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Assembly required: Soil test instructions

Reid Abbott, Field Agronomy Manager for AgroLiquid
Reid Abbott, Field Agronomy Manager for AgroLiquid

In my house, two words — “assembly required” — have come close to ruining more Christmas mornings than the Grinch himself. For many growers it seems the words “fertility program” can cause as much anxiety as staring down a Barbie Dreamhouse box under the tree. One savior that occasionally graces us with its presence both on Christmas morning and in the field however, is a good set of instructions. The right manual can ease the assembly process and increase the likelihood of a job well-done. When determining a fertility program, a soil sample report is that instruction manual and can very well keep you from running to the bowl of egg nog.

Upon first glance, a complete soil sample report can look like something a nuclear engineer carries in his briefcase. However, when each component is broken down, a picture of the soil conditions and needs start to come together. Organic matter percentage, cation exchange capacity, pH and base saturation percentages should be included on any standard soil test and are key to understanding the physical and chemical characteristics of a soil.

Organic matter percentage gives a grower an idea of the overall health of his soil as well as a rough estimate of certain naturally occurring nutrient releases (such as nitrogen) throughout the growing season. Cation exchange capacity (CEC) indicates the nutrient holding capacity of a soil and to an extent the soil’s texture.

As many are aware, the pH measures the hydrogen ion concentration (or acidity) of a soil. This value can give a grower insight in to how available (or unavailable) essential plant nutrients are in the soil. Finally, the base saturation percentages quantify the relationship between calcium (Ca), magnesium (Mg), potassium (K), hydrogen (H) and sodium (Na) in the soil. Depending on how these nutrients align with each other can widely affect the soil’s physical and chemical properties, nutrient interactions and a crop’s success. Agronomists use these values plus the soil’s CEC and pH when recommending soil amendments to determine the type of product (lime, gypsum, etc) and amount to be used to correct an imbalance in the soil.

After determining the opportunities and limitations due to the soil’s properties as discussed previously, a grower can accurately predict a yield goal and look to the individual nutrient levels to further develop a fertility program. On a soil sample report, individual nutrients are generally reported in parts per million (ppm).

Multiplying the values by a factor of two gives a unit of pounds per acre (lbs/A) which more easily correlates with application recommendations. In the case of nitrogen (N), a recommendation simply comes from the crop’s needs (based off yield goal) minus the N present in the soil (according to the soil test). Loss factors based on application options and climate must also be calculated when recommending N, but by-in-large, it is a fairly straightforward nutrient to recommend based off a soil test.

In contrast, however, due to the unique nature of phosphorus (P) and its habit of tying up in the soil, different tests and considerations must be taken into account. Bray1 and Bray2 P tests indicate readily available and slowly available P, respectively, in neutral and acidic soils. If the soil has a high pH, a bicarbonate test is performed to more accurately demonstrate available P levels. Also, phosphorus (P) can be affected by other nutrients such as zinc (Zn). For instance, high and low Zn levels each offer their own challenges for phosphorus efficiency in the soil and the plant. It is somewhat of a case of science meets gut-feeling when it comes to recommending phosphorus. Likewise, K is challenging because many variables have to be taken into account. For example, high Mg and Na saturations can negatively impact K efficiency and with low CECs, a soil’s ability to hold on to applied K is diminished.


Secondary and micronutrients are just as critical as the three macronutrients but are used by plants in much lower quantities. Regardless, they still require their own balance to function properly and for a crop to succeed. Paying careful attention to soil test levels, crop needs, nutrient interactions, and application options of secondary and micronutrients can produce big rewards at harvest.

When it comes to a proper fertility program, there will always be some “assembly required.” However, just as you would gather tools and instructions to put together a new Barbie Dreamhouse, obtaining a complete soil analysis and some trusted advice will ease the pain and set your farm on track for a successful year.

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One comment

  1. A very well written article. I would recommend using an independent lab for the testing.

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