Understanding and Utilizing Total Soil Nutrients

Now that harvest is wrapped up, planning for the next crop year begins.  And, the best way to start is by getting a better understanding of your field’s soil fertility by taking soil tests.

But, did you know that when you receive a soil test analysis only the estimated “plant-available” nutrients are shown, not the total amount of nutrients present in the soil?

With fertilizer reaching near record high prices, farmers are looking for ways to get more value out of the nutrients that exist in their fields.  It’s generally known that only a portion of the nutrients in the soil are plant-available.  Some portions of the nutrients are very slowly available, some are moderately available, and others are immediately plant-available.  

Our Study

To find out more about how much of our soils’ total nutrients are plant-available, in the spring of 2021 Granular Agronomy’s research team coordinated with 15 Granular Agronomy Certified Service Agents (CSAs) on 64 fields from the panhandle of Nebraska to southeast Indiana to collect key data. Two soil samples were taken per field and sent to Waypoint Analytical laboratories for two forms of analysis:

  1. S3M – The typical plant-available analysis of macro and micronutrients used for fertility recommendations.
  2. Total Nutrients (Digestion) – A stronger acid based extraction to determine the total nutrient levels in the soil.  This is more often used in environmental testing situations.
Locations of participating fields

The S3M analysis typically costs $8-12 per sample, whereas the Total Nutrients test costs $35-50 per sample, depending on the lab and volume of samples.

Results

The data from the two analysis types were merged by field and sample ID as they came back from the lab.  A summary of the results is listed below.

As the table indicates, the Total Nutrients results are many times higher than the plant-available.  Based on this data the relative nutrient plant availability can be calculated and expressed as a percentage.

Focusing on two main nutrients, the analysis indicates that in general, 9 percent of Phosphorus and 11 percent of Potassium from the soil samples taken are plant-available (using chemical extraction methods). The other ~90 percent are still unavailable to the plant. The results did show that in some sample locations, available Phosphorus could be as low as 1.5 percent and as high as 37.5 percent, and Potassium between 3.4 percent and 38.6 percent.  This provides some guidance as to the efficiency of each point’s ability to make total nutrients available to the plant.

Examples and Observations

  1. A sample from north-central Iowa had a relatively low plant available phosphorus value (19 ppm) but had 986 ppm of total nutrients phosphorus.  The sample also had 7.9 pH and 69,800 ppm of total calcium.  Upon further investigation it was determined this sample was taken in a Harps clay loam soil.  Harps soils are formed on the narrow rims or shorelines of depressions on till plains and moraines and are known for high pH.  While there was plenty of total phosphorus, only a small amount was plant-available.  This information was useful for the participating farmer to adjust fertilizer placement methods to increase P uptake efficiency.
  2. A field in Nebraska was the recipient of many loads of manure from a nearby cattle feedlot, and the soil tests, both total nutrients and plant-available, reflect elevated nutrient levels.  Percent Plant-Available Phosphorus is 22-37% and Potassium is around 17%.  However, plant tissue tests taken during the growing season showed deficiencies of both P and K.  The soil pH was elevated (8.5-8.7 pH) and sodium levels were also high.  This discovery helped the CSA and the farmer to determine a better strategy for increasing nutrient uptake into future crops.

Estimating Total Nutrients

The value of having Total Nutrients soil levels available to the farmer, along with Plant-Available, is to help determine where soils have more or less efficiency in making nutrients available to the plants.  However, given the extra cost for Total Nutrients tests, it would typically be cost-prohibitive in many situations to obtain both sets of analyses, especially under an intensive sampling protocol.

Fortunately, by using multivariate regression analysis of this data, it is possible to predict the Total Nutrients level (and range of potential error) for a given sample location based on the plant-available sample data.  The higher cost test can be reasonably estimated using the lower cost soil test. Upon running the multivariate regression analysis, these coefficients of correlation (R^2) values were found. (0 = no correlation, 1 = complete correlation), along with the mean absolute error, by nutrient.

Putting Total Nutrients to Work

With these regression calculations, estimated Total Nutrients can be estimated using spatially-intensive soil samples and interpolation. As an example, here are two maps of total nutrient Phosphorus and Potassium along with four sample points with known total nutrient measurements.

Estimated Total
Nutrients Phosphorus
Estimated Total
Nutrients Potassium
Plant-Available Phosphorus
Plant-Available Potassium
Estimated Percent 
Plant-Available Phosphorus
Estimated Percent 
Plant-Available Potassium

In this example, the lower estimated Total Nutrients areas are historically higher yielding. There are fewer Total Nutrients due to crop removal, but a greater percentage of what exists is plant-available.  Knowing this may be useful in fine-tuning fertility recommendations (areas of high Total Nutrients and Estimated Percent Plant-Available could require less due to better efficiency).

Next Steps

If you’re looking to gain a better understanding of the total soil nutrients in your field or just start planning for the next crop year then reach talk to your local Granular Agronomy certified services agent (CSA) today.

Find Your CSA

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