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The science behind intercropping

When plants can share nutrients, they can yield higher together than separately

Just as the adoption of no-till agriculture on the Prairies several decades ago was farmer-driven, the current shift to ecological (sometimes called regenerative) agricultural practices is no different, says Dr. Martin Entz of the University of Manitoba.

“In this fascinating carbon world, we’re now moving beyond just crop rotation and beyond just grazing management all the way to thinking about using spatial diversity such as intercrops,” said Entz during a presentation at an intercropping workshop in Brandon last fall.

Entz explained that crop rotation is diversity in time, meaning one year you grow soybeans, another year oats and so on. Intercropping is diversity in space. “If you think about how nature works, nature is always intercropping, there’s always more things on the land than just one thing,” said Entz. As an example, intercrops allow for plants with different root depths. “We do this in our pastures all the time. There is scientific evidence to show that some plants will actually lift water. They will not only use the water from down in the soil, but bring it up in the root and then deposit it into that upper layer of the soil where other plants can use it.”

There are many types of intercrops, like pasture mixtures, shelterbelts, cover crops and grain intercrops. These systems may help unlock yield potential as scientists and producers continue strive for more yield.

“Canola is a good example of where we have relatively low harvest indexes; there is lots of yield potential left there but with some other crops, there’s not,” said Entz. “When you have a monoculture, you risk disease because you create a lot of food for the organism that eats it, so the hypothesis now is that huge yields gains are going to come from mixtures.”

After years of experiments with intercropping, some researchers have found that the land equivalent ratio (LER) is greater with crops grown together than with crops grown individually. The LER is found by comparing yields of an intercrop by comparing monocrop yields on similar-sized land parcels. That is, does a 100-acre field of intercropped peas and canola yield more than a 50-acre field of peas and a 50-acre field of canola? If the intercrop yield is bigger than the monocrop yield, the LER is greater than one. “Canola is a very good intercrop partner, we get a lot of results where we’re harvesting more yield per unit area of land in a mixed crop environment,” said Entz.

Peas and canola is a common intercrop and has been reported to consistently over-yield by up to 21 per cent compared to either crop alone. Farmers in southern Saskatchewan have reported 120 to 130 per cent of the yields of monocrops when growing intercrops.

In-crop assistance

A lot of early intercropping research came from Scandinavia. In the 1980s Scandinavian work with barley and legumes showed that the grass roots tend to be bigger than the legume roots. “In this case, the growth rate of the barley root was about two to four times higher in the grass than the legume root,” said Entz. “That makes sense because the legume doesn’t need to build a big root system to capture nitrogen because it gets it biologically with nodules. The grass, on the other hand, has no choice but to build a big root system to go get that nitrogen.”

The same is seen in many involuntary intercrops such as canola/soybeans. Entz’s colleague, Dr. Rob Gulden has looked at these root systems and found that canola roots grow deeper to scavenge more nitrogen, while soybean roots don’t need to grow so deep. Gulden has also found that the canola and soybean roots intermingle, allowing opportunities for a synergistic relationship. “All plants exude sugars and proteins out of their roots into the rhizosphere, the area right around the root, so there is opportunity for one plant to take up what the other plant is giving.”

Not only are the roots intermingling, they are getting some help through something called the common mycorrhizal network (CMN), says Entz. Mycorrhizal fungi live in the soil as spores. When a plant sends a seed out, the spores will germinate. The fungi send a thread and infect that plant to get carbon from the plant. Then the fungi will bring nutrients to the plant — it’s a symbiotic relationship.

Entz cited a study that showed that in an intercrop of flax and sorghum, the sorghum increased flax growth by 46 per cent by shifting nutrients from its root system to the flax roots via the mycorrhizal network, costing the sorghum only seven per cent of its yield.

Why do plants help each other? It’s risk management, said Entz. “It’s a form of insurance. They want carbon to survive, so why not tap into two income streams as opposed to one,” he said.

Plants that are very mycorrhizal are corn, flax, sunflower, pulses, potatoes and all forage legumes. Plants that are mildly mycorrhizal are oats and barley. Canola and all the brassicas are not mycorrhizal. “That doesn’t make them bad, it just makes them different, and you can actually exploit that difference,” said Entz.

Intercrops in practice

How can producers use this knowledge to increase yields, improve soil and make more money?

Producers could use these root dynamics to reduce nitrogen needs. The best data in this area comes from pasture systems, showing plants shifting nutrients back and forth. In grain legumes, the nitrogen transfer from pea to barley is not usually detected in the field, but it happens. Chinese intercropping research has shown that intercropped legumes contribute about 15 per cent of nitrogen to a cereal.

Phosphorus in the soil is in different pools and only about half of it is in a plant-available form. Plant roots put a lot of compounds like enzymes and acids into the soil to solubilise phosphorus and make it available to plants. Canola and other brassicas are particularly good at this. An intercrop with the right choice of plant species can actually tap into different pools of phosphorus in the soil and make better use of it.

“For example, if you had an intercrop with chickpea that could tap into the organic phosphorus and you had another plant that could tap into the inorganic phosphorus, which is either your fertilizer or your phosphorus attached to the calcium, which is bound, then it’s easy to understand why this whole system might over-yield, why you might get an LER greater than one,” said Entz.

Soil health plays a big part, adds Entz. “The microbial biomass carbon is one of the best measures of the health of your soil,” he said. “Communities of bacteria that are involved in mixed species community increase production by 16 per cent versus bacteria from monoculture. If you have a mixture of plants in your system, you’re building up a healthier soil biological community and this is why sometimes weeds are your friend. They are sometimes the diversity in your system.”

The acidity of roots can also reduce disease such as root rots in cereals. For example, fababean produces a lot of acid, soybeans a moderate amount of acid and corn doesn’t produce any acid. So they can be complimentary. “Research in China showed fababean helped increase phosphorus by 12 to 56 per cent in maize when intercropped,” said Entz.

Plant conversations

Intercrops give above-ground benefits too. They’re collecting more energy from the sun as they fill in more spaces in the field. That can produce more plant biomass and yield.

And plants talk to each other. “There are a lot of volatile chemicals that are given off by plants that other plants react to,” said Entz. When German researchers clipped a sage plant it produced a chemical which caused a wild tobacco plant being grown near it to turn on its self-defense mechanisms.

Experiments at Clearwater, Man., in 2004 and 2005 demonstrated that intercrops not only over yield compared to growing just monocrops, but also help to reduce weed pressure, prevent disease and reduce fungicide use.

When wheat was seeded at half rate with other intercrops, weed numbers dropped and leaf disease in the wheat reduced to 30 per cent compared to 70 per cent on a wheat monocrop seeded at full rate. “If we’re going to try and increase our yields just by increasing our seeding rate, increasing the biomass of that one plant in that one field, we’re going to have diseases and we’re never going to get on top of that,” said Entz.

Entz has studied intercropping all over the world and believes that the way to food security is through intercrops and ecological farming systems.

He shared the story of a nitrogen-fixing tree called Faidherbia albida that grows only in certain parts of Africa. “It’s called a reverse phenology tree, so when the rainy season begins and people plant their maize, it’s roots are full of nitrogen,” said Entz. “During the growing season, once the crop is established, it starts to leaf out a bit and then when it’s really hot and dry and you’re trying to mature your crops, it leafs out and reduces the temperature on your field. You can modify the temperature of the field by the density of these trees. With all the technology for climate change, adaptation, here’s a natural solution.”

About the author

Contributor

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at http://alovell.ca or follow her on Twitter @angelalovell10.

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