Getting the Most out of Your Soil Test

By Dave Larsen, AAg

Conservation Agrologist

Soil testing is an important management tool that is often underutilized. An accurate soil test is a low cost way of determining the appropriate nutrient package for your crop. For less than one percent of the fertilizer cost, a field can have a fertilizer recommendation. Optimal fertility rates will maximize yield and economic return. Yet incidences of nutrient deficiencies are common. Under-fertilization will not meet the needs of the crop, while over-fertilization can be costly and inefficient. Without soil testing, nutrient application is merely a guess.

Crops require nutrients. Without a proper fertility package, the crops' nutrient requirements will come from soil reserves. The contribution from soil reserves may meet the requirements in the short term, but can have a long-term impact on future fertility of a soil. To maintain the long-term nutrient balance in a field, enough fertilizer must be added to meet the needs of the crop. Maintaining a nutrient balance leads to higher yields and quality, more efficient use of inputs, and increased profits.

If crop residue is returned to the soil, the amount of fertilizer that has to be added to maintain the status quo is equal to the nutrients removed by the crop. The amount of nutrients taken with the crop is summarized in Table 1. Long-term fertilization at recommended rates contributes to soil nitrogen supplying power. Nitrogen fertilizer that ends up immobilized in microbial biomass and soil organic matter contributes to a long-term reservoir of organic nitrogen that can be slowly made available through mineralization. (Schoenau et al. 1998).

Table 1. Nutrient uptake by the growing crop, and removal in the harvested portion of selected crops for western Canada.

Source: (Dr. Adrian Johnston, Potash and Phosphate Institute of Canada, 2002)

Yet less than 10% of the fields in Western Canada are currently managed based on annual soil testing practices (Karamanos, 2001). Amongst farmers that filled out survey forms at past SSCA Direct Seeding Conferences, only 35% reported soil testing on a regular basis. Something is obviously preventing producers from adapting this management technique.

So why isn't soil testing more common?

While the reasons for not sampling may vary, generally soil test results are considered too inaccurate to precisely follow the recommendations. Although testing labs are quite accurate, the greatest challenge in soil sampling is obtaining a sample that reflects the true fertility status of the field. Easy to say, not so easy to do. However, applying your knowledge of the field you are sampling will increase your soil tests accuracy.

Fields are inherently variable in nutrient composition. Mobile nutrients such as nitrogen, sulfate and chloride move with water and as a result, often accumulate in lower slope positions. As immobile nutrients, phosphorus and potassium are tightly bound to the soil and move through the landscape as a result of soil movement. Therefore, in a direct seeding system the phosphorus and potassium will be relatively immobile, as soil movement is minimal. Nitrogen, sulfate and chloride movement will not differ with direct seeding. Even in flat fields there is a wide range of nutrient levels throughout the field. The soil nutrient variability can make accurate sampling difficult. Therefore laboratory recommendations often do not match the crop's needs.

For example, a study in Alberta revealed a 40 acre field with soil potassium levels between 118 and 620 pounds per acre with an average of 270 pounds per acre. This would result in no potassium fertilizer recommendation for the field. With this recommendation, the field would be potassium deficient in 30 percent of the field with another 33 percent marginal.

The most common method of soil sampling is random sampling. Random sampling will typically generate numbers that are higher than the overall field's nutrient requirements. Typically the water-soluble nutrients, nitrogen and sulfate, will be more closely represented than the non-soluble nutrients, phosphorus and potassium. However, one or two cores that have a high concentration of sulfate can have a dramatic effect on the recommendation. For a crop like canola that is sensitive to S deficiency, the inflated S levels will create deficiencies in the crop if the recommendations are followed.

Variability of phosphorus and potassium will be greater than nitrogen and sulfate regardless of the sampling mechanism. Benchmark sampling has been shown to reduce some of the variability. Patterns of distribution of the immobile nutrients are harder to determine than the water-soluble nutrients. Fields under direct seeding will have a higher concentration of phosphorus and potassium in the top three inches. Increased nutrient concentration in the upper layer occurs from the fertilizer banding and nutrients released from residue decomposition. From a management point of view, the increased concentration of nutrients in the upper layer of soil is not detrimental. In fact, the increased concentration of roots in a zone of higher fertility can create greater nutrient availability. Samples should be taken away from the location of the nutrient band to avoid getting inflated levels.

