When something fits well, it’s much more likely to work.
And that's true whether you're wearing a bike helmet
to protect this resource
or growing a winter cover crop to protect and improve the soil.
My research journey with cover crops in organic vegetables began in 2001
And after thousands of hours in short and long- term experiments
I'm surprised to say that I've began to question
if cover crop mixtures of legumes and cereals
are well-suited for organic vegetable production.
Let me give you a little background on these mixtures
and then I'll explain why I question if they're a good fit for organic vegetable production in California.
The most common types of cover crop mixtures used in California have 90% legume seed and 10% cereal seed.
These mixtures have radically different root systems that can potentially complement each other.
The extensive fibrous roots of many cereals like rye
can quickly capture nitrogen left over from previous vegetable crops
and recycle it so that it doesn't leach into the ground water.
In contrast, roots of most legumes can develop nodules
In contrast, roots of most legumes can develop nodules were bacteria can capture nitrogen from the air
and convert it into forms that the plant needs.
We call this nitrogen fixation.
Theoretically these mixtures would fit well into rotations with organic vegetables
because I could potentially reduce the need for expensive organic fertilizers.
these fertilizers are commonly used in high-value organic vegetable production systems primarily to
add nitrogen to the soil and ensure high quality vegetable yields.
But here's the problem.
At typical seeding rates for cover crop mixtures
weeds grow extremely well and can
add huge amounts of weed seed to the soil.
We call weed seed in the soil
the weed seed bank.
It's a spooky thing
and unlike your regular bank
you want your weed seed bank to have a low or zero balance.
Because when this weed seed germinates during your next vegetable crop
it can increase weed management costs and therefore reduced profits.
This is a major problem in organic vegetable systems especially in regions like California
where many weed species and vegetables grow year round
and where hand labor for weeding is becoming more limited.
So how do you tackle this weed problem in cover crop mixture?
Well increasing the cover crop seeding rate to make the cover crop more dense or crowded
works really well because high density cover crops shade the soil quickly
which stunts the weeds and prevents them from producing all those unwanted seeds.
Over time this will reduce the size of the weed seed bank.
Unfortunately the seed costs for a high density, weed-suppressive legume-cereal mixtures
can be nearly 10 times higher than a high density, weed-suppressive pure cereal cover crop.
The mixtures are more expensive because they're dominated by relatively large seeded legumes that are more expensive than cereal seed.
Because of the large seeded legumes
the mixtures need to be planted at much higher
seeding rates than pure cereals to get an adequate cover crop density to suppress weeds.
So here's an important question.
Are the higher seed costs of these mixtures that are necessary to suppress weeds
worth the potential nitrogen benefits from the legumes?
For these benefits to occur the legumes need to thrive and produce large amounts of biomass
But my research over several years has shown that legumes often struggle to complete with cereals in mixtures.
An that even when legumes make 90% of the seed in a mixture,
legume biomass is often less than 25% of final cover crop biomass.
This has been the case with both legume-oat and legume-rye mixtures.
Several factors can make this situation even worse with the legumes.
Given this problem with legume survival and biomass production in mixtures
which in turn limits nitrogen fixation
rather than growing an expensive legume-cereal mixes it may be far more cost-effective
and environmentally sound to grow a pure cereal cover crop that's really good at recycling nutrients
and then purchase organic fertilizers to add nitrogen during the next vegetable crop.
But its important to mention that many common organic fertilizers are made from materials like manure
that have high amounts of phosphorous.
Often far more phosphorous than our crops need.
Recent information from long-term experiments in California
found that over fertilization with phosphorous can be a serious problem in some organic systems.
Ironically this brings us back to why it would be great if we could figure out how to efficiently integrate legume-cereal mixtures
into organic vegetable systems.
They could add nitrogen without adding other nutrients like phosphorous that we may not need to add.
My suggestion for cover crop management in organic vegetables is to take a long-term
systems approach that focused on frequent cover cropping and weed management.
Our long-term research with cover crops in organic vegetables
has shown that in terms of soil quality and vegetable yields
frequent cover cropping, every year if possible,
is far more important than whether a legume- cereal mixtures or non-legume cover crop is used.
I only suggest using legume-cereal mixes if they're consistent with this long-term approach to frequent cover cropping and weed management.
I hope this has helped you to understand the complexity of integrating legume-cereal cover crop mixtures with organic vegetable production.
There may be times and places where they're a good fit....
and others whether really not.
What you think?
Thanks for watching and stay tuned for more exciting sustainable ag research.