Soil Test vs. Plant Tissue: What NitroBoost Tells Us About Nutrient Cycling
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Why the next crop may not be starting at zero
One of the biggest questions we get when talking about food plots and cover crops is simple:
Can a diverse cover crop actually reduce fertilizer needs for the crop that follows it?
Instead of answering that question with theory, we wanted to back it up with numbers.
On one field, we had two pieces of information that created a very interesting comparison:
- A traditional soil test with fertilizer recommendations for the upcoming fall planting.
- A laboratory analysis of the standing NitroBoost crop shows the nutrients actually contained in its above-ground biomass.
The numbers tell an important story about the difference between fertilizer recommendation, nutrient availability, nutrient capture, and nutrient cycling.
What the soil test recommended
The soil test showed a pH of approximately 6.3 and provided fertility recommendations for the upcoming fall crop.
For a fall brassica-based planting, the recommendation was:
| Nutrient | Recommended application |
| Nitrogen | 80 lb/ac |
| Phosphate, P₂O₅ | 60 lb/ac |
| Potash, K₂O | 50 lb/ac |
| Sulfur | 0 lb/ac |
For a legume-based fall planting, the recommendation changed to:
| Nutrient | Recommended application |
| Nitrogen | 0 lb/ac |
| Phosphate, P₂O₅ | 60 lb/ac |
| Potash, K₂O | 50 lb/ac |
| Sulfur | 10 lb/ac |
At first glance, the recommendation is clear.
The soil test indicates that the next brassica crop could benefit from an 80-60-50 N-P₂O₅-K₂O fertility program per acre.
But then we looked at what was already growing on the acre.
What was actually inside the NitroBoost crop?
Immediately prior to the fall rotation, we sampled NitroBoost biomass and submitted it to Ward Laboratories.
The crop produced approximately:
4.34 tons of dry above-ground biomass per acre.
Laboratory analysis calculated that this standing biomass contained approximately:
| Nutrient | NitroBoost above-ground biomass |
| Nitrogen |
149.56 lb/ac
|
| Phosphate, P₂O₅ |
60.91 lb/ac
|
| Potash, K₂O |
226.44 lb/ac
|
| Calcium |
59.31 lb/ac
|
| Magnesium |
21.57 lb/ac
|
| Sulfur |
12.60 lb/ac
|
| Zinc |
0.26 lb/ac
|
| Iron |
1.03 lb/ac
|
| Manganese |
1.35 lb/ac
|
| Copper |
0.07 lb/ac
|
| Boron |
0.09 lb/ac
|
| Carbon |
4,034.69 lb/ac
|
And remember:
This analysis represents above-ground biomass only.
Now put the two reports beside each other
This is where the comparison becomes interesting.
| Nutrient | Soil-test recommendation | Measured in NitroBoost shoots |
| N | 80 lb/ac |
149.56 lb/ac
|
| P₂O₅ | 60 lb/ac |
60.91 lb/ac
|
| K₂O | 50 lb/ac |
226.44 lb/ac
|
| S, legume recommendation | 10 lb/ac |
12.60 lb/ac
|
The above-ground NitroBoost crop contained:
- Nearly 1.9 times the recommended nitrogen
- Approximately 100% of the recommended phosphorus
- More than 4.5 times the recommended potassium
- More sulfur than the legume recommendation called for
That does not mean those nutrients are equivalent to a bag of immediately available fertilizer.
But it does tell us something extremely important:
The next crop is not necessarily starting at zero.
Why can the soil test say we need phosphorus when the plants already found it?
This may be one of the most important questions raised by this entire experiment.
The soil test recommended approximately:
60 lb/ac of P₂O₅.
Yet NitroBoost had already accumulated:
60.91 lb/ac of P₂O₅ in above-ground biomass alone.
How can both things be true?
Because a routine soil test does not measure every pound of phosphorus contained in the soil. It uses an extractant to measure a particular operational pool that has been matched to historical crop demand and crop response.
That is extremely useful information, but it should not be confused with a measurement of the soil’s entire phosphorus reserve. This distinction becomes even more interesting when we look at the actual magnitude of soil phosphorus reserves.
