What type of lab analysis best correlates to what we observe in the field?
William Albert used to say, “Read books and study nature. And when the two don’t agree, throw out the books.”
Think about that in the context of soil analysis. You read the soil analysis reports and you study nature — specifically, in this context, you study what the crops are saying. And if the two do not agree, which are you going to believe? The crops are the final report card. They’re the ones that tell us what’s really happening.
If the crops and the soil analysis do not agree, then perhaps we should consider throwing out the soil analysis — or reconsidering our reliance on it.
There has been a tendency among some agronomists to use soil analysis as a sales tool for fertilizer sales. Sometimes this is necessary. But, in far too many cases, we have observed that the use of soil analysis results in the overapplication of products that actually create most of the nutritional imbalances that crops experience. That creates disease and insect susceptibility and causes reduced profitability and yield losses. We don’t normally think about a fertilizer application as reducing yield. But it absolutely can — particularly potassium and nitrogen overapplication in fruit and vegetable production, but also in the case of broadacre crops.
I believe that the purpose of agronomy is to balance nutrition and biology for optimal crop performance, to produce the highest marketable yields — and to have disease- and insect-resistant crops at the same time. The purpose of analytical tools for agronomy purposes — like soil analysis or plant analysis — is to accurately describe the nutrition available for plant absorption. In the case of sap analysis, this is to accurately describe that which has already been absorbed.
The key word here is “accuracy,” because if we don’t have accuracy, then we have to question whether bad data is worse than no data. Ultimately, the plants are the final report card of what nutrients are absorbed from the soil profile. They are the final indicator of what that soil had the capacity to deliver in that cropping season.
QUESTIONING SOIL ANALYSIS
Soil analysis has a long and venerated history. The cation exchange capacity (CEC) type of analysis that we’re familiar with today was pioneered by Dr. William Albert at the University of Missouri. Dr. Albrecht conducted foundational research on the importance of plant nutrition from a public-health and livestock-health perspective. His work on soil analysis is still considered a mainstay, particularly within the organic and biological agriculture community. Even many mainstream farmers use CEC-type analysis. There have been refinements and improvements in soil analysis methods, but the foundations haven’t really shifted in the past 70 years.
At AEA, we have done extensive work with sap analysis since 2011. Each year raises bigger and bigger questions about how we should use soil analysis. We find ourselves wondering, “Is it just that soil analysis is incomplete? Or is it that it is completely inaccurate?” Because the soil analysis reports are not correlating with what the plants are actually doing.
This takes us back to the quote from Dr. Albrecht. If the soil report and the plants do not correspond, which are you going to believe? We believe that the plants are the ones that are actually telling us the truth. And so, if soil analysis is significantly incorrect — frequently — and does not correlate with crop performance, at some point we have to ask the question, “Is it better to have bad data or to have no data?”
My perspective is that in many cases it would be better to have no soil analysis data at all. Because when we have no data, that means we rely on our other senses more — we rely on crop performance and other things that are happening. We may actually get a better perspective of what’s going on if we are more conscious of the overall ecosystem than if we just rely on a soil report.
Soil is commonly described as being composed of three primary components: the mineral, the physical and the biological. The soil analysis that we have built — our entire framework of managing plant nutrition — only measures one of these: it only measures the mineral aspect. There is no consideration given to the physical soil characteristics and aggregate structure and aggregate stability, for example. There is no consideration given to biological aspects in the soil reports that we get back from a typical soil laboratory.
Now, there certainly are biological assays available, and they’re increasingly being used. I’m a big advocate of using more biological assays. But the point is that what is considered a standard soil test is merely a mineral soil test — a chemistry soil test. We typically do not consider the biological or physical characteristics.
My perspective that this use of primarily focusing on a chemistry-based analysis has really led agriculture astray. I am not suggesting that there were nefarious intentions historically about the emphasis on chemistry-based analysis; it was simply the technology that we had access to at the time. But since we focused only on this chemistry-based analysis, we developed a very narrow perspective. We focused on mineral balancing, because minerals have been the only things we’ve been able to measure. We have lost sight of the fact that they are only a third of what we should be measuring. Fundamentally, this is the antithesis of a biological agriculture.
