Agronomic Testing Options
My titles have not become any more creative, but I get my point across.
This is the second to last post of the 52 Weeks of Agronomy Series. I would be lying if I didn’t say I’m slightly relieved. At the same time, I’m looking forward to having more time for other projects that do not require typing, or at least me doing it. Maybe one will require me wearing out a red pen ripping apart someone else’s grammar. Who knows!?
What I do know is that we have come to the time of year farmers have questions. They have them year-round, now they have more time to ask them.
One of the most recent questions I’ve gotten is regarding the different types of agronomic testing available to farmers and growers.
The following is part of an email conversation with a grower I work within Maryland-
The farmer asked,
“I was thinking about doing a sap sample of the grass and clover in the field and also a Brix's reading at the same time. What are your thoughts?”
My response was,
“As far as the sap and Brix's testing, from an agronomic standpoint they won't change any soil fertility recommendations I’ve made. Everything I look at and take into consideration is based on soil tests in conjunction with tissue testing depending on the type of crop, farming system, and whether the grower will see an economic return for the costs of the tests. I'm looking at the actual nutrient levels of nitrogen, phosphorus, potassium, calcium, etc. There are so many variables like time of day the sample was taken, what part of the plant was sampled, and what calibration standards were used, it would make it difficult for an agronomist to base a soil fertility assessment on sap and Brix's levels.
Tissue testing utilizes dry matter - it’s a picture at the moment at that part of the plant sampled as to what is there
Sap testing utilizes the fluid portion of the plant; it's similar to a blood test - keep in mind that fluids are constantly moving and every nutrient moves at different rates
There are always variables in any kind of testing; you can't mitigate for everyone. However, soil and tissue testing are the best tools currently an agronomist has to look at what is going on at that moment in time.
That being said, and due to the fact, you are testing sugar and protein levels in the plant with the Brix's test, from a forage quality standpoint, the results may be of value to an animal nutritionist when determining rations and taking animal health into consideration.”
Here are some common agronomic tests, followed with a brief description and some of my thoughts on them.
Standard soil test- I talk in detail about what a soil test should include in week 24
https://www.theaccidentalagronomist.com/news/2018/6/11/ag-101-week-24
Soil health assessments conducted by universities include soil typing and physical analysis of samples similar to what Cornell offers at the following link
http://soilhealth.cals.cornell.edu/testing-services/comprehensive-soil-health-assessment/
In general, I recommend a standard soil test and use the link at the Web Soil Survey
https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm
Using both of those is comparable to Cornell’s package costing considerably less
Soil paste test-
In general, this test shows what nutrients are immediately available to a plant because they are not bound to a colloid.
It is also one of the most accurate tests for measuring soil salinity which can be a common problem in soils in the central and western part of the country.
Some agronomist’s feel a standard soil test plus the paste test is the most accurate way to determine soil fertility needs. In my experience, it is farm, soil type, and management dependent as to whether you will see a return on investment for the extra cost.
Plant tissue test-
As stated before in my email response; tissue testing utilizes dry matter - it’s a picture at the moment at that part of the plant sampled as to what is there.
It is a good measure of the nutrient status of a particular plant at that moment in time giving you a picture of what may typically be happening. It is a good indication of the efficacy of your fertility program as well. Tissue tests are dependent on timing, plant temperature, and growth stage the sample was taken.
Typically, guides are offered for the plant stage at which a tissue sample should be taken based on crop type. The sample is taken when the plant is actively growing, so timing is critical.
Spectrum Analysis offers a detailed guide to plant tissue testing
https://www.spectrumanalytic.com/services/analysis/plantguide.pdf
Plant sap analysis-
This is the best analogy I have come up with for sap testing-
Sap testing utilizes the fluid portion of the plant; it's similar to a blood test - keep in mind that fluids are constantly moving and every nutrient moves at different rates
The following is an interesting article regarding sap analysis
https://www.specmeters.com/newsletter/if-plants-could-talk-vol-4/
I believe it can be a useful in-field test. However, there are variables such as operator error that would make me uncomfortable using it as the sole means of making fertility recommendations.
