Perhaps no other practice improves water movement into the soil surface more effectively than creating and maintaining a mulch layer. The primary benefit of a mulch layer is not that it slows the velocity of overland water flow, as is often assumed (though that is important); rather, it is that mulch absorbs the energy of falling raindrops and thereby prevents raindrop impact from destroying soil aggregates. If the aggregates remain intact, the water goes into the soil through the intact large pore spaces, and there is no runoff to slow down.
In my teaching days, I conducted a demo with two areas of soil. Over one area we suspended straw mulch on a frame of chicken wire a few inches above the soil surface. The adjacent area of soil had no protection. We sprayed both areas with a garden hose for several minutes. Even though the straw was not actually touching the soil, there was no runoff at all in soil under the mulch. The soil surface remained open and loose, while the unprotected area became sealed over and then crusted when it dried. Table 2.2 (below) indicates how surface mulch affects water infiltration.
Keeping the Residue We Already Have
So how do we create a mulch? The first and most obvious way is simply to avoid destroying or removing the mulch we have as a byproduct of our current land management, such as crop residue or the stubble of pasture grass.
Keep Plant Residues in the Field
Just as tillage is destructive, so is removal of crop residue from the field.
During the drought of 2012, a huge amount of crop residue was baled and sold for low-grade livestock feed in my area. I had neighbors who thought they had found a miniature gold mine, selling baled corn stalks for $60 a ton. I felt compelled to point out a few items to them. First, the cost of swathing, baling, and moving those bales amounted to around $30 a ton. So, their net above-harvest cost was only about $30. Then I had them figure the value of the fertilizer in those stalks. A ton of corn stalks contains about 20 pounds of nitrogen, about 8 pounds of phosphate, and about 60 pounds of potash. At $0.65 per pound of nitrogen, $0.75 per pound of phosphate, and $0.50 per pound of potash, that fertilizer value amounted to $49 a ton. Essentially, they sold $49 worth of fertilizer for $30.
The real loss, however, was the value of the stalks as mulch and organic matter. Neighbors who continued on this path for several years started to notice that their dryland fields were dropping in yields quite rapidly. They began to have crop failures while their neighbors were still pulling off average yields. They also had large amounts of potassium deficiency.
Potassium deficiency predisposes plants to stalk rots, which are fungal diseases that infect the lower stalks. Once the lower stalk is infected, it not only makes it difficult for the plant to take up water and nutrients, resulting in reduced yield, but the structural integrity of the stalk is compromised as well, and the plants begin to lodge (fall over) and become difficult to harvest.
No one fertilizes with potassium in my area, because our soils tend to be very high in that element. But when crop residue is harvested year after year, the most easily available potassium ions are largely removed from the surfaces of soil colloids and transported away in bales of stalks and straw.
The Amish have a saying that the man who sells hay is slowly selling his farm, and I have seen formerly rich soils become progressively impoverished by too much removal of crop residue. On top of that there is the issue of soil erosion. Removing the protective layer of natural mulch leaves the soil more vulnerable to erosion. I shudder to think of what might happen if cellulosic ethanol ever becomes a viable concern, if crop residue is the preferred feedstock. I am all for green energy, but only when it makes sense in the long run. Being able to fill our gas tank cheaply won’t do us much good if we can no longer grow food.
Similar to the removal of crop residue on cropland is the overgrazing of pasture lands. It is critical to leave a minimal amount of grass residue on the soil surface to promote water infiltration. The data below illustrate the effects of too much grass removal on water infiltration in a Texas study.
It may be helpful to point out exactly what “too much” grass removal consists of. In terms of pounds per acre, it is, roughly, grazing so much that less than 2,000 pounds of forage per acre remains. Once residue amounts drop below 1,000 pounds per acre, runoff rates (and evaporation rates, as the next chapter will describe) increase even more dramatically. In lieu of clipping, drying, and weighing the forage, however, it is much easier to “eyeball” the pasture. If there is any bare dirt showing (other than in high-traffic areas such as gateways), there has been too much forage removal.
Not only does bare, exposed soil reduce infiltration, but it also means that there is sunlight not being captured by green leaves, and not contributing to pasture productivity.
No-Till Cover-Crop Mulch
On cropland and gardens, we can add to the amount of mulch left by crop residue by growing cover crops in between cash crops. Cover crops can be managed to provide multiple benefits, but perhaps the most beneficial is the addition of surface mulch. As with all mulches, no-till is essential to maintaining this benefit.
A cover-crop mulch can dramatically improve infiltration. My very first experience in which I no-tilled (planted a crop without tillage, using a no-till planter) into a cover crop mulch demonstrated this in spectacular fashion. I planted a soybean crop into a killed cover crop of very thick rye. The mulch was so heavy that on 40 acres I flushed 29 hen pheasants out of it while planting (they were looking for a place to nest, and this was by far the best cover around).
During the planting process I had this nagging feeling that there was something I had forgotten. I kept getting out and checking the drill to see what I had omitted. Then it dawned on me: there was no dust. Every time I had planted into tilled ground there was always a cloud of dust that followed the tractor, and a layer of dust on the drill. This time, there was none. I wondered how much longer my bearings and engine would last without all that abrasive soil getting into them (not to mention my lungs) and how much money the lower maintenance alone would save me over the long run.
That summer it rained 11 inches (30 cm) in a two-week period from late July through early August. This is a perfect scenario for growing soybeans, and sure enough, this field produced the highest yield of soybeans the farm had ever grown. But the more astonishing thing about it was revealed later. You see, this field was shaped like a big bowl, and all the water drained into a small pond. During the two weeks when those 11 inches of rain fell, not once did the level of that pond rise. All the rain went right down into the soil.
Then the rains shut off in mid-August, and no rain fell until the following May 23. I planted wheat into this field following soybean harvest in October, and despite no rain for 10 months, my field raised a whopping 77 bushels of wheat an acre. My neighbor across the field access road, an excellent farmer and winner of many regional yield contests, drilled wheat into his soybean stubble the day before I did; his made 25 bushels an acre. All that excess rainfall that fell on my field during the growth of the soybean crop was stored for use by the wheat crop that followed, instead of running off.
The graphs below illustrate how cover crops improve water infiltration.
About the Author
Dale Strickler is an agronomist for Green Cover Seed, the nation’s leading cover crop-specific seed company, and a leader in the soil health movement. He is based out of Bladen, NE. This article is an excerpt from his book, The Drought Resilient Farm.