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In Pursuit of Humongous Pumpkins

A grower recently asked me which of our mycorrhizal inoculants contained the best type of spores for his giant pumpkin plants, and gave me a listing of what he was already putting in the soil. It seems like every year he has been experimenting with different additives to try to boost the final weight of his contest entries and his list had more than a dozen organic-nutrient items, kelp to molassas.

Couple points here: First, I could make some educated guesses about which mycorrhizal fungi would be the most likely to colonize the root system of his plants (Glomus intraradices is sort of a “generalist” average-performing species that can be used in nearly any situation); or I could guess which type would have the most powerful effect (Paraglomus brasilianum can make grape vines grow and mature superfast) but neither I nor anyone else can say for sure which would be the one “best” type for a specific plant/soil/climate situation. That’s why we market a multi-species product and let the spores sort themselves out, with the best-adapted types becoming dominant.

You’ll notice I said “types.” In nature it is common for multiple types of mycorrhizal fungi to be present on a root system, for unknown reasons, (but certainly there are reasons). Perhaps they have evolved some sort of division of labor, with one type being especially adept at foraging for phosphorus and another for moisture and a third for interacting with nitrogen-fixing bacteria, etc. This is all remaining to be researched some day. For now, the height of inoculant sophistication is to blend together several different types of spores, and this is something to look at on an inoculants label – which species, how many, and what are the spore counts?

The second main point is that by using so many additives the pumpkin grower was setting the stage for the mycorrhizal fungi to do very good things for his plants. In somewhat simplified terms, the beneficial fungi respond to the needs of their host plants and seek out deficient elements from the surrounding soil, or can even transfer them from other nearby plants. Any missing thing that causes stress for the fungi’s host plant, or keeps the plant from performing to its full genetic potential, becomes a foraging target. This “searching & regulating uptake” function is the result of millions of years of adaptation to a mutual-benefit relationship between plants and fungi.

I like this pumpkin grower’s odds of winning contests. He has done very well in the past, and by loading the soil with a wide variety of organic nutrients that gradually and continuously release nutrients as they are acted on by soil organisms, the mycorrhizal fungi should be able to find anything and everything that the pumpkin plant needs for exceptional vigor. If any essential minor or trace element is absent, the performance would have to be slightly lower.

Do you really need to add so many different kinds of nutrients and minerals to soil? Of course not, unless you grew an 800 lb. squash last year and really, really, want a 900 lb. one this year. Sounds like fun to me, and the general strategy of this grower – use the widest-possible mix of nutrients (small amounts of many things) plus a multi-specie mycorrhizal inoculant – are worth trying.

Cheers, and good growing, my friends,

Don Chapman

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