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Here's how easy it can be to screw up your soil

Updated: Apr 6, 2020

Today we have an informal experiment that shows just how easily things can go wrong.

We recently spent some time in the greenhouse, propagating landscaping plants to help with an upcoming project. In a small bunch of African daisies, just to see what would happen, we applied a dry organic fertilizer to the soil surface of a single plant. For all the others, we drenched the soil as usual, with a water microbial solution.

In the picture above, you probably noticed the fluffy white fungal growth in the soil of that one clone. Yup, that’s the one with the fertilizer.

The fungus appears to be a saprotrophic type (it feeds on dead organic matter) commonly found on soil surfaces. There are plenty of beneficial saprotrophic fungi in healthy soil, but this is not one of them. It’s an anaerobic type, meaning it thrives in reduced-oxygen conditions. For plants and humans, nature’s anaerobes are at best indirectly harmful, at worst downright pathogenic.

You may also notice that the fertilized plant appears larger than the others. The main idea here was to compare growth rates between fertilized and microbe-inoculated plants, and the fertilized plant did exhibit a faster growth spurt than the others.

But that doesn’t mean it was healthier. Quite the contrary, it shows several wilted and misshapen leaves, and its colors and textures appear less vibrant than those of the other plants.

Conditions in this propagation greenhouse were relatively humid and warm, and the soil may have been kept a tad too wet. Which means conditions were ripe for promoting this kind of fungus, with high moisture and warm air combining to throttle down oxygen flow to the soils around these plants’ roots.

So what’s the connection between fertilizer and mold? The process that created this growth was triggered by the sudden rush of nutrients from the fertilizer. That rush was all it took to tip near-anaerobic soil conditions over the edge.

The important point to remember here is that it’s not only plants that use water-soluble plant nutrients. Soil microbes, most readily bacteria, can also consume them – and they do so quickly, because the water-solubility means those nutrients can be digested much more easily than nutrients that must first be separated from organic and mineral matter.

The soil we used here was a typical market product, not containing much biologically, although certainly some bacteria. Those bacteria grew in number as they feasted, and soon began using up more of the oxygen available among soil particles. Once oxygen concentration dropped enough (below around 6 parts per million, depending on measuring method), dormant anaerobic microbes came alive. In unhealthy soil, it’s easy for a single species to exhibit runaway growth when conditions are right and natural competitors are absent. And that’s what happened here, in our fertilized nursery pot.

We often perform these mini-experiments as a means of orienteering. The practice of science doesn’t always require collecting data. Part of the process involves trying small things, just to see what happens. It helps maintain one’s perspective and focus among all the variables inherent to a natural system.

Stay curious out there.

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