shaz":2dbyv92w said:
Stocker Steve":2dbyv92w said:
fargus":2dbyv92w said:
The other thing to keep in mind is that perennial forages are the best topsoil builder out there, better even then forest cover.The fastest way to increase soil OM is to lie about it, the next fastest is to raise yields.
I saw an article recently where they had continuous no till corn for a number of years but could not show any significant OM increase. Did not seem right, but there was some speculation about the litter on top was oxidizing. Maybe they need trained cows to walk up and down each row for incorporation. ;-) I guess the good news is that OM did not go down.
I have seen several mob grazing presentations where they claimed huge OM increases. One claimed over 1% per year. I think if you pound down enough forage and then soil sample shallow that may work. How may pounds/acre of litter/roots/crap do you need to get a 1% OM increase in the top 6 inches?
I've seen the same massive increase 2 months after I used chicken litter but it may have been a sampling anomaly?
More to come...
Our levels were pretty average 20 years ago, around 3.5-5% OM. We sample to a depth of 6 inches (assuming the guy running the probe is doing it right. Seeing as that's me, I shouldn't complain too much.) To get a 1% OM increase you need a pile of stuff. I'll try and do the math, but I'm not digging out my soil science notes, so I apologize for any mistakes I might make.
1 acre furrow slice is 43560 sq feet by 6 inches deep. The book value is an estimate of 2 million pounds per acre. A soil with higher bulk density (your clays and clay loams) will be heavier, while your coarser loams and sands will be less. 1% of that is 20,000 lbs. So the quick answer is 10 tons. That doesn't really answer the question though, as you need to increase the soil OM pool by 20,000 lbs/acre, and have it stay that high. A large percentage of soil OM (I think it's about 45-48%) is "short term" and turns over in the soil rapidly.
Rough math again, but if a REALLY good pasture or hay field can produce 8 tons/ac of forage per year, and IF we only harvest half of the above ground portion on each grazing pass, you are adding 8000 lbs of "stuff." Grasses are roughly 80% top, 20% roots, and legumes are 75/25. Most pastures are predominantly grass, so we'll go with 80/20. That's a further 4000 lbs (16,000/0.8 - 16000) of root growth each year. Now, 20% of a plants "carbon assimilation" each year is lost in the form of root exudate. That's the combination of root slime sloughing off, respiration in root tissues, etc. So 16,000 lbs of top + 4,000 lbs = 20,000 lbs of biomass / 0.8 - 20,000 = 5,000 lbs of root exudate. Not all of that is retained in the soil, so call it 4,000 lbs that doesn't immediately gas off. That is a total soil contribution of 16,000 lbs of plant material. So a spike of 0.8 - 1% OM immediately after a mob-grazing pass is certainly possible. I doubt that it stays that high for long. My guess would be if you went and re-sampled that same area 3 months later you would find a negligible impact. (The explanation for that is simple. The way soil labs "measure" OM is to burn the carbon out of a portion of the sample. They measure the CO2 released, and use it calculate OM based on the mass of the sample. If soil OM is 45% carbon the math is simple. The problem is, it doesn't measure what kind of OM you have, only that on that day, in that spot you had X amount of carbon contained in that sample. That's why sampling regularly really helps. It takes out some of the anomalies and lets you see trends. If something seems really out of whack with previous results, it probably is.)
Our most dramatic shift in soil OM over a 15 year span came in a field that got a lot of solid "cow" manure before our system switched around. It had a lot of straw in it, which has a much higher C:N ratio then fresh green plant material, and more lignin. That results in more medium-term storage carbon going in to the soil pool.
It does stand to reason that once-over mob grazing could add more lignin to the mix, as later in the year you are hitting "mature" grasses that are well past their prime and have had a chance to lignify the stalks holding up seed-heads. I still think that good rotational grazing creates more benefits by raising your total forage production per acre, per year more than a once-over mob grazing system can.
On the corn thing I have a simpler explanation. In Ontario's corn belt (which is south of Minnesota) corn on corn suffers a yield penalty, and no-till corn tends to lag tilled acres, typically from cool soil temperatures. No-tilled corn on corn always looks like hell, and yields accordingly in this part of the world. If you give up 40 bu/ac of yield, the book value is you give up 2200 lbs/ac of stover, 900 lbs of root mass, and 1000 lbs/ac of root exudate. That's a lot of stuff NOT going into the soil pool. Tillage burns carbon, but if it's done properly it can result in more yield gain, and more biomass return than the carbon it releases from the soil pool, especially in the northern corn belt.
I think we have probably maxed out around 8% OM. In other words, we've reached a new equilibrium, and barring another major change in management (like going back to row-cropping as a part of our rotation) it should bounce around in that area going forward. The only prediction I will make (and it's safe to make, because I have no soil sample data to disprove me) is that our deep soil OM will continue to go up. Apex vegetation here is northern lowland hardwood. It makes great topsoil, but it's shallow (typically 10-14 inches) and perennial grasses have a much deeper, fibrous root system. It's still going to take us years to build our topsoil any deeper, but it's far more likely under this management plan then under a row-crop rotation.
Hope that's clearer than Mississippi mud.