University of Illinois researchers have reported that Illinois soil profiles contain approximately 15,000 to 25,000 pounds of total phosphorus per acre, whereas routine extractable soil tests may measure only tens of parts per million.
That does not mean all 15,000 to 25,000 pounds are available to a crop. Most clearly are not. But it demonstrates the tremendous difference between total geological/mineral phosphorus and the relatively small pool represented by a routine soil test.
This is where the work and educational presentations of soil ecologist Dr. Christine Jones become particularly relevant. Jones has referred to this as the “phosphorus paradox”: soils can contain substantial quantities of phosphorus, yet crops remain limited by how effectively that phosphorus is converted into biologically available forms.
Her emphasis has been on the relationship among living roots, root exudates, bacteria, mycorrhizal fungi, and phosphorus acquisition.
Additionally, broader plant-nutrition research supports many of the mechanisms behind this concept.
Plant roots and rhizosphere organisms can release organic acids and other compounds capable of:
- Chelating calcium, iron, and aluminum associated with phosphorus
- Competing with phosphate for mineral binding sites
- Desorbing phosphorus from mineral surfaces
- Altering pH within portions of the rhizosphere
- Increasing microbial activity
- Expanding nutrient exploration through mycorrhizal hyphae
These processes can increase phosphorus acquisition from pools that a simple soil extraction does not fully describe.
That doesn’t mean a plant can suddenly access every pound of geological phosphorus in the soil.
It means the biologically accessible pool is dynamic rather than fixed.
And that may help explain what we observed.
A soil test suggested adding approximately 60 lb/ac of P₂O₅, yet the existing NitroBoost crop had already biologically acquired approximately the same quantity in its shoots alone.
Potassium may tell an even bigger story
The potassium result may be the most dramatic number in the entire analysis.
The soil recommendation called for:
50 lb K₂O/ac.
The NitroBoost shoots contained:
226.44 lb K₂O/ac.
That is more than four times the fertilizer recommendation.
And potassium provides another excellent example of why we should distinguish between a soil-test number and the soil’s total mineral inventory.
The Potassium Paradox
Researchers S.A. Khan, R.L. Mulvaney and T.R. Ellsworth at the University of Illinois published a peer-reviewed paper titled “The Potassium Paradox.”
Their work examined long-term field experiments, seasonal changes in soil-test K, and the relationship between exchangeable and non-exchangeable potassium.
One of their central observations was that conventional soil K tests measure only the exchangeable portion of a much larger, highly dynamic potassium system.
They concluded that exchangeable-K testing does not fully account for the movement of potassium between:
soil solution → exchangeable K → non-exchangeable K → mineral reserves.
This distinction can involve enormous quantities of potassium.
Historic Illinois work reported roughly 25,000 to 45,000 pounds of total potassium per acre in the upper six to seven inches of many Illinois soils.
Again, that does NOT mean there are tens of thousands of pounds of immediately plant-available potassium.
The overwhelming majority is held in mineral and non-exchangeable forms.
But that’s exactly the point.
A soil test may report a relatively small extractable K pool, while the geological reserve beneath the crop can be measured in tens of thousands of pounds.
The agronomic question becomes:
How quickly can the soil-plant-biological system move some of that reserve into forms plants can use?
Roots are not passive straws sitting in soil waiting for dissolved fertilizer.
They change the environment around themselves.
Root activity, acids, microbial metabolites, weathering, clay mineralogy, and moisture cycles can all influence movement among different potassium pools.
That doesn’t mean K fertilizer never has value.
Soil mineralogy matters enormously. Sandy soils, highly weathered soils, different clay types, low-CEC soils, and crops with large K removal can respond very differently.
But the NitroBoost tissue result gives us another piece of actual field evidence.
Whatever the soil test said about immediately extractable potassium, the plants successfully accumulated:
226 lb K₂O/ac above ground.
The plants found the potassium somewhere.
Once NitroBoost captures nutrients, the cycling opportunity changes
This may be one of the most important functions of a diverse, high-biomass cover crop.
NitroBoost is not creating phosphorus or potassium.
It acquires nutrients from the soil system and concentrates them into living biological tissue.
That changes both the location and form of those nutrients.
After termination, those nutrients begin cycling back toward the next crop.