(Doug Keyes, Norwest Labs, 1999)

Sampling one area is all that is required if you can find a representative area. This is called benchmark sampling. This method of sampling is simple and effective. Benchmark sampling reduces the inherent variability of a field by reducing the area sampled. A small area (generally about ¼ of an acre) representing the majority of the field is sampled. The same number of cores is taken, but it is taken from a smaller area. This is treated as the reference area from which fertilizer recommendations are made. It is marked with GPS and returned to for subsequent years. Sampling from the same area will reduce sampling variability, and create a better picture of year to year changes.

(Westco Fertilizers)

Analyzing a few separate benchmark areas in the first year will reduce the risk of getting a sample not representative of the field. Although you will have higher analysis costs the first year, it will help determine what area to use as a benchmark.

Another way to reduce the risk is to take a composite sample of a few identified areas. This form of managed random sampling will average the variability of the sampled areas, reducing the effect of a single unrepresentative sample. This is different from completely random sampling. Managed random sampling only samples from areas you identified as average production areas. Random sampling will be an average of all cores taken throughout your field. Managed random sampling or creating more than one benchmark is recommended if you cannot identify a dominant production area on your field.

Applying your knowledge of the field will help decrease the soil test variability. Your soil sample should be representative of the field. Therefore, by sampling from an area of the field where yield is typically average, your 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.

Areas to avoid include:

When picking a location, use observable features such as soil colour and landscape to roughly identify where different soil types occur. Select a site that has characteristics similar to most of the field or the dominant soil type. Often the best time to identify the different soil characteristics is through crop development. At the beginning of the growing season when crop establishment and vigour differences can be seen, a typical location may be easier to pick out.

If you are not comfortable in picking a location to sample or do not have the time to go out with the person taking the soil samples, there are a couple of options available. Maps of your field's productivity can be obtained from either yield monitors or satellite images. Areas of average production can be identified from the maps and geo referenced using GPS to the corresponding location in your field.

Yield maps require a yield monitor on the combine. Some custom combining outfits will offer this service if you do not have the equipment yourself. However, if you are not already planning to have a yield map made it is unlikely to be economically feasible. Yield is also affected by many factors other than nutrient deficiencies. If another factor is influencing yield (when isn't it?) yield monitors won't be as effective for nutrient assessment. A cheaper and more effective technique is to get a satellite image of the field you want tested. Previous crop years can be viewed at different dates throughout the year. The images display the vegetation growth on your field through infrared photography. The pictures will accurately depict management zones to help determine average production areas. This is a very effective and cheap technique, however it does require some technical ability to read the maps and operate the software.

With growing sophistication you can take the benchmark process even farther. Establishing a couple of benchmark areas in different areas will allow customization of your fertilizer rates. By identifying a primary benchmark area and a secondary benchmark area and perhaps even a tertiary benchmark area, you can further fine-tune your fertility package even without variable rate technology. Analyzing a couple of production zones will provide you with a good understanding of your fields' fertility levels. If there are deficiencies in the secondary benchmark area that do not occur in the primary benchmark area, then a decision should be made as to whether the extra yield on the secondary benchmark is worth the over application on the primary area.

Dividing your field into management zones allows you to get an understanding of different conditions within your field. This is particularly effective in rolling 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, rates can often be easily increased through other adjustments.

There are 3 companies in Western Canada that will provide soil analysis. They are Western Ag Labs, Norwest Labs, and Envirotest Labs. Some will also provide field testing services or work through an input supplier to provide the service. While each lab will provide consistent and accurate analysis they do have some differences in their techniques and recommendations. You must decide which one you are most comfortable with. Finding out what your soil has available and how you can tailor your fertilizer package to optimize yield will take a lot of the guesswork out of your spring fertilization.