Corn roots (Credit Blendspace.com)

I’ve long been fascinated by corn yield and did my Masters and PhD research on this subject. The physiology of corn yield was a prime research interest during my former academic career in Crop Science at the University of Guelph.

Most of the research on yield has featured above-ground plant parts – eg. higher and sustained rates of leaf photosynthesis, canopy morphology, timing of pollen shed and silk emergence, and higher harvest index (portion of above-ground dry matter in the grain at harvest). All of these are highly important. Improvements achieved both genetically and agronomically have meant that 300 bushels/acre (19t/ha) is now a realistic goal for some farmers, and 450 bu/acre (28t/ha) has been exceeded.

During my tenure in Crop Science, I became increasingly interested in the role of corn roots and their interactions with soil. This was triggered, in part, by another personal research interest, soil tillage – and part by some anecdotal observations and quasi-research findings which were difficult/impossible to explain by conventional crop physiology.

My career in research ended when I became full-time chief of staff for the Ontario Corn Producers’ Association in 1985. I did return briefly to the University of Guelph as adjunct professor in 2002, hoping I could pursue my interest in corn roots. But the system had changed: there were new (justifiable) charges for the use of almost anything – greenhouse bench space or whatever – and I had no enthusiasm for resuming the ‘chase for research dollars’ which I had left years before. So I did a lot of reading (actually relatively little literature on corn roots), and ultimately returned to more non-academic activities including increased attention to our family farm business.

It’s obvious that further research on corn roots is no longer in my future, but the fascination remains. I’ll spell out below why and how I think roots are so critical to superior yields. Maybe others will pick up the challenge.

Case 1. During our initial 24 years of farming, my wife and I harvested corn with an old-style picker, carefully screening out loose corn kernels as the ears went up a conveyor elevator into the corn crib. The loose kernels fell onto the ground a few metres from the crib, year-by-year adding to the soil organic matter in a band parallelling the crib.

Eventually I noticed that the corn grown in this strip grew and developed much more quickly than did nearby plants. It silked about a week earlier, was much taller and yielded more. Initially, I assumed this was somehow a wind-barrier/warm-temperature effect. But one year the crib was not filled and the effect continued. It was not caused by warmer soil temperature since the soil in the favoured strip was wetter and likely colder. And it was not caused by higher fertility or better weed control. The only obvious explanation: higher soil organic matter meant a significantly higher rate of both growth and development in independent of temperature. We have not cribbed corn now for 18 years and the effect has largely disappeared. I suppose much of that organic matter has been oxidized, or spread elsewhere by tillage implements.

Case 2. Nigel Fairey was my PhD student from 1972 to 1976 and his research project involved studying C¹⁴ labelled assimilate movement within developing corn plants. He grew corn plants outdoors but seeded in buried 22-litre perforated containers containing a granular baked-clay medium called ‘Turface,’ plus nutrient culture. Nigel also planted corn in the soil around the pots so that the treated plants would behave as if in a normal corn stand. One of his problems was that the corn plants in Turface grew and developed much more quickly than their neighbours. They silked 10 days earlier. The reason was unknown. Nigel checked and it was not differences in root-zone temperature. He solved the problem in year two by planting the bordering plants two weeks ahead of those in the pails. He got the data he needed, graduated and moved on to a productive career. But the puzzle of the accelerated growth remained.

Corn roots (Credit: Dr. Amélie Gaudin, Plant Agriculture, University of Guelph

Case 3. In the late 1970s, I filled a large growth-room bench with corn grown in perforated pails containing Turface and regularly replenished nutrient solution. The plant density was high, about 10/m² (equivalent to 100,000/ha) as I recall, and I recorded a yield (border plants near the bench edge not harvested) of nearly 200 bushels/acre. This was despite the fact the amount of daily visible radiation to which those plants were exposed was only about 20% of what field-grown plants would receive outdoors on a clear sunny day in mid July. This was back in an era when we struggled to grow 100 bu/acre on the Daynard farm.

Case 4. In the mid 1980s, Dr. Madhava Reddy, a post-doctorate, grew corn plants indoors, suspended in pails containing aerated nutrient solution (no Turface), with half of them being subjected to a weekly regime of partial root tip removal using fingernail clippers. This was only a preliminary trial but the results showed a much slower rate of foliar growth and development for the tip-clipped plants despite the ample supply of nutrients, water and nutrients. The roots were obviously ‘annoyed,’ and this affected foliar growth and development in unknown ways. Both Madhava and I left U Guelph soon after so the work was not continued.

Case 5 is more generic and involves the generally better rates of growth and development, and grain yield I and others see with corn grown in well-manured fields. Our neighbours, the Dupasquiers, grew great crops when they had dairy cattle manure to spread. Now with chicken manure, the crops are simply superb. I am also intrigued at how many high yield reports elsewhere are associated with ample applications of manure, especially poultry manure.

