Nutrient Cycling on Laterite Soils in the Mid-South

People look at the lush jungles and cannot believe that the soils in wet, tropical areas are extremely infertile. It is counter-intuitive to believe that almost all of the nutrients are captured in the living plants and animals and almost none of those nutrients are in the soil.

In tropical regions, organic matter that hits the surface is very quickly broken down by termites, fungi and bacteria. Feces is scavenged by ants and beetles. The humidity and temperatures make that happen in a matter of hours or days.

Buttressing of trunks is indirect evidence of a tree with shallow roots. Trunks "girth up" based on strain triggering growth hormones. The individual buttresses align with large, roots near the surface. Individual trees with little buttressing have deep root-systems.

Tree roots in jungles are typically very close to the surface and very finely divided as there is a life-and-death race to capture those newly released nutrients before another neighbor does. The tree that is slow to capture those nutrients is over-topped by its neighbors and languishes in their shade.

That compares to dry grasslands where dead roots decay-in-place and captured nutrients accumulate over the centuries. Cool, temperate regions also lack the super-aggressive decay mechanisms seen in the tropics and can accumulate nutrients in the soil.

Since science mostly evolved in temperate Europe and North America, we tend to think all soils are like, or should be like, the soils of Geisenheim, East Malling, Saint-Aubin or Champaign or Ithaca.

The soils of Tennessee, in particular the soils of the Cumberland Plateau are more like tropical soils than they are like the soils of central Illinois.

N-P-K

The three macro-nutrients for plant growth are Nitrogen, Phosphorous and Potassium. There are several important "minor" nutrients like calcium, magnesium and sulfur.

Nitrates are leached from tropical soil in a matter of months and years.

Potassium is very soluble and is leached out of tropical soil in years-to-decades.

Calcium and magnesium carbonates like limestone and dolomite, and calcium sulfate (gypsum) and most phosphates dissolved out of tropical soils in a matter of decades-to-centuries.  

Silica (sand) also dissolves, but more slowly. Little known fact, rice stubble and chaff is very rich in silica and rice-straw can be used to make silicon carbide whiskers.

That leaves clay.

Over time, the clay also changes. Young clay is slippery/slidey because cations of sodium and potassium act like ball-bearings between the flat clay particles. Imagine the shingles torn off of a roof and tossed to the ground. The shingles tend to lie flat-to-flat. That is what clay particles look like. The cations nest between the shingles, held there by ionic attraction. Water fluffs-up the shingles and lets them flow freely.

Over time, water leaches out the sodium and potassium ions and the clay become blockier at the molecular level.

The blockier clays (like kaolin) are generally preferred for pottery over the shingle-like clays (like bentonite and montmorillonites) while the shingle-like clays are preferred for kitty-litter and for sealing bore-holes and the bottoms of ponds.

The mid-South...like eastern-Tennessee are directly down-range of the vast amounts of water that evaporate from the Gulf of Mexico. The centrifugal action of the earth spinning causes dry, dense air to move toward the equator down the spine of Mexico and displace the less dense, wet air and allow it to move north toward the poles. 

Because the air has eastward momentum due to be farther from the axis the earth spins around, the storm tracks arch eastward as they head north on the weather conveyor belt where they smash into the Cumberland Plateau. As Larson succinctly stated, "Bummer of a birthmark, Hal". 

Another factor that comes into play.  is that the Cumberland Plateau rises approximately 1000 feet above land to the west. Especially in late-summer and early-to-mid fall when the surface temperatures of the Gulf are hottest, the Cumberland Plateau are the bowling pins and storm tracks are the bowling ball.

They get a lot of rain.

More recent history

 

Before white settlers showed up, the forests of the Cumberland Plateau burned every six-to-eight years.

Settlers between 1750-and-1929 bent the land to the plow and hoof, girdling and burning to clear the land.

The ashes from that burning were the only amendment to the soil.

The low pH and dearth of potassium and phosphorous limited the growth of clovers and other legumes that could have naturally enriched the soil.

During the Depression, the marginal areas of the mid-South were depopulated of farmers. They moved to Detroit or to TVA projects in the valleys.

The trees grew back as second-growth forests.

Is it hopeless?

Humor me and let me start with a story.

A young fellow was driving in Wyoming during a white-out blizzard. He drove for fifty miles and could barely see the road. Finally, he spotted a gas-station/convenience-store beside the road and pulled off.

Going into the store, the young flatlanders complained to the proprietor "I thought you didn't get a lot of snow in this part of Wyoming."

The proprietor responded "We don't"

"Well what about that?" the young fellow objected, pointing outside.

The proprietor shrugged. "We don't get a lot of snow here, but we get a hellova lot of mileage out of the snow we do get."

Point being, one inch of snow can look like a lot if it is dry and the wind picks it up and blows it at 30mph.

The story applies to Sig and Blain's challenges at Copperhead Cove in two ways. They need SOME fertility just like Wyoming needed an inch of snow. The second point is they can make SOME fertility work as long as they rapidly cycle it back into growing plants just like the tropical forest does.

That cloud of blowing snow probably didn't extend more than fifty feet in elevation.

The key to rapidly cycling nutrients is to spread it thin and to spread it on growing plants.

Even though the "bump" in crop yields will be primarily due to the nitrogen in the chicken-shit and bedding, the long-term benefits will be the phosphorous and potassium that will allow red clover to germinate and grow in future years/decades thereby making fallow-years far more effective at creating fertility.

Minor issues

One minor issue with the way the residents of Copperhead Cove are managing their fertility is that potassium is mostly excreted in the urine while phosphorous is mostly excreted in the poop.

Long-term, throwing the poop from the pasture into the garden will result in phosphorous migrating from the pasture to the tilled area. The tilled areas will not be able to exploit the increased phosphorous because it will not be balanced by the necessary amount of potassium.

Similarly, the potassium which is cheerfully cycling in the pasture will not be able to stimulate clover growth because of the growing lack of phosphorous.

The productivity of both areas will diminish (especially the pasture) even though there is NO NET OUTFLOW of nutrients. The problem is that the nutrients are segregating.

The exception to this general flow will be the garden plots closest to the buildings which will get dressed with wood-ash which is rich in potassium. Those plots will get both manure (nitrogen and phosphorous) and wood-ash (potassium). A person who does not walk all of the "fields" but only looks at the ones closest to the buildings will think everything is peachy-keen.

One "fix" that can happen is when the small-holder supplements the cows on the pasture with corn-stalks in the winter and/or corn (grain) in the summer when their milk production sags as they graze the least fertile parts of the pasture. That returns phosphorous to the pasture as long as all poop is not collected and put on the tilled areas.