Getting to the Root of the Matter

When we talk about crops and crop production, our discussions seem to gravitate towards the plant parts that we can see – the part that is above ground. However, at least half--and often much more--of every crop plant is invisible. This portion consists entirely or largely of roots which extend far into the soil profile.

Cultivated plants obtain their entire water supply from the soil. Thus, the root system, as well as the amount of water within the rooting zone, ultimately dictates the yield and quality of the crop.

Over the past week, many farms in the area have received excessive rainfall. Rain seldom seems to come at a convenient time or in proper amounts. Although most area crops are planted, many farmers are trying to get their crops sprayed while others are trying to make hay. Although it’s easy to get frustrated with these excessive rainfall events, it’s a necessary part of crop production.

Last fall, and early this spring, our soil water reserves were at dangerously low levels. Many farmers reported powdery dry clay extending five feet into the soil profile. With this degree of deficit, most soils need an additional eight to ten inches of rain just to bring the soil profile back to field capacity (the amount the soil can hold once excess water has drained away). Without this water getting replenished, crops will likely suffer later this summer when dryer, warmer conditions are likely to prevail.

Although the top five or six feet may seem deeper than necessary, most crops we grow use that amount for root development. Furthermore, most rainfall events are not totally effective in replenishing the soil; a growing crop will intercept only a portion of the rainfall. The balance is lost to run-off or evaporation, the actual amount that replenishes the rooting profile depends on above ground vegetation, terrain and the soil type.

In East Polk County, the three predominant crops are soybean, wheat and forage crops with approximately 80,000, 60,000 and 42,000 acres of each, respectively.

With recent excessive rainfall, the greatest water absorption, and thus replenishing soil water reserves, is dictated by these crops -- but in the reverse order. Because of the above ground vegetation and root development, forages will capture most of the rainfall; wheat would be next, while soybean would have the least capture, and thus, the greatest runoff.
Wheat is the most commonly grown cereal grain in our area. Both spring wheat and winter wheat have a fibrous root system which penetrates deeply into the subsoil. Winter wheat, perhaps because of the longer season for growth, tends to be more extensive and tends to capture more moisture simply because the groundcover is in place to winter snow and frequent spring rainfalls which minimize run-off.

After wheat seed germination, the primary root takes the lead, but very, soon, two other roots appear on opposite sides of the first. To this whorl of three, still others may be added, and together they constitute the primary root system (Figure 1). In some cases, there may be as many as eight roots. Under good growing conditions and lack of soil compaction, the root structure of wheat will often extend to five feet!

Alfalfa is a perennial that plays a critical, but often under appreciated, role in our crop rotation. Alfalfa stands tend to be the most productive for the first three or four years, though many stands often exceed 5 to 7 years. The effective productive life is dependent both on the variety and on the environmental conditions.  Alfalfa is deeply rooted and is just one reason it has such value in a crop rotation.

During the seedling year, alfalfa gives rise to a single taproot which takes a vertically downward course with considerable variation in the number of side roots. In fact, it’s not uncommon for mature alfalfa roots to extend beyond ten feet! This is the reason alfalfa is able to withstand long periods of dry weather; it’s has a tremendous root system in which to draw water from deep within the soil profile.

But, this is only half of the story. The macropores created by the large taproots also serve as a conduit for internal soil water movement for the following crops. Moreover, the extensive root system significantly improves the all important soil aeration as well as transporting deeply held nutrients closer to the soil surface when the alfalfa is plowed under (Figure 2).

The benefits of deep rooted legumes in your crop rotation extend well beyond its high value forage and deep into your soil. It’s the underappreciated, extensive root system that likely explains the excellent development of other crops following alfalfa, sweet clover, or red clover. The roots penetrate deeply, providing not only increased nitrogen supply from nitrogen fixation, but also better aeration, which is especially important on clay subsoils. Since aeration is a not a crop input being sold, we hear little about it, but aeration may be as important as many of the other crop inputs.

So, keep in mind, that into each life a little rain must fall. Without replenishing our soil moisture reserves, the root systems could not provide the moisture and nutrients necessary to harvest a bountiful crop. As you look at your crop rotation, look not only at what’s happening above ground, but also what’s happening beneath the soil surface.

Reference: Root Development of Field Crops, by John Weaver. http://www.soilandhealth.org/01aglibrary/010139fieldcroproots/010139toc.html

Jim Stordahl, Clearwater/Polk County
U of Minnesota Extension