So what’s going on here? Higher soil organic matter (manure addition, kernels dropping beside our corn crib) obviously has advantages in increasing available water storage and that will increase yield in most years. Nutrients from manure are also important, though that should not have been a yield factor in the cases listed above. The boost came using the biologically inert Turface as well as organic matter. The effect on rate of development is of special interest to me as I had long believed that heat accumulation to be the primary driver of rate of development (as compared to rate of growth – plant height, for example). But in some of the cases described above, above-ground rate plant development was affected by the root environment via some mechanism apparently independent of temperature – and sometimes in a dramatic way.

Plant physiologists know that stressed roots send chemical/hormonal signals to above ground parts which cause various physiological responses including reduced rates of growth. Abscissic acid is a key hormonal messenger of this type and there are others.

But are there positive signals which roots send upward when they are essentially stress free? Signals which say the biological equivalent of “put the pedal to the metal”? Signals which will tell the plant to grow and develop more quickly, set more kernels, even more ears? And maybe which stimulate increases in rates of photosynthesis?

In the months I spent as an adjunct professor, I spent much of my time in the library and on line, looking for published evidence of a “good times” hormonal signal – sadly, without success. Perhaps that’s because it’s hard to find, or perhaps no one has looked. Or perhaps the positive signal is simply the absence of stress-signal messengers.

If it’s the latter, perhaps the “brakes” are almost always on, at least in part, for real-world corn plants. That’s a good thing for plants to survive in the normal world. But it’s not so good if the goal is record high yields.

I recall an experiment performed by the late Bev Kay in Land Resource Science at Guelph, who grew young corn plants in stable soil aggregates (from a field growing red clover) of various size categories. Plants grew more quickly in soil consisting of fine aggregates (about 0.5 to 1 mm in diameter, as I recall) than with larger aggregates, and this was an effect independent of fertility or moisture supply.

In checking the roots, Bev and his team found something intriguing: Corn roots in the finely aggregated soil grew straight. Those in larger-sized aggregates were wiggly. With smaller aggregates, the soil appeared to move out of the way as the roots lengthened. With larger aggregates the roots had to grow around.

And that’s when, and why, I developed a (now long-held) hypothesis: Plant roots like to grow unimpeded without needing to turn or grow around anything. Perhaps each turn or impediment triggers a negative signal. “We’re not sure what we’re encountering here,” say the root tips to the rest of the plant, “but better you’d slow up a bit – just in case it is bad or gets worse.”

Farmers all know that compaction hurts yield. The most obvious means is by restricting root growth and access into areas of the soil containing needed water or nutrients, or by impeding water and air flow. But perhaps it also reduces yield just by impeding root growth itself, even if water, air, and nutrient supplies are fine.

How much it yields depends so much on the roots

My wife is an excellent gardener, outdoors and in, and she regularly has to repot plants into larger pots or else they won’t grow and flower as they should. These plants are well watered and well fertilized, but still they do poorly when “root bound.” And what does root bound mean? Roots which are constantly running into each other or the container walls, and can only grow by turning – growing around things.

Or consider Bonsai plant cultivation with trees permanently dwarfed by restricted root growth.

Ontario farmer Dean Glenney at Dunnville Ontario has produced some really high corn yields and he plants his corn and soybeans in the same row positions year after year. He says that roots can grow easily down existing root channels. This fits my conjecture well. I’ve not met Dean personally, but understand that he also uses lots of poultry manure. The manure will also be great for reducing soil density and strength, making it easier for roots to grow unimpeded.

Old root channels may be one of the biggest benefits for cover crops, i.e., in addition to drainage and aeration benefits – provided, of course, that these channels aren’t destroyed by subsequent tillage and heavy equipment tramping on wet soils.

My hypothesis is that unimpeded root growth is highly important for top corn performance – independent of any effect on providing soil moisture or nutrients. (Not that the latter aren’t also highly important.)

It’s an untested hypothesis. I’ve spent hours thinking about ways to test this. Dr. Amélie Gaudin completed a PhD thesis recently in Plant Agriculture, University of Guelph, growing corn plants aerobically with roots dangling in a mist of nutrient solution. Her study was about genetic differences between corn and ancestral teosinte, but maybe the same system could be used to study the effects of impediments to root growth independent of water, nutrient and air supply. The challenge would be to introduce barriers/impediments in such a way that they force roots to bend around obstacles but don’t lead to stressful oxygen deprivation. Maybe there are other creative ways to test my conjecture.

To close: I’ve no proof – nothing would stand up to good peer reviewing – only a collection of anecdotal and quasi-research results pointing in a certain direction. But if you really want to be the farmer who breaks above 500-bu/acre corn yield – or even 50 bu/acre better than what you’re doing today, figure out how to encourage unimpeded root growth. And if you can sort out how to do this in a cost-effective manner (not everyone has huge poultry manure supplies, and Turface is way too expensive), you could become a very wealthy person.