This article was published by No-Till Farmer Magazine and shared with their permission.
Top 10 Lessons About Soil Fertility
By Emily Schultz, Editorial Intern
Doug Miller, vice president of Midwest Bio-Tech, discusses 10 lessons
he has learned about the interaction of soil fertility
and microbial activity over 40 years.
CERTAIN LESSONS about microbiology can only be learned under
unusual conditions, says Doug Miller. The vice president of Midwest
Bio-Tech, founded in 1981, shared some of these lessons, learned over
the past 4 decades, in a recent webinar hosted on www.No-TillFanner.com.
1. EVERY SITUATION IS DIFFERENT
Every farm has its own situation, even farms that are near each other. Not only are the natural environments different, over time a farm is likely to make operational changes - such as changing tillage practices or rotations - that can affect biology and how the fields perform.
When changes occur, Miller recommends using the Solvita burst test, because it can show the effects of the changes sometimes within a couple of weeks of making them. The test detects how much carbon dioxide (CO2) is respired by the microbes in the soil, which in turn causes carbon to be sequestered in the soil.
"It's a leaky process," Miller says, "in which we measure the amount emitted
rather than what's being sequestered. Soil with high activity is a good sign and this test helps us measure that."
2. MOISTURE MATTERS
The amount of moisture in soil is crucial for the soil's biological activity
levels. But if the conditions are too dry or too wet, the microbes go dormant until the situation improves.
To measure microbial activity based on moisture, Midwest Bio-Tech tested silt loam soil samples at different moisture levels. On the low end, they saturated the soil at a level that represented 20% of field capacity (roughly equivalent to having ¼ inch of rain soaked into the top foot of soil) and they went up to 120% of field capacity, which represents a saturated soil.
"Soil with high activity is a good sign ... "
The temperature was held constant, leaving water to be the only variable in
What Miller found is that the more saturation, the more biological activity - up to a point.
At 20% field capacity, the biological activity was about 70 parts per million (ppm) and then ramped up to about 140 ppm at about 80% saturation. After that point, biological activity began to decrease, going back down below 70 ppm at 120% saturation (See chart I).
3. TEMPERATURE MATTERS
Extreme temperatures also inhibit microbial activity. When temperatures get above 85-90 degrees F or below 40 degrees F, microbial activity drops off dramatically. While it's commonly believed there is no microbial activity when the temperature is below freezing, research has shown that there is some bacterial activity down to about 20-25 degrees F. Surprisingly, Miller
says, there's also a small amount of bacterial activity above 110 degrees F.
Miller says for every 10-degree increase in temperature above about 40 degrees F, biological activity doubles.
So why does this matter?
''If you're considering things like residue decay or nutrient availability and
you're thinking about doing something late in the fall or early in the spring, there's just not going to be as much microbial activity as you would get during warmer temps," Miller says.
"So if you 're using a residue decay product, for example, and you put it on
when it's cooler, as the temperature warms, the microbes will go to work and will really ramp up as the temperature rises in the spring" (See chart 2).
4. EXCESS N CAN HURT CORN YIELDS DURING A DROUGHT
Many studies have shown there's no relationship between the amount of moisture available to crops and the level of nitrogen (N) applied. Others show that N produces more yield when it gets dryer because greater biomass (produced by feeding more N) shades the soil and reduces evaporation, making more moisture available to the plant.
However, Miller says Midwest Bio-Tech has evidence that N can hurt com
yields during drought years. In 1988, the company experienced dry conditions, and they found that plots that had higher levels of N typically had lower yields. In retrospect, Miller believes the timing of the lack of moisture may have had an impact, as it was dry early, which may have prevented the com from growing large enough to create the shade that would be needed to preserve moisture.
In addition, a recent study indicated that in dry conditions, a larger amount of nutrients does more harm than good to crops because there's not enough moisture for both vegetation and grain production.
5. YIELD GAINS FROM FOLIAR & SIDEDRESS FERTILIZER
Adopting foliar feeding and sidedress fertilizer programs is beneficial for
farmers, especially those with lighter soils.
Although weather conditions may be restrictive, farmers can change their nutrient program to fit the needs of the current weather conditions.
6. BIOLOGICALLY ACTIVE SOILS MINERALIZE MORE
One Solvita tool is a calculator that indicates how much soil N has been
mineralized by microbial activity.
If soil has high microbial levels, then more N is mineralized, Miller says. This
can affect fertility decisions, as the calculator measures soil health and nitrogen.
Historically, the general rule of thumb is that for each percent of soil organic matter in the soil, you can expect 15-20 pounds of N to be mineralized. But in Miller's experience, soils with a lot of biological activity will often mineralize 10-20 more pounds of N than what is expected
(See chart 3).
7. FERTILITY REQUIRED TO CYCLE RESIDUE, COVERS
Nitrogen is needed for residue decay to occur. The amount of N required
depends on the type of crop and the environmental conditions, but the estimate is that for every ton of com stalks decayed, 13 pounds of N is needed to support the biological activity needed to properly decay the residue.
In addition, most cover crops in the fall need 15-20 pounds of N to germinate and produce growth.
"When you add these amounts together, and you're trying to decay about 2 tons of residue in the fall and grow a cover crop, you might need about
45-50 pounds of available Nat once," Miller says. ''And if you don't have that
available, you're either going to limit the amount of decay activity or your covers are going to suffer. So, it might be the case that you need additional N in the fall."
Furthermore, a recent history of heavy rainfall may require extra N because some could get lost to leaching.
8. RESIDUE IS NOT TRASH
Decaying residue is a valuable resource - it helps with increasing organic matter, improving water infiltration, nutrient bonding capacity, buffering
the soil from heavy rainfall and improving root penetration.
Residue is also crucial for carbon sequestration and is a source of recycled nutrients farmers can use in place of purchased fertilizer prices.
Every year, Miller analyzes the cost of various commercial fertilizers and comes up with a composite value for N, phosphorus (P), potassium (K) and sulfur.
Knowing roughly how much of each of those nutrients is in a ton of residue, he can then put a value on that ton of residue based on the crop, and it shows that if a farmer is able to recycle the nutrients in the residue,
he could potentially save approximately $30.90-47.75 per acre in commercial fertilizer purchases. Not all nutrients will be available at once, of course, but it will get into the mix, Miller says (See chart 4).
9. YOU CAN MAKE MARGINAL SOILS INTO GOOD SOILS
By focusing on soil biology, any soil can be made useful by changing to better practices, Miller says. You can't change the soil type - a sandy soil won't ever become a silt or a loam, for instance - but by reducing tillage, changing fertility programs, using cover crops and adding diversity, even very degraded soils can become healthier and more productive.
10. SOIL HEALTH IS BUILT OVER YEARS
BUT CAN BE HARMED IN MOMENTS
Improving soils can happen by transitioning from tillage to no-till, adoption
of cover crops and using biologicals or manure. Most farmers who have done this say it can take up to 3-5 years for soils to change. However, damage to soil can be done quickly.
Even activities such as riding ATVs on fields can damage soil. Miller says he saw a field experience a "8-10-bushel com yield loss in places where the soil got compacted from light activity- it wasn't heavy equipment, but it was repeated."
Farmers should keep in mind how easily soil can be damaged and avoid practices that will have harmful effects.