Getting more fertilizer value from manure: organic vs. ammonium nitrogen

Manure nitrogen comes in two forms. Ammonium nitrogen is plant-available the day you apply it. Organic nitrogen is locked in undigested feed, bedding, and microbial solids, and soil bacteria have to mineralize it before a crop can touch it. That happens on the soil's schedule, not yours, and often over several seasons.

What forms is nitrogen in when it leaves the barn?

Your manure test reports a total nitrogen number and, if you ordered the right test, an ammonium nitrogen number underneath it. Subtract the second from the first and what is left is organic nitrogen. Those two lines behave like completely different products in the field.

Ammonium nitrogen (NH4-N) comes mostly from urine. Urea hydrolyzes to ammonium within hours of hitting the pit, and it stays in solution as ammonium. It is an ion. A crop root takes it up directly. So does a soil nitrifier, which converts it to nitrate. Either way, it is in play as soon as it is in the ground.

Organic nitrogen is nitrogen still bound up in something. Protein in undigested feed. Bedding. Bacterial cells. Fiber. None of it is available to a plant in that form. Something has to take those molecules apart first.

The ratio between the two is not fixed. It moves with species, ration, bedding, storage type, storage time, and how much biological activity the pit has had. That is the part you have some control over.

Why can't a crop use organic nitrogen directly?

Roots take up nitrogen as ammonium or nitrate. That is it. A root cannot digest a piece of corn fiber.

Getting from organic nitrogen to ammonium requires mineralization, which is soil bacteria and fungi eating the organic material and releasing nitrogen as a byproduct. It is the same kind of process happening in your pit, just running in soil instead of slurry.

Four things control how fast it goes, and you do not control any of them:

• Soil temperature. Cold soil, slow biology. A spring that stays cold means nitrogen that stays locked up past the window when the corn wanted it.
• Soil moisture. Too dry stalls it. Saturated stalls it too, and adds denitrification losses on top.
• Carbon to nitrogen ratio. This is the one that catches people. Material heavy in bedding and undigested fiber is carbon rich. When soil microbes get a big carbon load, they need nitrogen to process it, and they will pull nitrogen out of the soil solution to do it. That is immobilization, and for a stretch it works against you. The nitrogen comes back later, but "later" is not a growth stage.
• Contact and particle size. A fine, well-digested material mineralizes faster than chunks.

State nutrient management guidelines handle this by assuming only part of the organic nitrogen becomes available in the first year, with the remainder credited over following years. The exact figure varies by state, manure type, and application method, so use your own state's number rather than a rule of thumb. Iowa State University Extension and Outreach and University of Minnesota Extension both publish the tables for their states.

The practical version: organic nitrogen is a savings account with a withdrawal schedule somebody else sets. Ammonium is cash.

What does a stalled pit do to your nitrogen?

The same fiber problem that causes crust and hardpan is a fertilizer problem.

A confinement pit stalls when the bacterial population that digests cellulose fiber collapses. Undigested feed and bedding accumulate. That fiber floats as crust or settles as hardpan, and the nitrogen bound up in it goes with it. Two things follow.

First, more of your total nitrogen sits in the organic pool, on the slow release schedule, instead of the ammonium pool you can actually credit this year.

Second, and this is the one nobody budgets for, the nitrogen stratifies. Fiber-bound nitrogen concentrates in the crust and in the hardpan. If the hardpan does not resuspend during agitation, that nitrogen never gets in the tanker. It stays in the pit. You are hauling the liquid fraction and leaving a portion of the fertilizer value on the floor, year after year, while your manure test tells you about the part that moved.

Helping break down the fiber addresses both. PitCharger products are formulated for specific manure-management problems. Depending on the product, formulations may include aerobic, anaerobic, and facultative bacteria, enzymes, or other specific modes of action. Not every PitCharger product uses the same ingredients or works the same way, and the product is selected based on the problem being treated. As fiber breaks down, solids become easier to suspend and the material becomes uniform enough that what you pump reflects what is in the pit. Treatment does not create additional nitrogen; the point is to convert more of the existing nitrogen into a plant-available form, which raises the fertilizer value of what you haul. More at how PitCharger works, and the connection between fiber and pumpable volume is in what changes when solids liquefy.

What happens to ammonium after you spread it?

Here is the honest tradeoff, and it is important enough that skipping it would be dishonest: more ammonium means more to lose if you surface apply and walk away.