But they do not all behave the same way.
Potassium: fast cycling
Potassium exists primarily as K⁺ in plant tissues rather than being incorporated into complex organic structures.
Once plant tissue dies, rainfall and moisture can release K from the residue relatively quickly.
That means a meaningful portion of the 226 lb K₂O/ac captured in our NitroBoost biomass can move out of the residue and back toward plant-available soil pools relatively rapidly.
NitroBoost has essentially:
Acquired K → concentrated it in biomass → relocated it near the soil surface → placed it for the next crop.
Nitrogen: slower, more sustained cycling
Nitrogen behaves differently.
Much of the nitrogen in plant tissue is incorporated into proteins, amino acids, and other organic compounds.
That means the 149.56 lb N/ac measured in the NitroBoost shoots does not simply become nitrate the day we terminate the crop.
Microorganisms decompose the residue and mineralize portions of that organic nitrogen over time.
That slower biological release can be useful.
Instead of relying entirely on one large pulse of soluble nitrogen, decomposing residue can contribute nitrogen progressively as the biological system processes it.
How quickly that occurs depends on moisture, temperature, residue maturity, microbial activity, and C:N ratio.
Our NitroBoost sample contained:
46.49% carbon
1.723% nitrogen
1.723% nitrogen
That gives the whole above-ground biomass a calculated C:N ratio of approximately 27:1.
But NitroBoost isn’t a monoculture of mature cereal residue.
It contains legumes, brassicas, broadleaves, and grasses.
The softer, nitrogen-rich components can decompose relatively quickly, while more carbonaceous material can persist longer.
That creates a more continuous spectrum of decomposition and nutrient turnover, rather than a single release event.
Phosphorus: the phosphorus does not disappear. It cycles.
In the soil, phosphorus can become strongly associated with calcium, iron, aluminum, and mineral surfaces, limiting its movement and immediate availability.
Yet our NitroBoost plants had already acquired:
60.91 lb P₂O₅/ac in the above-ground biomass alone.
Once the plant captures that phosphorus, it moves from the soil nutrient pool into a living biological pool.
When NitroBoost is terminated, that phosphorus does not disappear.
It cycles.
As the residue breaks down, phosphorus moves through plant residue, earthworms, soil organisms, microbial biomass, and organic matter, and eventually returns to forms that growing roots can take up.
The pathway can look something like:
Soil P → NitroBoost roots → plant biomass → residue → worms and microbes → mineralization → Carbon Load roots → plant biomass → repeat.
Some phosphorus may temporarily become immobilized in microbial biomass.
Some released phosphorus may reassociate with soil minerals.
But it remains part of the soil-plant nutrient cycle.
Calcium, magnesium, sulfur, and micronutrients cycle too
The same concept extends well beyond N, P, and K.
Our above-ground NitroBoost crop also contained approximately:
59 lb calcium/ac
22 lb magnesium/ac
13 lb sulfur/ac
22 lb magnesium/ac
13 lb sulfur/ac
plus zinc, iron, manganese, copper, and boron.
Roots acquired every one of those nutrients, transported them into the plant, and concentrated them into biomass.
When the crop terminates, those nutrients also enter the residue-decomposition cycle.
So when we talk about cover crops recycling fertility, we are not simply talking about nitrogen.
We are cycling an entire nutrient package.
Then we terminate NitroBoost
This is when capture becomes cycling.
Once NitroBoost is terminated, the nutrient inventory does not disappear.
The residue begins to decompose, and nutrients begin to move through various biological and chemical pathways.
Potassium can cycle relatively quickly.
Nitrogen generally requires microbial decomposition and mineralization, creating a slower and potentially more sustained release.
Phosphorus moves through residue, microbes, soil organisms, and new roots. It does not disappear. It cycles.
Calcium, magnesium, sulfur, and micronutrients enter the same broader decomposition cycle.
This is why residue is much more than simply “ground cover.”
It is a nutrient reservoir positioned directly above and throughout the root zone of the next crop.
And we still haven’t counted the roots
Everything discussed so far is based on above-ground biomass.
A cover crop has an entire second system operating underground.
