Data collection is only useful if you can use it to make management decisions. ‘Manage-able’ data in other words.
Soil analysis is still somewhat of an imprecise science. If you disagree, split a soil sample, send it to different labs and observe the results. This doesn’t mean we shouldn’t conduct soil analysis. It does mean we should understand what we are getting, and know the difference between the various extraction methods.
I credit much of our success in our agronomy work at Advancing Eco Agriculture to the fact that we avoid guessing about anything we have the capacity to measure. We look at soil nutritional profiles, irrigation water quality where that is relevant, and actual plant absorbed nutrients through sap analysis.
Our approach to soil analysis has continued to evolve. A decade ago, we would run an ammonium acetate extraction in early fall, August – September time frame, and saturated paste extraction every two weeks during the crop production season. You get an education really quickly about fertilizer performance when you measure what is happening in the soil with plant available nutrients every two weeks.
After the development of reliable plant sap analysis in the lab, we replaced the biweekly saturated paste samples with biweekly sap analysis. After all, the plant is the final report card, and it can tell us precisely what it is finding abundant or missing, regardless of the soil levels.
Sap analysis informed us rapidly that there is very little correlation between the presence of nutrients in the soil and actual absorption by the crop. In fact, in the case of some nutrients such as iron and manganese, there is zero correlation. (for reasons of soil redox, dysfunctional biology, and more.)
As a result of constantly learning and improving our understanding of what is happening with the soil’s nutritional profile, I would suggest that growers should collect at least three different types of soil samples to understand what is happening with soil nutrients.
- A ‘geochemical assay’ type soil analysis that measures the total mineral content within the profile. This will be the assay that shows in black and white the tens of thousands of pounds of phosphorus and potassium, and the hundreds of pounds of manganese and other trace minerals contained within many soils. Minerals that biology can tap into over time. Several samples should be pulled at different depths. One sample should be as deep as the A horizon, the topsoil layer, or as deep as the upper mass of roots generally reach. Usually somewhere in the neighborhood of 6-12 inches. A second sample should be collected immediately below the first, down to a depth of 24-36 inches, the B horizon. These samples only need to be collected once, to give us evidence of what reserves we have to work with, or not. If there is no molybdenum, selenium, cobalt, vanadium, or some of the other ultra trace elements showing up, crops will benefit from adding some. This type of sample can be run through AGAT Labs, and possibly others.
- A more familiar ‘CEC’ analysis with ammonium acetate or Mehlich III extraction. Mehlich III or Olsen extraction are preferred for phosphorus, Bray extraction can be unreliable in some soil types. We typically use a Mehlich III extraction for all the nutrients across the board. With this analysis it is also valuable, I would suggest necessary, to measure cobalt, selenium, molybdenum, and nickel as a standard, at least on some fields. We typically conduct this analysis in late summer/early fall, every year on high value crops, and every few years on broad acre commodity crops.
- An organic acid, ‘H3A’ or Haney analysis to identify the nutrients the soil is capable of releasing in the coming growing season. We have just begun including this test as a standard, and are still learning it’s ins and outs. We are experimenting with both spring and fall samples, but it makes sense to me to collect these samples in the spring to most accurately identify what is happening closest to the crop season.
When you collect all three of these samples, you can form an accurate perception of what is really going on with your soils mineral profile, and the resources you have available to work with. Now you can make informed decisions about nutrients actually need to be applied, and what you can tap into from the soil reserves.
Saturated paste tests are still a very useful tool for special situations. Very sandy soil, a fast-growing crop such as spinach, muck soils, and in artificial media are all places where saturated paste tests give us valuable information. They are not the right decision-making tool to determine soil amendments though. Their biggest strength is also their biggest weakness. Their strength is they show you what is available the next few weeks. Their weakness is that they only show you what is available the next few weeks.
I am still looking for a microbial assay that gives us ‘manage-able’ data. This is a bit of a challenge, because we need an assay that both identifies the presence of a species or group, AND the degree of presence. It is not enough to know that we have both pseudomonas and fusarium species. We need to know that we have enough of the right pseudomonads to suppress possibly infectious fusarium.
PFLA tests are ‘interesting’ but we haven’t figured out yet how to make the data manage-able. Non-actionable means we don’t run them very often.
A topic for another post is the importance and usefulness of qualitative in field soil analysis such as water infiltration and aggregate stability tests.