In a biological agriculture, the primary emphasis should be on biology, not on chemistry. I have observed over and over that biology supersedes chemistry. In other words, you can have a completely imbalanced soil analysis, from a chemistry perspective, and still produce high-yielding, high-quality crops — when you have robust biology.
But the reverse I have never observed to be true. If you have a soil analysis that is perfectly balanced — everything is exactly at the desired values — but if you do not have good biology, you cannot produce healthy crops. Perfect chemistry with poor biology will not produce healthy crops, but abundant biology with imbalanced chemistry can still produce perfect crops.
SOIL TEST CAVEATS
If it were true that soil analysis correlates with plant nutrient absorption, then the nutrients that are high in soil should also be high on plants, and nutrients that are deficient, as indicated on the soil report, should also be deficient in plants. But neither of these is the case. Because of that, we cannot make reasonable and effective agronomic recommendations for soil amendments from soil analysis alone.
Let’s use magnesium as an example. A soil test shows that we have a magnesium deficiency and need to add 50 pounds per acre. If we were to add 50 pounds per acre of additional magnesium, that should give us all the magnesium required for a really healthy crop. Except that that’s not necessarily true. It depends on calcium levels, and it depends on whole lot of other factors in the soil profile that aren’t accounted for in the lab report.
Learning to interpret soil analysis reports is a bit like learning to speak English. When we learn to speak English with rules, we have to remember all the exceptions to the rules. In some cases, the list of exceptions is very long. The same is true of soil analysis — in many cases, there is a long list of exceptions to the rules. This is common knowledge and has been from the beginning — that these chemical assays do have limitations, and we have to understand the nutrient interactions to be able to make recommendations from them, and we have to remember all of the caveats.
For example, we know that when we have really high magnesium levels, that actually limits magnesium availability; we know that we when we have excess phosphorus levels, that limits phosphorus availability; we know that what are typically considered to be adequate and desired values of calcium are not enough in lighter, sandy soils. For every nutrient, there’s a list of rules or exceptions that we have to understand — the interactions between geology and other nutrients in the profile, as well as soil biology — and we have to take those into consideration when we’re making recommendations.
We know that potassium levels that would show up to be very deficient are actually adequate in most of our agricultural soils, because they already have abundant potassium levels in the soil-mineral matrix that can supply the crop — when we have good biology. We also know that soil analysis does not report the nutrient redox state, and it doesn’t accurately report nutrient availability. The typical CEC analysis does not report bicarbonates and chlorides, which are extremely important for managing biological activity and managing overall nutrient availability.
COMPARING SOIL AND SAP TESTS
Sap analysis data does not correlate to soil analysis data. Sap analysis shows us the nutrients that are actually inside the plants, and since I choose to believe the plants — because I believe plants are an accurate indicator of what’s happening — I choose to believe sap analysis over soil analysis.
We’ve worked with tens of thousands of soil analysis reports from different laboratories and tens of thousands of sap analysis reports on 50-plus different crops, in all types of growing environments and soil types. When we compile this information in a database, we can use it to see how nutrients are interacting with one another — sometimes in surprising ways.
For example, we have observed that sap analysis reports for a variety of different crops show a strong positive correlation between ammonium and nitrogen, and a strong negative correlation between ammonium and sugar. In other words, every time total nitrogen levels increase, there is also an increase in ammonium, and when you have higher ammonium levels, sugar levels tend to trend down. This is not a surprise, because we often observe ammonium levels to be higher when plants are in photorespiration mode — they’re stressed because of high temperatures and aren’t photosynthesizing well, so they consume the sugars and proteins that they have built in the past. You end up with plant that is low in sugar and high in ammonium, which is the perfect environment for spider mites and similar types of pests to show up.
