It has been observed that a mixture of plants often performs better than a monoculture of the best performing plant in the mix, an observation that defies “common sense”. Dr. Norman Gentsch from the Institute of Soil Science at Leibniz University Hannover says, “In biodiverse mixes, specific species or varieties which are adapted to specific stress conditions such as drought or specific pathogens act as buffers, thereby reducing losses among the less well-adapted plants. This ensures yield stability. The more diverse the mix, the more stress-resistant is the cropping system.”
The secret to making this work lies in microbes. Soil microbes thrive under many of the same conditions as rumen microbes, so feed them like a cow: a balanced and diverse diet on a steady basis. We used to think the microbes lived by eating crop residue, but now realize that in a healthy soil, the majority of microbes live by feeding on the nutrient rich exudates from plant roots. Scientists have found that plants leak as much as 40-50% of their energy out of the roots and into the surrounding soil to feed the soil biology.
Each plant species has a unique chemical composition to its exudate. Warm-season grasses have high sugar and high energy exudates. Legumes have exudates very high in protein building amino acids. Buckwheat and lupine produce organic acids that make phosphorus more available in the soil. A diverse mixture of plants (cover crops) growing produces a balanced diet of sugar, energy, proteins, and nutrients and the microbial population increases dramatically.
Dr. Gentsch agrees and points out, “Root biomass increases as the diversity of the cover crops increases. This is because the different species can tap into different root depths, which enables them to make the most of nutrient absorption and storage. As a result of this, mineral levels in the cover crop residues increase, thereby increasing the availability of the essential nutrients nitrogen, phosphorus, potassium and magnesium to the follow-on crops. Diverse cover crop mixes also have a higher leaf area index (LAI), which boosts the rate of photosynthesis. The products of photosynthesis such as sugar are transported more rapidly to the rhizosphere, where they promote microbial biomass growth. This boosts microbial activity and is of great benefit to mycorrhizal fungi. Bacteria in the nitrogen cycle in particular benefit from the energy-rich cover crop residues.”
At this point, you may be asking, “Why should I care about microbes? I am trying to raise crops and I want grain yield, not microbes.” Have you ever wondered why plants donate such a large percentage of its hard-earned energy to soil microbes – isn’t that foolish? Perhaps it is not so foolish after all as science shows us that plants that give off high levels of root exudates tend to be more successful than plants that do not. Just as in human interaction, generosity tends to be rewarded. Plants surrounded by healthy, abundant microbial communities are more drought tolerant, are better supplied with plant nutrients, and more resistant to disease; in addition, all this microbial activity increases soil organic matter and improves soil structure.
This is one of the reasons we try to create diverse cover crop mixtures containing several plant families, instead of just picking the highest yielding or the “best one”. Plant diversity also provides different root types for better use of soil resources, a layered canopy for better capture of sunlight, better livestock nutrition for grazing, and far lower risk of any one insect or disease taking out the stand. While there may be some situations in which a monoculture cover crop is the best choice, in general you get far more long-term soil benefit by choosing to plant a diverse mixture of several plant families for your cover crop.