Rainfall Simulator Results at Milan No-till Field Day, 2016
One thing different about the 2016 Milan No-till Field Day was rain. The field day began early with heavy rain in the Jackson and Milan areas. The Natural Resources Conservation Service (NRCS) and Tennessee Association of Conservation Districts (TACD) had two rainfall simulators with five treatments each. The natural rainfall provided immediate results as we arrived at 7:00 am the morning of the field day. All day long we simulated rainfall with treatments receiving more than 12 inches of rainfall. Farmers and visitors could easily distinguish the differences of the treatments. The rainfall simulators were managed by Greg Brann, NRCS' State Grazing Lands Specialist and Soil Health Specialist, and Mike Hubbs (author), and TACD Soil Health Coordinator/Specialist, Matthew Denton, NRCS' District Conservationist, Trenton, Tennessee, and NRCS' Soil Conservationist, Martha Griffin, Trenton, Tennessee. We had assistance from several NRCS' District Conservationists from West Tennessee.
In addition to the rainfall simulator, we showed the results of dried soil aggregates being placed in water. This test is called a slake test. It shows the ability of the soil to hold together when in water or when water hits the soil such as a raindrop. When soils are improving in soil health, soils are covered, not tilled, roots are continuously growing, and diversity is present then soils are aggregated and are held together by glues such as glomalin from soil life. Below you see the soil on left has been tilled. It falls apart as soon as water enters the pore space. The aggregate implodes due to a low aggregate stability. The water becomes turbid. The soil on the right has been in cover crops and has been no-tilled. Its structural integrity remains intact all day long sitting in water. We placed the soil samples in water at 8:00 am. Picture taken two hours later. When soils have low aggregate stability, they seal up after a rain reducing the ability to infiltrate water.
On the first rain fall simulator, we had five treatments. All treatments from Rainfall simulator one was sampled the day before from Milan except Treatment 3. Treatment 3 came from a farm near by farm in Gibson County. Treatment one was a bare area between fields that had no green plant growing. The area was planted but had no vegetation this season. The soil in the sample pan was much heavier which we speculate from the weight that the bulk density was highest in this pan. No roots were present and no cover was evident. Soil crusts were present with a depth of approximately 1/4 inch. Treatment 2 was conventional soybeans this year with corn last year. The soil sample pan was dug between rows. The soil had been worked two to three times and had soil crusts. The pan's weight was heavier than most of the treatments, but was lighter than treatment one. Treatment three was from a double crop field of wheat and soybeans this year. The field was tilled with a vertical tillage implement prior to wheat and soybeans. There was not any significant residue on the surface. Treatment 4 had five multi species cover crops followed by corn last year and soybeans this year. Residue completely covered the soil. Treatment 5 was between fields on a fence row. The treatment was deciduous forest. The natural cover in the pan had not been disturbed and represented the forest floor. The picture shows the results.
Treatments 1 - 3 showed similar results. There were high amounts of runoff evident immediately in front jugs after simulating the rainfall. The front jars are for runoff and one can see the turbidity in the water with all jars with high percent of runoff and very little infiltration as shown by the jars in the background. Treatment 4 and 5 behaved similarly. Both jars had 90% plus infiltration and little runoff. The natural forest is the standard that we want to mimic in a cropping ecosystem. We want diversity like the forests with as many species as practical. We want to emulate the natural cover by maintaining all residue on the soil surface. We want a root growing as continuously as possible. Green plants produce carbon through photosynthesis. Keep plants growing continuously intercepts energy from sunlight. The plant produces carbon which feeds soil organisms through exudates leaking from roots. Soil organisms aggregate the soil providing soil voids or pore space. This builds soil structure that is crumbly. Soils that do not have continuously growing roots tend to have horizontal plates that inhibit infiltration. This structure is known as platy structure. Our goal as conservationists and farmers are to have crumbly and blocky structure. This type structure is similar to marbles in a jar. The marbles take up space in the jar with porous space between marbles. This results in better infiltration, less erosion, and runoff.
Rainfall simulator 2 also had five treatments. Treatment one was a continuous corn and soybean rotation that has been no-tilled and left fallow in winter with residue left on the surface. The soil structure was better than tilled treatments from rainfall simulator one, but had platy structure. Treatment 2 was 3 multi species cover crop mix in a corn and soybean rotation. The field had been no-tilled many years. Treatment 3 was the same as Treatment 4 from rainfall simulator one. It was a 5 multi species cover crop mix in a corn and soybean rotation, no-tilled. Treatment 4 was a continuously grazed pasture from Greene County. There were some areas that were bare due to over grazing. The roots were stunted due to over grazing. Treatment fives was a soil sample taken from a prescribed grazed pasture field from Greene County, Tennessee. The field is intensely grazed for short periods of time and rest for a minimum of 30-40 days. Like the treatments with cover crops the properly grazed pasture has something growing most days of the year. The rested grass has good leaf index to intercept sunlight and produce carbon from the atmosphere. The sugars in the plant eventually becomes soil organic matter or soil carbon. The roots from the properly grazed pasture emulates the top growth. When grasses are rested and growing a leaf then the roots are also growing and sloughing off providing an abundance of organic material to feed the soil biology.
The results from rainfall simulator 2 were expected. The no-till had some infiltration but did experience higher amounts of runoff than treatments 2, 3, and 5. Treatment 1 had less runoff than the tilled treatments from Rainfall simulator 1. Treatment 2 and 3 functioned similarly with excellent infiltration and little runoff. Treatment 4 functioned poorly with high amounts of runoff and little infiltration. Predictably, Treatment 5 functioned well with excellent infiltration similar to the woodland treatment from rainfall simulator 1. Runoff was almost nonexistent.
So what is the take away message from this simple but very evident demonstration. Soils must be covered to reduce the high potential of erosion in West Tennessee. Roots must be present and kept growing to provide good soil aggregation and void spaces for good infiltration. Tillage destroys soil aggregates regardless of depth. Tillage always leaves a restriction zone which causes some compaction and restricts infiltration after soil settles. If one is tilling, one must continue to till to take care of the problems causing tillage. Diversity adds soil biology diversity which improves soil aggregation and nutrient cycling. Just because a field has grass on it does not guarantee good soil function. Pastures that are over grazed will function poorly as shown by erosion, runoff, and poor infiltration.
Livestock Farmers should consider these findings to follow a grazing plan by NRCS to better their soils for better grazing and production of livestock. Crop farmers can better utilize both natural rainfall or irrigation by following a no-till system with cover crops. Better soils mean better profits with a better environment.