That includes:
- Living roots
- Root turnover
- Root exudates
- Root nodules
- Mycorrhizal fungi
- Rhizosphere bacteria
- Microbial biomass
- Microbial necromass
- Nutrients acquired below the normal soil-test sampling depth
- Normalized OM mineralization
A soil sample taken from a shallow depth and exposed to a laboratory extractant cannot reproduce the full season-long activity of:
thousands of pounds of growing roots + exudates + fungi + bacteria + moisture cycles + biological acids.
Again, that doesn’t invalidate the soil test.
It simply means the soil test and a living root system measure and interact with the soil in very different ways.
And that makes the measured 149-61-226 even more interesting.
What about soil organic matter?
Soil organic matter also provides a much larger nutrient reservoir.
To illustrate the scale, consider a hypothetical soil containing only 2.5% organic matter in the upper six inches.
Using the common estimate of approximately 2 million pounds of soil in an acre-furrow slice, that represents roughly:
50,000 lb of organic matter per acre.
Average estimates suggest substantial quantities of N, P, K, sulfur, and carbon can be contained within that organic fraction.
These are reservoirs, not annual fertilizer credits.
Only a fraction becomes available during any individual growing season.
And this organic nutrient reserve sits alongside the much larger geological and mineral reserves of phosphorus and potassium discussed above.
That gives us several different nutrient pools operating simultaneously:
Mineral/geological reserves
↓
Non-exchangeable / fixed pools
↓
Exchangeable pools
↓
Soil solution
↓
Microbial biomass
↓
Plant roots
↓
Plant biomass
↓
Residue
↓
Back into the cycle
↓
Non-exchangeable / fixed pools
↓
Exchangeable pools
↓
Soil solution
↓
Microbial biomass
↓
Plant roots
↓
Plant biomass
↓
Residue
↓
Back into the cycle
The size of the total nutrient bank and the amount immediately available to a crop are two very different things.
Biology helps connect them.
This changes how we think about fertilizer
The conventional fertilizer model often begins with:
Crop needs X; therefore, apply X.
A nutrient-cycling system asks another question first:
How much of X can the biological system already provide?
That includes:
- Nutrients in the preceding crop
- Nutrients released from residue
- Soil organic matter mineralization
- Geological and mineral nutrient reserves
- Previous thatch
- Root turnover
- Root exudates
- Microbial biomass and necromass
- Exchangeable nutrient pools
- Legume nitrogen fixation
- Mycorrhizal nutrient acquisition
- Nutrients obtained from deeper in the profile
That is the concept we are working toward with the NitroBoost → Carbon Load rotation.
Reason #1: NitroBoost can reduce fertilizer demand through nutrient capture
NitroBoost puts living roots into the soil during the spring and summer growing season.
Those roots acquire nutrients.
Different species explore different soil depths and occupy different portions of the rhizosphere.
Roots and their microbial partners can also influence nutrient chemistry in the immediate vicinity of the root.
The cover crop then converts those acquired nutrients into living biomass.
In this example:
150 lb N/ac
61 lb P₂O₅/ac
226 lb K₂O/ac
61 lb P₂O₅/ac
226 lb K₂O/ac
were stored above ground.
Instead of thinking only about biomass production, we can begin thinking of that biomass as a temporary biological nutrient bank.
Reason #2: NitroBoost both adds and scavenges nitrogen
Nitrogen is different from phosphorus and potassium because legumes have access to an additional source:
The atmosphere.
Nitrogen gas makes up the majority of the atmosphere, but most plants cannot use it directly.
Legumes form symbiotic relationships with rhizobia that convert atmospheric N₂ into biologically useful nitrogen.
Meanwhile, non-legume species can capture nitrogen already present in the soil.
So within a diverse mixture:
Legumes fix nitrogen.
Grasses and brassicas scavenge nitrogen.
That helps explain how this NitroBoost crop accumulated nearly:
150 lb N/ac above ground.
Some of that nitrogen came from the soil.
Some may have come from biological nitrogen fixation.
Either way, it was captured in biomass and retained within the biological system rather than leaving the acre.
Reason #3: Biomass feeds the biological system that cycles everything again
The NitroBoost analysis also measured approximately:
4,035 lb carbon/ac above ground.
Carbon is microbial food.
When NitroBoost breaks down, soil organisms begin consuming the residue.