There are a few labs in the states that conduct sap analysis. One of them being
https://www.crophealthlabs.com/
Brix test-
A brix test is the measure of the sugar content found in a plant tissue sample that has been pulverized, and the aqueous solution from that is then put in a refractometer and read.
Once again as with the sap testing, I believe it has in field use especially in a grazing operation. However, the variables associated with it make it suitable for field use and not what I would base a sound soil fertility program on.
I work with growers that use it in the field to help with harvest decisions with a tremendous amount of success. Using well-calibrated equipment with consistent protocols for testing has saved them time and money ensuring they harvest at that crop’s peak quality and nutrition levels.
Just as with plant sap analysis I believe it can be a useful in-field test. However, there are variables such as operator error and calibration protocols that would make me uncomfortable using it as the sole means of making fertility recommendations.
Soil Biological testing-
Co2 Burst test-
This test is a soil health indicator, measuring the amount of microbial respiration.
The following pdf is the best explanation I have found
https://www.wardlab.com/download/biotesting/Respiration_Information.pdf
I would recommend growers use this in conjunction with soil nitrate tests to make a comparison and better calibrate any side-dress nitrogen applications that might need to be applied during the growing season.
The following is a well-written extension guide from Penn State explaining pre-side dress soil nitrate testing
https://extension.psu.edu/pre-sidedress-soil-nitrate-test-for-corn
Microbial Identification-
Microbial identification is done with a microscope.
There are several consultants that provide soil microbiological identification services at the following link
https://soilfoodweb.mykajabi.com/p/find-a-consultant-lab
Water testing-
Water can be the carney in the coal mine especially when it comes to greenhouse management. pH is the most critical aspect of water testing in term of a soil fertility program and soil health. However, with water and its natural pH due to its source, comes with its dissolved elements that could be beneficial or not depending on the situation. The best measure to determine the impact irrigation water will have is through testing.
The following are links to labs I regularly use for soil and water testing
https://www.spectrumanalytic.com/
http://www.waypointanalytical.com/
Entomological/ Pathological testing for field crops-
Before I started the 52 Weeks Series, I wrote a post titled “What’s an Agronomist, Anyway?”
Here’s an excerpt-
“On any given day as challenges and questions are posed to me, I have to think like a biologist, a chemist, a botanist, sometimes a pathologist, and if I’m really confident an entomologist. Usually, I defer to experts on the pathology and entomology. I know enough to be dangerous and kill stuff. “
As an agronomist, I kill stuff or keep it alive depending on what the farmer wants me to do.
What I can also say is I have been doing this long enough to make a definite diagnosis like thrips, aphids, blights, rusts, etc. However, when it goes beyond the obvious, I send a sample to the local land-grant university for identification. That’s what they are for, as well as other services.
Some good references to have on hand to make common identifications easier are-
Introduction to Plant Diseases: Identification and Management by George B. Lucas & Lee Campbell
Garden Insects of North America by Whitney Cranshaw
The Ortho Problem Solver by Michael D. Smith
Forage test-
Ask an animal nutritionist or forage specialist. I can’t be an expert at all things.
In week 44 I discuss potassium and admit what my dad refers to me as
https://www.theaccidentalagronomist.com/news/2018/10/29/ag-101-week-44
With that in mind here are basic ratios I look at when interpreting a soil test for forages-
Calcium to phosphorus 1.7:1
Calcium to magnesium 2.7:1
Calcium to sulfur 2.1-1
Potassium to calcium 1.5:1
Potassium to magnesium 4:1
Protein to sulfur 50:1
Nitrogen to sulfur 8.8:1
Potassium should be between 3.0-3.5% Calcium between 70-75% of the base saturation
One of the best agronomists who has an extensive forage background and one I’ve had the pleasure to be around and learn from is Dave Wilson from Penn State Extension.
Here’s a link to two articles he has written
https://extension.psu.edu/shopby/dave-wilson
In summary-
This is in no way an exhausted list. There are several tests that are similar but have different names based on the lab offering them.
I am not suggesting that every farm needs to be doing all the mentioned tests. Each one mentioned is another layer of management in the overall plan of how you are running your farm. They are not meant to be used in every case all of the time. Every farmer needs to weigh if the results from the analysis will yield an adequate return on investment for their operation.
Just as the title states, they are agronomic testing options.