Foresight for Successful Cropping Systems

By Zachary Fore
U of M Cropping Systems Specialist
forex002@umn.edu

I have heard ‘dirt’ defined as “soil out of place.”  As soil, it is extremely valuable and useful. As dirt, it is at best an annoyance and at worst an expensive and destructive problem. 

Exactly how valuable is our soil?  Putting a price tag on it can be difficult (though some would argue that land rent/prices would come close) but one could also say that the value of our soil is priceless, as alluded in the following quote by John Seymour, author of “The Erosion of Civilizations.”:

“India, China and Indonesia all have to import North American grain in times of shortage. We in Europe are heavily dependent upon North American soya for the protein part of our stock rations. So the soil of the American Great Plains is probably the most important thing in this world.”

Most people would probably not rank the soil of the American Great Plains as the most important thing in this world, but it is very important in terms of global food production, and even more important in terms of its contribution to the economy of the country and the region. 

Indeed, it is difficult to over-estimate the value of soil and the consequences of its loss.  Major civilizations have literally been destroyed by soil erosion. Have you ever wondered when you see those pictures in National Geographic of ancient cities being excavated out of sand dunes, why they built a city in the sand dunes in the first place?  Most likely, it wasn’t sand dunes when they built it.  There are many examples, particularly in the Middle East, of major civilizations that built cities in agriculturally fertile areas. The increase in population caused the intensification of agriculture, which created practices that caused excessive soil erosion.  Eventually the land was no longer able to produce food for the population, and the civilization collapsed. 

The bottom line is that soil erosion is a very bad thing, and that we should expend great effort to prevent it.  Not only does soil erosion reduce the productivity of our soils, it reduces water quality and fills up our ditches, rivers, and lakes. Taking care of our soil is a stewardship issue, and everyone that drives by on a blustery prairie day can see if we are being good stewards or not. 

Preventing Erosion
In the Northern Great Plains we have two types of soil erosion – wind and water.  Wind erosion tends to be most important on flat soils, and water erosion on soils with greater slopes. Erosion is preventable on all of our soils. Little erosion generally occurs when we have a crop in the field with 50% or more canopy cover. Most of the soil erosion occurs after harvest in the fall, and before 50% canopy closure in the spring. 

The most important practice we use to prevent soil erosion is crop residue management.  Soil erosion is greatly reduced if 30% or more of the soil surface is covered with crop residue. Figure 1 shows that erosion is reduced by at least 50% (compared to bare, fallow soil) if 30% of the surface is covered with residue.

Small grains, corn, and soybeans generally give us enough residue to work with, as shown in the table at right.

Every tillage operation after harvest reduces the amount of residue cover remaining. Soybean residue is particularly fragile. To estimate the percentage of residue surface cover remaining after future tillage operations, calculations may be made using the data from the table at the bottom of this page. Additional information is available on nearly every type of implement.

Managing crops that don’t leave much residue
Some of our crops leave us very little residue to work with after harvest. Sugarbeets and potatoes are of particular concern. These crops are harvested late with little opportunity to establish a fall-planted cover crop. Managing soil erosion with these crops is truly a challenge. Here are a few practices to consider:

•  Till fields creating large ridges that run perpendicular to prevailing winds.

•  Plant shelterbelts.

•  Plant in strips with high residue producing crops.

•  Spread manure (where available) or residue from another crop (this works, but isn’t very practical).

•  If harvest is early enough, plant a small grain or other crop to establish cover.

What about the effect of too much residue on suppressing soil warm-up in the spring, and increasing the potential for disease such as scab and white mold? The key factor here is “too much.” Ground cover of 30% is not too much in terms of effect on soil temperature, and we generally leave 30% or more cover on wheat ground anyway with normal practices such as chisel plowing. 

The greater issue, particularly with potatoes, sugarbeets, and soybean ground, is tilling too much.  With potatoes and sugarbeets, you don’t end up with 30% residue cover even if you don’t till.  Soybean residue is fragile, so you will start with adequate residue cover (30% or more) but if you till aggressively, you will not maintain 30% or more cover. 

	Percent Residue Remaining
Implement	Non-fragile (corn)	Fragile (soybeans)
Moldboard Plow	0-10	0-5
Paraplow	80-90	75-85
V-Ripper/Subsoiler 12-14 inches deep, 20 inch spacings	70-90	60-80
Disk Subsoiler	30-50	10-20
Chisel Plow with Sweeps	70-85	50-60
Field Cultivator Sweeps or Shovels 6-12 inches	35-75	50-70

Therefore, I don’t have concerns with too much residue and cold soils or with increases in disease potential.  I believe we can manage residue to prevent erosion without increasing these concerns. 

We also can’t ignore the benefit of field stubble on soil moisture recharge, a particular concern in drier growing conditions.  The difference between leaving upright stubble and leaving no stubble made a difference of between 1.1 inches and 2.15 inches of extra moisture, according to North Dakota research. The studies averaged about 1.5 inches of extra moisture due to snow catch, which is influenced in part by surface roughness, but mostly by stubble height. Stubble left at 13 to 15 inches increased spring soil moisture by 1.45 inches compared to a 2 inch stubble height.

Soil erosion is not just a nuisance; it has serious effects wherever it is lost from and wherever it is deposited to.  It is also a very visible problem—many are watching to see how we manage it. We know a great deal about managing soil erosion, and many programs are in place to provide support and funding to those who want to take advantage of them. Generations to come are depending on us to do all we can to keep our soil where it belongs. Figuratively and literally speaking, our soil really is the root of production agriculture.