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Stepping Up Your Management Game: Soil Sampling

You’ve probably been hearing it from us a lot lately: its time for soil sampling, get your soil sample acres booked, have you made your soil sampling plan? So what is all the hubbub about? And why, in tight financial situations, should you make the investment in soil sampling?

Why Soil Sample?

Soil sampling allows us to take an unknown variable and quantify it for use in decision making down the road. Without soil sampling we are making fairly expensive decisions each year based on minimal to no information. Too much fertilizer could be applied resulting in unnecessary expenses, and conversely, too little fertilizer could be applied resulting in not enough nutrients for optimal yields. An article by AgProfessional states that 2.5 acre grids will often pay for themselves in the reduction in excess fertilizer application, or the increase in yield from targeted applications. This really is one of the pivotal benefits of soil sampling; knowing if you are fertilizing the correct amount.

When we are taking soil samples we are trying to discover all the different variation and fertility levels within a field. We achieve this by sampling across the whole field in a dense pattern. Traditionally this can be done by grid or zone sampling, however we would recommend grid sampling as the best way to quantify all variability within the field. Historically, if soil samples were taken, it was primarily concerned with comparing levels of fertility field to field. Now we are focusing on all the variability possible within a single field. Often times this variability is even higher within a field than between fields.

In addition to soil test results, the use of past yield data and imagery can help make and verify decisions. This includes past soil test data. Multiple years of soil sampling can really prove to be of benefit. Using current soil test results in addition to past results helps provide a trendline explaining how the field has reacted to fertilizer applications as well as crop withdrawals. Using historical data can help provide additional accuracy to current soil test results. This also allows us to begin to calculated the amount of nutrients withdrawn for each year and what we can expect for nutrient removal in an average year.

Why Grid Sample?

With various options available, why should you grid sample? Grid sampling is traditionally more expensive than whole field or zone sampling but provides a much more detailed look at what is going on in the field. This is particularly of benefit in fields that haven’t been sampled before and for maintaining trendlines for fields that have been grid sampled in the past. Grid sampling also allows for variable rate fertilization which really optimizes the overall fertilizer use on a field. Grid sampling is also known for its accuracy and unbiased representation of the field.

Grids are traditionally taken on a 2.5 acre grid. While this is not the highest level of accuracy, it provides the highest density for the most economical cost. Interpolation is then used to determine the values between the grid points. 2.5 acre grids can then be used to create variable rate fertilizer maps.

The University of Nebraska has conducted research showing that sub acre grids are really optimum for determining field variability that affects crop growth. However, sub 1 acre grids are very cost prohibitive. They suggest conducting 1 acre grids for a balanced approach and have determined that at this density, samples would be valid for 5-10 years for pH and 4-5 years for phosphorus, potassium, and zinc. They do note that in fields with larger scale variability or in situations where some of the field history is known, sampling at 2.5 acre grids is feasible. However, much larger than this, you begin to lose resolution and miss patterns and hot spots within a field.

Variable Rate Fertilization

“Variable rate fertilizer application involves the application of different rates and/or types of fertilizers on uniquely different soil areas within a field according to a pre-set field map that is developed based on various types of information.” As mentioned before, this practice reduces fertilizer costs in portions of the field needing less fertilizer, and while potentially increasing fertilizer costs in other areas, will also result in optimum yields and a higher return on investment for the applied fertilizer. If a field is known to be extremely uniform, it is less likely that VRF will be beneficial. If the field has known variability in soil types and management history, VRF can help.

Its all based on this data

Many of the decisions we make throughout the growing season rely on this issue of fertility. If we do not have up to date information, each fertilizer decision has the potential to further compound issues we are already seeing in the field, result in excess money spent, or cause a reduction in yield. We can create the perfect hybrid and seeding plan, have the ideal growing season with optimal rainfall, and get to the field on time for harvest, but if nutrients are not available to the crop to optimize growth from the get go, we have compromised all of our hard work and decision making.

Look for future articles from us covering some case studies for soil sampling and variable rate management, and the impact it made. This will all be part of a new blog series that will be presented off and on for the coming months: Stepping Up Your Management Game. We hope you will join us!

Do you have questions about soil sampling or are curious about a plan for your acres? Contact one of the Pederson Seed team for more info!


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