Some carbon is respired back to the atmosphere as CO₂.
Some enter microbial biomass.
Some contribute to longer-lived organic compounds.
And substantial carbon has already entered the soil through living roots and root exudates before termination ever occurs.
At the same time, nutrients contained in the residue move through those organisms.
This is why the objective isn’t simply to produce biomass.
The objective is to create a continual cycle:
Living roots → nutrient acquisition → biomass → residue → biology → nutrient release → new roots.
Then Carbon Load captures those nutrients again.
And the process repeats.
Where do small biological and organic inputs fit?
This is also where we believe small, targeted inputs can make more sense than attempting to meet the entire crop requirement from a single fertilizer bag.
There is an important distinction here:
A biological product measured in ounces per acre does not replace 150 pounds of nitrogen.
And a relatively low rate of organic fertilizer does not physically contain the same N-P-K quantity as hundreds of pounds of synthetic fertilizer, especially when applied in small amounts.
That is not the argument.
The argument is that we’re using those products within a system that already has substantial fertility, using them as a catalyst.
Our strategy may include:
Revive RX at termination
Applied around termination as part of our residue and soil biological program.
The objective is to support biological activity while several tons of nutrient-rich NitroBoost residue begin cycling.
Bio-Charge
Used to support biological and rhizosphere activity as the next crop develops.
Seed Feed
Delivers a relatively small amount of nutrients directly to the seed during germination and early root development.
Spectrum + Myco
Introduces beneficial biological partners at the seed/root interface, including mycorrhizal fungi that can expand the effective nutrient-exploration area of roots.
This may be especially relevant to phosphorus because fungal hyphae and rhizosphere organisms can explore soil and nutrient pools outside of the immediate root surface.
No-Till Nurture
Provides organic N-P-K and other organic inputs at a relatively modest application rate.
Rather than expecting No-Till Nurture to supply the entire nutrient requirement, we’re using it to supplement a much larger biological and mineral nutrient pool already present in residue, organic matter, and soil.
The goal is not zero inputs.
The goal is to achieve more because the biological system does more of the work.
Carbon Load becomes the next nutrient capture crop
After NitroBoost is terminated, Carbon Load enters the system.
The fall mixture develops a new network of living roots.
Its legumes begin establishing nitrogen-fixing relationships.
Its grasses and brassicas capture available N and other nutrients.
Roots explore the soil profile.
Microorganisms process the preceding NitroBoost residue.
Potassium moves relatively quickly from decomposing residue.
Nitrogen is progressively mineralized.
Phosphorus continues cycling through residue, biology, and new roots.
Calcium, magnesium, sulfur, and micronutrients are recycled alongside them.
Roots and their biological partners continue interacting with the much larger mineral nutrient pools in the soil.
Carbon Load captures those nutrients.
Then Carbon Load becomes residue for the following NitroBoost.
That is why we call it the 1-2 system.
It isn’t simply:
Spring seed → fall seed.
It is:
Capture → cycle → recapture → repeat.
What this experiment does NOT prove
It is equally important to be clear about what these results do not tell us.
This tissue analysis does not prove that:
- 100% of the nutrients in NitroBoost will become immediately available to Carbon Load.
- Every soil can eliminate fertilizer.
- Every NitroBoost planting will accumulate exactly 150-61-226.
- Biological amendments can replace hundreds of pounds of fertilizer on their own, specifically when applied in small amounts.
- Every pound of geological phosphorus or potassium is available to a crop.
- A soil-test recommendation should simply be ignored.
Soil mineralogy, CEC, biological activity, rooting depth, soil moisture, temperature, rainfall, species composition, maturity at termination, and crop demand all influence nutrient acquisition and release.
The University of Illinois Potassium Paradox work itself has generated scientific debate, and current fertility recommendations still recognize situations where crops respond economically to added potassium.
But none of that changes what our tissue analysis directly measured:
There was substantially more fertility moving through the living NitroBoost crop than the soil-test fertilizer recommendation alone would have suggested.
That's what we’re interested in understanding, measuring, and learning how to cycle more efficiently.
As the system repeats from year to year, it should only become more efficient, and the soil itself should continue to improve right along with it.