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Five Questions to Ask About Your Soil Testing Program

By Dr. Rob Mikkelsen, West Region Director, PPI and Dr. Mike Stewart, Southern and Central Great Plains Region Director, PPI

Five Questions to Ask About Your Soil Testing Program

Soil testing techniques and procedures continue to evolve. But the goals of the individual grower must be considered in making fertilizer recommendations.

Regular soil testing is one of the best tools for determining which nutrients are present and how much will be available to your crops.Soil testing can predict which nutrients and how much you need to add for plants to reach their yield potential. Decades of improving lab chemistry techniques and extensive field calibration provide a measure of confidence in knowing what is happening in the soil. Nevertheless, there are several things worth considering before samples are sent to the lab and after the results are returned. Here are some questions and responses.

Do my soil samples adequately represent my fields?

Getting a good soil analysis totally depends on submitting a representative sample. Considering that the rooting zone of an acre consists of well over 1,000 tons of soil and that the laboratory will analyze only a tablespoon of this soil, the importance of getting a representative sample is obvious. A soil analysis can only be as good as the sample the lab receives. The greatest errors in soil testing typically occur as a result of taking a poor sample.

Why do I often need to add more nutrients than I harvest in crops?

Whenever farmers remove crops from the field during harvest, the nutrients in the harvested portion of the plants are also removed, thus leaving the soil a bit more depleted than before. Returning these harvested nutrients is a bare minimum for maintaining soil fertility levels. However, because of a variety of chemical and biological reactions that make many nutrients less available to the plant, the addition of more nutrients is usually required than just the amount harvested. Failure to replenish the soil supply of nutrients will result in a gradual depletion of soil fertility and can ultimately lead to diminished crop yield and quality.

But where is the fertilizer I added last year?

 In a perfect world, the crop would take up all the added fertilizer and use it for growth. Unfortunately, many soil reactions reduce the availability of applied nutrients ­— whether you add the nutrients as fertilizer, manure, or from other sources. Since these reactions differ for each nutrient, carefully consider where last year’s fertilizer became diverted from your plants and decide what practices you can adopt to improve fertilizer efficiency, boost yields and avoid undesirable losses. Although many practices can improve fertilizer recovery, nature will always remove a portion of the added nutrients and keep some from being available to crops.

How much fertilizer should I add?

Example of P recommendations based on the “sufficiency” and the “build and maintenance” approach for Kansas crop production. Source: Leikam et al. 2003. Better Crops with Plant Food, Vol. 87, No 3, p. 6-10.

General fertilizer recommendations are based on the results of the soil analysis and the expected crop response. But the goals of each grower need to be considered in making a specific fertilizer recommendation. One common approach, especially for phosphorus (P) and potassium (K), is called the “sufficiency” concept and is based only on the predicted crop response to each added nutrient. Fertilization occurs only when a profitable response is likely that year. This could be compared to adding just one gallon of gasoline to the car in order to get to the store and home again.

The other common approach to P and K fertilizer recommendations is the “build and maintenance” concept. With this system, nutrients are added in excess of the minimum crop requirement in order to build the nutrient concentrations in the soil to the point where they won’t limit growth potential. Liken this approach to “keeping the gas tank full.” Once nutrient concentrations reach the target range (the blue zone in Figure 1), add only enough additional fertilizer to maintain soil levels and  “top off the tank.”

So, what’s the right amount to add?

There’s no single answer to this question. The “sufficiency” approach may be best for a farmer who’s pinched for cash and can meet only the minimum nutrient requirements for a given year. Also, if the land is leased for short periods of time without the landowner and farmer sharing build-up costs, then the long-term benefits of nutrient building may not be realized.

When a longer period of tenancy is available, it’s often a wise investment to build and maintain soil fertility levels to the point they’ll no longer be a limiting factor. In these conditions, it may be possible in some years to reduce fertilizer application rates and draw on the nutrients already in the soil. Already-fertile soil gives you more application flexibility in good and bad years.

Consider the value of the crop and compare the costs of potential yield loss with the expenses associated with fertilization.The most appropriate philosophical approach will vary from farm to farm, thus the final decision is best left to the grower and his or her trusted advisers. Some soil testing labs have recognized the importance of providing this flexibility and offer P and K recommendations based on both the sufficiency and build and maintenance approaches to general fertilizer recommendations. The graph ( Figure 1) shown here provides an example of P recommendations based on both approaches for Kansas crop production.

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