or benchmark area. More sites means the samples will be more representative of your field. 


A common mistake is only taking six or seven soil cores from a field or management zone. This is not enough and can result in unreliable information and inaccurate fertilizer recommendations. 


Related: Taking fertilizer beyond yield

5. Sample at three depths

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Various sampling depths are suggested by agronomists. 


My preference is to separate each soil core into depth intervals of zero to six inches, six to 12 inches, and 12 to 24 inches. Place samples from these three depths into three clean plastic pails. Do this at each of the 20 sites sampled.


Sampling three depths will give a good picture of the amounts of each nutrient and where the nutrients are located. Most of the soil test correlation research for nitrogen and sulphur in Western Canada has been done with sampling to 24 inches. For phosphorus and potassium, most soil test correlation research has been with the zero to six-inch depth, as these nutrients are much less mobile in soil. 


Some agronomists prefer to only sample one or two depths making the sampling process simple and faster. If only the zero to six inch and six to 12 inch depths are sampled, you don’t know of the amount of N or S that may be present in the subsoil. This information is important.


After the 20 soil cores are taken, be sure to thoroughly mix each composite sample and layout the soil samples to completely air dry to stop nutrient changes. If moist soil samples are sent directly to the lab in sealed bags, soil microbes can alter the levels of plant available N, P and S causing incorrect estimates of soil nutrient levels. Or ship the soil samples in coolers to the lab over night.


Related: Soil test right after the combine

6. Test for the right nutrients

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The important plant available macronutrients to test a soil sample for are nitrate-nitrogen (NO3N), phosphate-phosphorus (PO3-P), potassium (K+), and sulphate-sulphur (SO4-2-S). Determine plant available N, P, K and S in the zero to six and six 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. 


You can also test for 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 five years is fine. Also, the tests for B and Cl are not that reliable. Often soil analysis levels are interpreted as low for B or Cl, but crops do not respond to added fertilizer. The problem is with the soil test methodology and critical levels used, resulting in unnecessary fertilizer recommendations.


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, Alberta samples should be sent to a lab that uses this method. For Manitoba farmers, all soil test P calibration has been with the Olsen method (also referred to as the Bicarb method). For Manitoba farmers, use a lab the uses the Olsen method. For Saskatchewan farmers, either method can be used to determine soil P. Other soil test P methods, such as the Bray method, have never been calibrated to Western Canada soils. It is my opinion that other methods that have not been calibrated for Western Canadian soils should not be used.


Related: Soil health scores spark interest

7. Get unbiased reccommendations

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The process of soil test interpretation is the next step in the process. A fertilizer salesperson working on commission may not be your best source of unbiased advice. Make sure you seek the advice and opinion of one or two independent agronomists when developing your fertilizer plans for next spring.

Related: New soil test company on the block

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