Sap analysis demonstrates other correlations that are more surprising, though. For example, the correlation between silicon, iron and aluminum is fairly pronounced. I find this to be really interesting, because aluminum is usually considered to be an anti-nutrient. The popular narrative is that it only accumulates in plants when you have compromised root systems — if you have nematode damage or fungal infections in the root system, then aluminum goes into the root. A similar narrative is also considered to be true of iron: that when you have higher iron levels — this was particularly when we were looking at tissue analysis instead of sap analysis — plants primarily absorb silicon in the form of mono-salicylic acid, which is released from the abundant silicon in the minerals comprising the soil when you have abundant microbial activity. The popular narrative has been that when you have really good biological activity, silicon levels go up and iron and aluminum levels go down. Except the sap data shows that that isn’t actually true — that these three tend to correlate together. It is worth mentioning in this context that, for whatever reason, sap analysis apparently only reports the iron that is in the reduced form, or that is physiologically active inside the plant — it doesn’t report the oxidized iron that might be stored in the plant vacuoles.
Similarly, if we look at calcium, we see that it correlates very strongly with the presence of magnesium — which is not a surprise — and that there are also correlations with sulfur and with manganese. But sap analysis actually reveals a negative correlation between calcium and phosphorus. The historical knowledge has been that these two nutrients have a synergistic effect within the plant — when one increases, it increases the presence of the other. However, at least for sap analysis, the data shows the opposite to be generally true.
We can also look at side-by-side soil and sap reports of the same crops to determine how closely they align. The soil report in Table 1 shows an example of results from tomato plants and the soil they were grown in. The results, in my experience, are typical. They demonstrate a disconnect between what these tests are telling us. Both cannot be correct.
| Soil test (ppm) | Sap test (ppm) | |
| Calcium desired/detected | 1,322 / 1,502 — adequate/high | 3560 / 570 (young leaves), 2789 (old leaves) — low |
| Potassium desired/detected | 151 / 83 — low | 3644 / 3920 (young leaves), 4095 (old leaves) — high |
| Iron desired/detected | 60-80 / 258 — high | 2.7 / 1.1 (young leaves), 1.18 (old leaves) — low |
| Manganese desired/detected | 80-90 / 93 — adequate/high | 24.75 / 3.45 (young leaves), 3.98 (old leaves) — low |
Table 1. Example soil and sap test comparison
CONSTRUCTIVE USE OF SOIL TESTS
I believe that we should approach plant nutrition from a biological perspective, not from a chemistry perspective. We need to adopt the framework that biology feeds plants — that we should try to get all of our plant nutrition from biology and not from chemistry.
Chemistry-based soil analysis is like a set of blinders that has forced us to focus on the chemistry alone. But if we want to manage soil well, we should try to get soil analysis reports that also describe biology, soil structure and geology. The only nutrients that tend to correlate between the soil analysis and the sap analysis reports are sulfur, zinc and boron. That’s a very short list — three nutrients out of a total of the 12 or so that are reported on a common soil analyses.
The growers we work with do still pull CEC soil analysis. We find that this is still useful. These tests are familiar to the grower, and they’re useful in tracking how things improve over time. We use them to manage our calcium-to-magnesium balance, which we still find to be very important. We also use them for sulfur, zinc and boron.
But that’s about it for the soil test. We pull soil CEC analysis once a year for high-value fruit and vegetable crops and once every 36 months for broadacre crops. Everything else depends on context: the soil geology, the biology and the soil structure — the other things that we should be measuring.
Instead of only collecting CEC analysis via a soil test in order to manage soil mineral balance, we collect two other types of analysis. The first is a total geological assay. This is essentially an assay for miners — for geological prospectors — to determine everything that is contained within a rock. We run this type of analysis only once. This is not something you need to repeat year after year — you only do it one time, and it will tell you the total nutrient content of your soil. This can give you very valuable perspective. Soils often contain all the nutrients a plant needs — they’re just not available to the plant. The soil only requires biology, in order to make those nutrients that are present available. We don’t need to add more nutrients to such soils.
