Soil Health, Erosion and Fall Field Management

Fall harvest has begun and this is a good time to look ahead and plan for better soil health. Soil health has several important aspects, but first and foremost is soil erosion prevention. Soil loss equals topsoil loss and topsoil is where the largest portion of soil and plant biology occurs. Eroded soil is typically higher in the smaller mineral particles (clay, silt) and organic matter/humus – the ‘good things’ that contribute to soil water and nutrient retention/cycling and crop productivity.

In Kentucky, most soil erosion is caused by water/rainfall. Water erosion soil losses are driven by storm length and strength, and Kentucky is receiving fewer, but stronger, storms. Figure 1, below, shows the tons of soil lost per acre, by rainfall event, for 2 years in a monitored no-tillage field area with a 5-6% slope and under a corn/full-season soybean rotation. The focus is on winter/spring rainfall when most erosion happens, so the timeline begins in the summer when corn is being grown, starting July 1 of Year 1, and ending June 30 of Year 3 (Fig. 1).

Corn was harvested, and the wheat cover crop planted, in early to mid-September of Year 1 (Fig. 1). In the next winter/early spring (Year 1/Year 2) period there is very little soil loss. Corn residues and the wheat cover crop acted like a close growing grass sod (Fig. 2), providing excellent soil protection against erosion despite several winter/early spring rainfall events that were often equal to or greater than 2 inches (Fig. 1).

After cover crop termination and soybean planting at the start of Year 2, corn and cover crop residues continued to protect the soil and late spring/early summer soil erosion losses were low (Fig.1). Summer rainfall can be heavy, and a strong storm (7 inches) occurred mid-summer of Year 2. The soybean crop was at full canopy (Fig. 3), which intercepted the rain, dissipating its energy and minimizing erosion. Soil loss was small, about 0.2 tons/acre (Fig. 1).

Soybean was harvested, and the wheat cover crop planted, in early to mid-October of Year 2 (Fig.1). In the first part of second winter/early spring (Year 2/Year 3) period, rainfall events were generally light/small, and the soybean residues and wheat cover crop provided adequate soil erosion control. Just prior to cover crop termination in mid-March of Year 3, a 2.2-inch rain resulted in a large soil loss (9 tons/acre). The soil protection provided by the wheat cover crop and soybean residues was not adequate, given the conditions (5-6% slope, soil already wet, residue breakdown, thin cover crop) that combined (Fig. 4) to favor soil erosion in this event. The second cover crop, after soybean, was planted about one month later than the first cover crop, after corn (Fig.1). An example of a thin, patchy cover crop that was established into soybean residue is shown in Figure 5.

So, while no-tillage soil management can help reduce soil erosion, these stronger, heavier storms present a serious challenge, even to fields under notill soil management. But there are some things that can be done to enhance no-till soil erosion control.

Use existing resources – crop residues. The crop residue remaining after harvest is the ‘frontline’ resource available to prevent/slow soil erosion between harvest and canopy closure of the next crop grown in the field (Fig. 2). Residue protects soil aggregates from raindrop energy and impact, lessening soil particle detachment, the first step in the water erosion process. Avoid fall field operations that diminish residue’s erosion control effectiveness. Don’t ‘size’ residue with a rotary chopper, disc, or vertical tillage tool. Those small residue pieces are more likely to float/move downhill during strong storm events, leaving soil areas uncovered and unprotected (Fig. 4). Bigger residue pieces are more likely to get hung together and dam water movement and soil particle transport, the second step in water erosion of soil. Don’t enhance crop residue decomposition, biologically, chemically, or physically – soil erosion protection needs to last through the winter into early spring. Kentucky winters are relatively mild and residue breakdown continues slowly but surely all winter long.

Use existing resources – soil aggregates. Soil agresidues. gregates also contribute to soil erosion resistance. Strong, water stable aggregates enhance water infiltration rather than runoff. At the end of the season, soil aggregation is at a maximum. Aggregation is enhanced by fresh carbon additions and the seasonal growth of the crop has provided both exudates and root biomass. Soil biology (both macro and micro) acts all growing season long on these new carbon sources, forming organic matter/humus and bringing organic and mineral particles (clay, silt, and sand) together to form aggregates. Aggregate damage happens when aggregates are crushed (compaction) or disrupted (broken). Fall tillage, especially any kind of surface tillage, causes aggregate damage by both mechanisms. The surface disruption that occurs with vertical/disc tillage breaks up aggregates near the surface, while the compaction of aggregates happens due to considerable weight/pressure at the blade tips.

Consider using additional resources – winter cover crops. Fall surface tillage and/or residue sizing, either chopping or cutting, means that additional resources might be needed to prevent/slow erosion. Full-season soybean residue levels often look adequate for erosion control in the fall but degrade quickly and leave these fields vulnerable to springtime erosion. In these instances, a winter cereal cover crop, especially winter cereal rye, is needed to reduce erosion risk. The more dense, fibrous winter cereal root system helps maintain aggregation as it holds soil in place. If the winter cereal cover crop planting date is sufficiently early, the shoot biomass also provides some additional aboveground protection against raindrop detachment and further aggregate destruction

Soil erosion remains a major threat to long-term productivity in many areas of Kentucky. Most of our agricultural lands exhibit slope and many of our field soils are covered by silt loam topsoils. These conditions, combined with our rainfall patterns, make Kentucky agriculture vulnerable to soil erosion. No-tillage soil management does a lot to reduce soil erosion risk, but no-tillage practices can be strengthened to guard against erosive precipitation events that result in catastrophic soil loss. Improved soil health begins with soil erosion control.