Nutrient Cycling in Small Pasture Paddocks in Rotation

Spring has come to East Texas. March, and it is hot already between episodes of freezing. The fruit trees have all blossomed out and are laden with pollen which the bees are busy hauling back to their hive. Spring is a lovely time to walk about and enjoy the new life rousing from its winter slumber. Spring is also a time to get ready for the hard work of gardening and field planting if we expect a decent summer harvest.

Our veggie gardens are in good shape and the soil is ready for compost and plants. The tricky issues evolve around planting grain patches and making sure the pastures are in prime condition for forage. We are committed to grass-fed beef with supplements of grain when the mama cows are fresh. Grass, with the exception of alfalfa, and clover, just do not provide all the nutrients a cow needs for steady milk production along with a good body score. We’ll grow the grain (corn and sorghum) near the fish ponds so we can “fertigate” these heavy feeders with fish waste. Getting pasture soil fertility right—and I don’t mean in theory—is the hard part.

We know that we must build up soil organic matter to create the soil structure and fertility it needs for its own sake. Soil is a living biotic community in its own right. To grow a rich pasture forage crop on top of that means even more nutrients must be added. The problem is relatively simple to state: the mass balance of organic matter (OM) in the soil must be maintained, so whatever you take from the soil, you must put back. Here’s an example—Coastal Bermuda-grass in Texas producing a stand of grass equaling 6 tons of weight over the course of a year will remove about 258 pounds of nitrogen (N) if it has a crude protein of at least 12%. That’s pretty good coastal hay. You will also lose 60 pounds of phosphorus (P) and 280 pounds of potassium (K).

Lucky enough, the small cows we have weigh about half of a normal cow and eat about half. Our mama’s require about 40 pounds of grass a day and 2 pounds of richer supplements. Given the ideal three head herd (2 mamas and 1 bull), they together would consume 120 pounds of Coastal a day for 365 days or a total of 44,000 pounds, which is 22 tons. If we get 6 tons per acre, we will need about 4 acres in pasture. This we have already set up in paddocks for rotation.

You can begin to see the problem we face. We will remove 4 A @ 258 lbs. = 1000 lbs. of N, 4 A @ 60lbs = 240 lbs. of P, and 4 A @ 280lbs. = 1120lbs. of K out of those four acres this year. How will all that macro-nutrient get returned to the soil along with enough extra to keep the soil OM going for its own sake?

Fortunately, with the right rotation through half-acre paddocks, we can keep all the dung and urine from the cows on the pasture. Some of it volatilizes and leaves as gas, but most of it is buried underground by dung beetles and other critters. Our paddocks are contoured, so we get very little to no run-off and uniform infiltration of water and effluent. For the sake of this discussion, let’s say we get back 80% of what the animals ingest. We will need to find the lost 20%, which is not small change--200 lbs. of N, 48 lbs. of P, and 200 lbs. of K to make up the loss and maintain the forage mass balance—or maintain the soil organic matter (OM) annual equilibrium.

But the forage loss is not the only loss in the system. The soil by itself, without any kind of tillage or crop, will lose 2% of its organic matter annually. Let’s assume we begin with 40,000 pounds of humus per typical acre. We have less, but a 2% loss would leave 39,200 pounds. That’s 800 pounds of new humus to add per acre per year--just to keep up. To get that 800 pounds per acre back in the soil, I must add over a ton of plant residue per acre per year in addition to the loss from forage.

I can make up the humus loss by importing large, one ton bales of Bermuda hay in the winter. I can also mow the grass the cows leave and allow it to rot in place, reducing the nutrient demand a little. However, to get a forage crop without ruining the soil, I must buy and spread chemical fertilizers carefully, based on regular testing. No one has yet showed me how to make up the losses organically. I get lots of theory and advice, but I have not seen a system that works without chemical inputs that has functioned for longer than ten years.

The Amish get around this mass balance dilemma by using zero tillage, all their manure, and herbicides instead of fertilizer. It’s a sustainable practice, obviously, since they have been doing it a long time and are economically and ecologically successful. I cannot afford the equipment for zero tillage, nor do we have the space. The average Amish farm is over 100 acres. Mine is 12. While I understand herbicides, I choose to err on the side of limited chemical fertilizer when soil and forage tests show a need. Small amounts of chemicals will not burn the microbial biomass and do not lead to salting.

Some folks would argue that all the nutrients I need can be found in the soil microbial biomass, held in microbe bodies. If I broke them down, the nutrients they hold would be available. Let’s look at nitrogen. Say I have an optimistic 4% total biomass in my soil and the total OM (humus and microbial biomass) is 80,000 pounds. My loss is 2% or 1600 pounds of organic matter per acre per year. Of that organic matter 5% is N. I would lose 80 pounds of N per acre just from soil respiration—not a crop!. I must input over 2 tons of detritus (biomass) per acre per year just to maintain the original OM equilibrium. What would it take after that to get a crop?

I now need to know what I will lose to any crop. If I plow or till, I will lose even more OM as CO2 to the air. Just to grow pasture grass with decent forage quality, I will need to find over 200 pounds of N per acre per year. 80% of it will cycle back in waste, but I have to have the N available before that. If 2 tons of detritus yields 80 pounds of N as above, then I will need to import over 5 tons per acre to get the N I need organically. Furthermore, I will lose the N in the soil to microbial decomposition of the detritus for about 4 weeks after every application.

It would seem that if I put 8 acres to grass pasture and mowed, hayed and manured all of these, I could conceivably meet all my nutrient needs organically. That’s an IF I don’t have the luxury to assume.

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