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Getting to the root of the matter

In the third of a three-part series, Les Henry looks at roots of field and garden crops

This is the final of a three-part series. In part 1 (April 11, 2017) I talked about the folks that provided very detailed diagrams of many plant roots to the depth needed to get the complete picture. Part 2 (April 25, 2017) was perennial pasture and hay crops and weeds and part 3 is field and garden crops.

Wheat

Wheat was the crop that built our farming base and is still an important crop. Many still think that wheat roots end at a foot or two. Not so —more like five feet (See figure 1 below).

Wheat (figure 1)
Wheat at flowering with roots to five feet.
photo: From: Weaver & Bruner

Weaver went on to explain that the root system at harvest showed no great change from that at flowering. In my many years of soil probing, I’ve found that a wheat crop is done extracting soil water by late July when flowering is complete. From then on it is a matter of redistributing what is in the plant to the seed in the head. Many times we hear about a late rain being good for “crop filling” but late in the season, wheat’s water use is from depth, not the soil surface.

Winter wheat was found to have a well-developed root system to a depth of 3.5 feet when 55 days old so it goes in to winter all set to “suck from the deep” when spring growth starts.

Barley

At the University of Saskatchewan, Pavlychenko compared wild oats to barley (see figure 2 below) and wheat. He made detailed measurements of root type and length at five, 22, 40 and 80 days after emergence in the field. At 22 days, when they are duking it out for water and nutrients, barley had 3.7 times and wheat 1.6 times the seminal (primary) root length of wild oats.

That is why barley can manage to come through with a crop with wild oat competition but wheat not so much. With current cropping and herbicide availability, wild oats have been all but eliminated on many farm fields but vigilance is needed to keep them at bay.

Barley (figure 2)
Barley (left) and wild oats. The wild oat roots are marked with 1, 2 and3–22days after emergence.
photo: From: Pavlychenko, T. K. 1937. Ecology 18: 62-79

Canola, lentils, peas

Much of the income from many farms comes from canola, lentils and peas but no root diagrams yet exist for those crops. With modern backhoes, trackhoes etc., it would be much easier to do the excavation now than it was decades ago but it has not been done. It should be.

The Agriculture and Agri-Food Canada Swift Current station has recently conducted field experiments comparing root habits of wheat to oilseed (canola, mustard, flax) and pulse crops (pea, lentil, chickpea). The crops were planted in cylinders with diameter of 15 cm, one metre in length. The cylinders were removed at four plant growth stages from seedling to maturity.

All pulse crops had much less density (mat) of roots than wheat but pulse root diameter was larger. Canola matched wheat for density with mustard less and flax much less. Root diameter of wheat and oilseeds was similar.

The restricted nature of the cylinders does not allow for a full evaluation of what the plants will do in a field crop situation.

Vegetable crops

The document for information on these crops is: Weaver, J. E and Bruner, W. E. 1927. Root development of vegetable crops. McGraw Hill Book Co. All information here comes from that source.

I have a 15-year old plantation of 1,000 crowns of Jersey Knight hybrid asparagus that is just now in its prime so I have a special interest in this crop. It’s rooted deep, and the horizontal roots of at least eight feet are the reason for the longevity.

Asparagus (figure 3)
Six-year old asparagus (not all roots shown). Depth is 11 feet; horizontal spread is more than eight feet.
photo: From: Weaver & Bruner

Weaver’s book also shows a diagram to let us know that our good old friend rhubarb has rooting habits much like asparagus. No wonder farmyards that disappeared many years ago show a few rhubarb sprigs as the last evidence of habitation. The farm I was raised on was bulldozed in mid 1970s and sprigs of rhubarb were still there in 2008 when we erected the cairn. Imagine all those years of summerfallow and all those herbicides — and it still motored on because of the huge and deep root system.

It was a great surprise to me to find that tomatoes root to at least four feet in just two months. Carrots by harvest root to seven feet, beets to 10 feet and good old horseradish to 14 feet. The biggest surprise was the width to which the roots extend. A total width of six to eight feet is not uncommon.

In my farm garden I use at least six feet between rows mainly to facilitate passage between rows with the rototiller. No wonder such good crops grow with so little water. I had no idea that roots could spread that wide. It also means that I should keep inter row cultivation depth to a minimum.

Summary

To summarize what we have found out about roots:

  1. Plant roots in general extend to much greater depths than generally believed. For the annual crops we grow, four to five feet is common and for perennial crops like alfalfa, grasses, vegetable crops and weeds, 10 feet or greater is common.
  2. Many crop plants extend roots laterally to several feet. That means that row spacing is more a matter of weed control than access by the crop to moisture and nutrients.
  3. Rooting potential is determined by the genetics of the plant but environmental controls modify rooting greatly. Moisture is the big one. Plants do not root through dry soil so if we have only a foot of moist soil that is the potential rooting depth until rain or irrigation happens.
  4. As research on crop roots gears up, it must start with a thorough review of the literature and an understanding of root architecture and responses to environmental variables. More than greenhouse studies of roots in cages will be needed to enhance future field crop production.

About the author

Columnist

J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.

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