Steps To Understanding A Soil Report And How To Use That Information To Take Corrective Action
Now before we begin, we need to clarify the scope and type of soil report we are using today.
The sample report we are using will be reflecting values in parts per million (ppm) derived from a Mehlich 3 (M3) extraction process. We are assuming we are utilizing AgriGro’s prebiotic technology to get maximum nutrient efficiency from those results.
A good comprehensive soil report will have results for the macronutrients phosphorus (P) and potassium (K), the secondary macronutrients calcium(Ca), magnesium (Mg), sodium (Na), sulfur (S), and also essential micronutrients such as boron(B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn). Some additional nutrients you may want to check as you get more thorough are chloride, cobalt, moly, nickel, and silica.
Let’s start with one of the most confusing or misunderstood numbers on this report. The CEC, which is the Cation Exchange Capacity (CEC) that was either measured, calculated or estimated for the soil being tested. Sometimes it is shown as ECEC for estimated CEC or CCEC for Calculated Cation Exchange Capacity. Regardless, the number represents the total milliequivalents (meq) or positively charged cations that 100-grams of this soil can hold. In other words, you could say it reflects how big of a sponge your soil has. The more clay in your soil the bigger your sponge. The higher the CEC number the more nutrients are retained on a soil particle.
There is not necessarily a good or bad CEC reading, it just tells you what type of soil you have and will indicate how much material will be required to add or remove to balance the soil based on its holding capacity for cations.
The primary cations are hydrogen, calcium, magnesium, potassium, and sodium.
This chart copied from an agronomy handbook produced by A & L Labs shows the desired levels of three of these cations in balanced soil. To compute the desired balanced level of sodium you can multiply the CEC by 3.42 to give you the ppm desired. To convert ppm to lbs./acre, multiply by two.
You may have noticed that hydrogen is also missing from this chart. Hydrogen can be looked at like a placeholder on the soil. When calcium, magnesium, sodium, or potassium are missing, hydrogen takes its place on the soil particle. The more hydrogen on the soil particle the lower the pH . As you replace hydrogen with a missing cation like potassium or calcium the pH will move up. When the pH is at 7 you will not have any hydrogen showing on your soil report. It is desirable to have 10 to 15% hydrogen for good exchange and availability of phosphorus and micronutrients.
The next section of the soil report we want to observe that will quickly tell you the condition of the soil will be the area that summarizes the percentage of saturation of each of the five main cations: Potassium, Calcium, Magnesium, Hydrogen and Sodium.
If your CEC is showing higher than eight then you will want calcium percentages to be around 70% and magnesium around 10%, followed by potassium above 2% and sodium less than potassium.
If your CEC is below eight then you can have adequate calcium around 60% and magnesium around 20% with a minimum of 3% potassium and sodium less than potassium.
After these percentages are investigated and you have determined what cations are deficient or in excess you then can look at the extraction results to determine quantities needed to correct the soil.
Phosphorus is not a cation. On an M3 test for adequate amounts, it should be around a minimum of 25-ppm or if we multiply by 2, 50 lbs./acre and remember these figures are only valid when utilizing IgniteS2® or SeedMaxx® prebiotic technology. There is virtually no return from pushing P levels higher than 30-ppm.
Next if we have potassium less than 2% on a CEC 14 soil, we need to look at our results and then check the chart to see what level the potassium should be in order to be balanced. For example, our chart shows 164-ppm for a normal 2-5% range on a 14-CEC. Our results indicate we have extracted 132-ppm. 164 minus 132 = 32-ppm of Potassium deficient. 32-ppm x 2 = 64 lbs/acre of lacking potassium (K).
We now have to decide how we are going to add this 64-lbs. of potassium. There are many options for adding potassium either liquid or dry to the soil and you could also decide to just feed the plant now that you know it is deficient and yield-limiting nutrients in this soil.
The same determination and method should be followed for all the nutrients that have been tested.
For an M3 extraction process, you should use these values for a minimum desired level of all elements that are not shown in the cation percentages…
Manganese: 40-ppm and less than Iron
This information should put you well on your way to becoming proficient at analyzing a soil report and putting a plan in motion to improve or maintain your soil when paired with AgriGro’s prebiotic program.