How much topsoil loss is enough?

Yesterday's windy conditions caused considerable topsoil loss if the tall grey-brown clouds coming out of the west and deposition of ultra-fine soil particles were any gauge. The article below, titled "How much topsoil loss is enough?" was written by North Dakota State University Extension soil health specialist Naeem Kalwar, Extension soil specialist Dave Franzen and professor of soil science Larry Cihacek and shared with me by Mr. Kalwar. 

Wind and water are the main causes of topsoil loss. However, recently tillage has been identified as an additional reason that not only removes topsoil from agricultural fields but it actually increase the intensity and rates of wind and water erosion (Figure 1). Loss of topsoil due to tillage can increase to severe levels if weather is windy, especially at high speeds and soils are dry. Tillage equipment breaks down soil aggregates into smaller, finer particles which then become airborne in wind and blow away.

Figure 1. May 12, 2021. Tillage in operation causing a cloud of topsoil blowing away in Cavalier County, ND; photo credit: Naeem Kalwar, NDSU.

A recent use of tillage in fall and spring was to dry the soils faster in spring for early plating. However, not only was it dry in fall-2020 but so far in spring-2021 it has been very dry as well. Tilling soils this spring has resulted in evaporation of the little bit of moisture which was there, increased wicking up of groundwater that has deposited excess salts and sodium at or near soil surface and has produced clouds of precious topsoil blowing away. This airborne soil ends up in fields or areas where the wind velocity is lower compared to the original wind velocity that made the soil particles airborne. Some coarser particles drop into neighboring fields or ditches, however, the finer mineral and organic matter particles may go up to hundreds of miles away (Figure 2).

Cihacek et al., (1992, 1993) reported an average of 150 parts per million or ppm (about 300 lbs. per acre) of nitrogen (N) in wind erosion sediments deposited at 34 sites in road ditches of the Red River Valley. Most of this N appeared to be from spring broadcast applications of N fertilizers. The fertilizer materials were abraded by wind movement of soil particles into particles that were carried along with the soil particles from the field. In some instances, pre-plant herbicides were also found in the sediments.

Figure 2. May 12, 2021. Finer particles of topsoil and organic matter blowing away during windy and dry weather in Cavalier County, ND; photo credit: Naeem Kalwar, NDSU.

When settled in ponds, lakes, rivers and streams, the soil sediments produce poor water quality for humans and livestock alike. In nutshell, topsoil should stay where it belongs!

Long-term effects of tillage disturbance are especially evident on hilltops (Figure 3). Consistent dragging of topsoil from hilltops to the lower grounds devoid hilltops of organic matter resulting in poor stands and yields. This is called “tillage erosion”. The movement of soil is nearly always downhill due to speed of the tillage equipment and gravity. The movement of tillage equipment lifts the soil into the air while gravity pulls the soil particles downhill. The speed of the tillage equipment also provides a forward velocity to the soil particles moving the soil in the direction that the equipment is moving.

 

Figure 3. May 17, 2021. Long-term tillage resulting in dragging of topsoil from hilltops to the lower ground causing tillage-erosion, in addition to wind-erosion in Grand Forks County, ND; photo credit: Naeem Kalwar, NDSU.

If planting no-till was ever an option, it was 2021. Understandably, tillage cannot be eliminated overnight. There could be a need for some tillage for incorporating pre-plant herbicides required for controlling resistant weeds that have already germinated or to manage residual straw for a better seedbed if planters are not designed to handle it. However, producers need to seriously consider the loss of moisture and topsoil versus other objectives in a spring like this. Rock rolling on tilled, loose soil is another management decision, which can cause havoc when it comes to topsoil loss versus potential equipment damage in fall due to rocks. Rolling crushes the soil roughness and aggregates to a point that the wind velocity at the soil surface is not reduced. Thus, the energy imposed by the wind on the soil surface can lift and move soil particles more easily. A tilled, smooth soil is more likely to blow away compared to a tilled, rough soil.

• So the million dollar question is: how much topsoil we are losing per acre?

Roughly, topsoil having thickness of a dime is five-tons per acre. Considering the erosion events that have happened this spring, some fields have lost several inches of topsoil. Since topsoil depths in our fields are seldom more than six-inches deep to begin with (a sorry shadow of the two to three feet present in 1880) a loss of any topsoil is serious. As per a general rule with the present topsoil depth on most fields of soil productivity by soil depth, if a field lost half of its topsoil, it will lose 35 percent of its relative productivity. One might consider it as losing 35 percent of the land value if land value is related to soil productivity.

So what can producers do to minimize the loss of topsoil?

• Avoid unnecessary tillage. If you think that you can do without another tillage pass or operation, do not do it. Everything helps!  
• If planting no-till or strip-till is an option, think seriously about it. 
• Once you are caught up with field work, consider planting shelterbelts.
• If fall is dry, avoid any tillage in the fall. A lot of producers tilled dry fields in 2020-fall thinking that 2021-spring will be wet. They are forced to till now as ground is cloddy.
• If weather stays dry in summer and fall, let volunteers grains grow after harvest. In some cases letting non-resistant weeds can also help as long as they do not go to seed. Roots of volunteer plants and crop stubble will hold the topsoil at its rightful place.

In summary, reducing tillage, leaving a cover of crops or crop residues or strips of standing residue, leaving the soil surface rough after tillage (such as after chisel plow tillage), establishing and maintaining shelterbelts are all practices that reduce topsoil loss to wind erosion.