This can give us a valuable perspective on how we should manage soil amendments. Our aspiration as regenerative agronomists should be to develop an agricultural ecosystem that is so robust and so healthy that we do not need to constantly add inputs. We need to remove the need for inputs. There’s one important caveat to this: we may need to occasionally add the minerals that are not present in the native soil profile. For example, if you are grazing livestock and you want to have healthy livestock that are immune to parasites and diseases, you will want to have adequate levels of selenium, molybdenum and zinc in your soil. What if your native soil geology doesn’t have enough of one of those? You may need to apply what’s missing once every three or five years, or maybe only once every decade.
The additional soil test that we pull is the Haney analysis. The Haney test does correlate with actual measured plant nutrient absorption as reported on a sap analysis — much more closely than the CEC analysis. It also correlates with crop response and the way the crops are behaving in the field much more closely than the CEC analysis.
When you take this different approach, agronomic recommendations change. One thing we’ve noticed — and this is of course context specific — is that, overall, using sap analysis together with a mineral assay and the Haney test has led us to need to recommend fewer fertilizer applications. Potassium applications have dropped as much as 70 percent. We typically see nitrogen applications drop in the neighborhood of 60 to 70 percent. And, almost universally, we recommend foliar applications of manganese and iron. It’s very common for us to also add copper, cobalt, molybdenum and boron.
These are very broad brushstrokes of what we see happening in most agricultural contexts when we begin focusing on biology. When you add and build biology, it can supply a tremendous amount of nitrogen and potash to a crop — if you have that potash in your native bedrock.
THE NEED FOR BIOLOGY
What has changed from when William Albrecht developed his CEC soil analysis back in the ’40s and ’50s? Carey Reams and William Albrecht both reported adding a couple tons per acre of limestone, a ton or two of rock phosphate and a ton or two of poultry manure and seeing incredible recoveries — yields on alfalfa and forages would jump by 50 to 60 percent in a single year. But we don’t see these results from soil balancing today. I have very seldom observed the types of results and improvements that Dr. Albrecht reported. I’ve come to the conclusion that they were able to see these tremendous turnarounds because they still had very active soil biology — unlike today.
My hypothesis is that our soil biology, collectively, has degraded extensively in the last seventy years, since that foundational research was done, because of the widespread use of pesticides and herbicides. Additionally, we are using much larger equipment today and have much greater compaction than they did seventy years ago. This has changed the way our soils respond.
Here’s a fundamental question: If your crops produce high yields, have exceptionally high quality and are completely disease and insect free, do you actually care if your soil analysis report is ideally balanced? I don’t think so. We care about producing healthy, high-quality crops. Historically, we’ve been told that if our soils are optimally balanced from a chemistry perspective, we will achieve healthy, high-quality, high-yielding crops. I suggest that this isn’t completely true — that even if even when we have an ideal, balanced soil, we still must have vigorous biology.
Read your lab reports and observe your plants, and when the two disagree, find better laboratory analysis! Choose a lab analysis — like sap analysis — that does truly correlate to field observation.
John Kempf is the founder of Advancing Eco Agriculture and the executive editor of Acres U.S.A.
| Albrechtisms The following comes from Volume I of the Albrecht Papers — Albrecht’s Foundational Concepts — published by Acres U.S.A. Don’t lime to fight soil acidity. Use lime to feed the plant. Insects and disease are the symptoms of a failing crop, not the cause of it. The use of sprays is an act of desperation in a dying agriculture. It’s not the overpowering invader we must fear but the weakened condition of the victim. The excessive use of chemical salts in fertilizers is upsetting plant nutrition. Manure forms an organic shield around the salts. It is a buffer against salt injury. As soils become lower in organic matter, we will not be able to use salts so directly. Fertilizer placement is the art of putting the slats in the ground so the plant roots can dodge it. To help them maintain their soils, farmers should be given a depletion allowance on their income tax, the same as owners of mines, oil wells and timber tracts. We are exhausting the quality of our soils. As we do so, the quality of our plants goes down. And we are accepting this. Those who teach must constantly hold up the challenge to study nature, not books. |