Ammonium in solution sits in equilibrium with ammonia gas. Warm, windy, dry, and high pH all push that equilibrium toward gas, and gas leaves. Manure lying on a surface on a warm windy afternoon is losing nitrogen the entire time it lies there, and the losses climb the longer it sits.

What holds it:

• Injection. The nitrogen goes below the surface where the ammonium binds to soil exchange sites. This is the most effective option available.
• Incorporation, fast. Tillage the same day, ideally within hours, not the next morning. The interval matters more than the implement.
• Timing. Cool conditions, and not immediately ahead of a hot windy stretch.
• Application when the crop can use it. Once ammonium nitrifies to nitrate in the soil, it is mobile and subject to leaching and denitrification. That is a water quality issue as well as a fertilizer issue. EPA's agriculture pages and USDA NRCS cover the regulatory and conservation side.

None of that is new advice. It just gets more consequential as the ammonium fraction goes up. Better manure rewards better handling and punishes careless handling harder.

How do you sample a pit so the test means anything?

A manure test is only as good as the sample, and pit sampling is where most of the error lives.

• Sample during agitation, when the material is mixed. A sample off the top of a quiet pit tells you about the top of a quiet pit.
• Take multiple subsamples through the pumping event, not one grab at the start. Combine them and submit a composite.
• Fill the container part way, not full. Manure samples produce gas. A full sealed bottle can rupture.
• Keep it cool and ship it fast. Ammonium volatilizes out of a warm sample sitting in a pickup for three days, and your test comes back understating the number you care about most.
• Ask for ammonium nitrogen specifically. Some standard packages report total N only. Without the split you cannot separate what is available now from what is not.
• Test every year. Book values are a starting point when you have nothing else. They are not your pit.

Sample the same way every year or the year-over-year comparison is noise.

What does this mean for a nutrient management plan?

Rate calculations run on the available nitrogen, not the total on the label. If your plan uses a first-year availability factor for organic nitrogen and a near-full credit for ammonium, then a shift in the split changes the effective rate of everything you spread.

Practical consequences:

• Credit it properly. If more of the nitrogen is ammonium, and you inject or incorporate it promptly, the acre is getting more usable nitrogen per thousand gallons than it did before. Adjust the commercial fertilizer accordingly instead of stacking on top of it.
• Watch the phosphorus. Plans limited by P rather than N do not get more permissive because the N picture improved. Same acres, same limit.
• Keep the records. Test results, application dates, rates, method, and weather. If your state or your permit requires a plan, this is the documentation, and it also happens to be how you find out whether anything you did actually worked.

Hog operations commonly start around one gallon per month per 1,000 head, about four gallons a month for a 4,000-head finishing barn. Cattle and dairy commonly start around one gallon per 75 animals. Lagoon rates may be adjusted based on the manure already in the lagoon and the amount expected to be added each month. Most producers should notice a difference within the first month, with continued improvement through the second and third months. Significant results are normally measured after about six months. That is not a minimum program; it is the point where broader changes in the manure system are measured. Run your own numbers through the treatment calculator or request a treatment plan.

Frequently asked questions

Will treating my pit raise the total nitrogen in my manure?

No, and be careful with anyone who implies otherwise. Nitrogen comes in with the feed. Biological treatment changes the form it is in, moving organic nitrogen toward plant-available ammonium, and it changes how much of what is in the pit actually makes it into the tanker instead of staying in the hardpan. Total nitrogen entering the system is a ration question.

If ammonium volatilizes, am I better off leaving nitrogen in the organic form?

Only if you are going to surface apply and leave it lying. Organic nitrogen is not lost so much as it is unavailable on a schedule you do not control, and a share of it never leaves the pit at all if it is bound in hardpan. Ammonium plus injection or same-day incorporation beats organic nitrogen plus hope.

How do I know the split changed?

Test. Same sampling method, same lab, same point in the pumping event, before and after. Watch the ammonium nitrogen line specifically, not just total N. Significant results are normally measured after about six months, and longer if you want to see what happens to material that has been in the hardpan for years.

Does this change my agitation and pumping?

Yes, and the two are connected. Liquefied solids agitate faster and pump more consistently, which also means the load leaving the pit is uniform enough that a manure test represents it. Details at what changes when solids liquefy and on the additive categories at biological vs. chemical vs. masking agents.

Is the product safe around the crop and around livestock?

It is biological, not caustic. Safe around cattle, horses, pets, children, and vegetation. Handling in the barn is the same as handling any other liquid you pour into a pit, with the usual manure gas precautions when you are near the opening.