Mycorhizzae (“fungus root”) are fungi that have a symbiotic relationship with plants, growing into or surrounding a plant’s roots. Like almost all fungi, mycorhizzae consist of branching filaments called hyphae that extend much further into the soil than a plant’s roots can, allowing them to gather water and nutrients otherwise unavailable to the plant (nitrogen, phosphorous, potassium – the N, P, K on fertilizer formulas). The fungi then deliver these essential substances to the plant in exchange for sugars produced by photosynthesis.
- Plants attract mycorhizzae by secreting signaling molecules from their roots that trigger the growth of the hyphae to the roots.
- Mycorhizzae are obligate symbionts – they cannot grow without a host plant; if plants are not attracting them, they will die off (some spores remain in the soil).
- Synthetic fertilizers provide fast-release nutrients to plants. The plants have what they need for the moment so they don’t secrete molecules to attract mycorhizzae. The mycorhizzae die off.
- Synthetic fertilizers provide fast-release nutrients to plants. The plants have what they need for the moment so they don’t secrete molecules to attract mycorhizzae. The mycorhizzae die off.
- The result is plants that are dependent on continued fertilizing and require more water than when mycorhizzae are present.
Plants were grown indoors under grow lights set to a 10-hour photoperiod. They were watered twice a week. The experiment lasted a little over 5 weeks.
Plant growth/appearance was monitored, roots were analyzed for mycorhizzal structures, and soil fungi were cultured.
NOTE: Due to space limitations, the sample size was fairly small – 10 control plants, 10 with synthetic fertilizer, 10 with organic fertilizer. However, there was consistency in results within each treatment.
Synthetic fertilizer (MiracleGro) produced faster plant growth than organic fertilizer (MicroLife). This is what was expected since MiracleGro, like most synthetics, is a fast release product. All organics are slow release. The control and organically fertilized plants were faster to flower (as has been demonstrated by research at TAMU). Therefore, fruit would have formed earlier than in the synthetic group.
The synthetically fertilized plants became infested with spider mites during the last couple of weeks of the experiment while the organically fertilized plants and the control remained healthy. We transferred some mites to the leaf of an organically fertilized plant. They infected that leaf but did not spread. This supported what published research has found – that the mycorhizzal association results in hardier plants that are more resistant to parasites/pathogens. The mechanism of resistance involves some fairly complex plant physiology which was beyond the scope of the project.
The plants treated with synthetic fertilizer became more dehydrated than either the control or the organically fertilized plants. This was assumed to be the result of the spider mite infestation. These plants died the week the experiment ended. It was notable that when the experiment concluded and plants were no longer being watered, the organically fertilized plants lived almost 2 weeks more without watering and looked fine when we pulled them up due to the semester ending.
After 3 ½ weeks, 2 plants of each group were sacrificed and their roots sliced and stained to reveal mycorhizzal structures. We were looking for arbusculae and vesicles.
Microscopic examination of the roots revealed significant mycorhizzal infiltration of the roots of organically fertilized plants. As expected, the synthetically fertilized plants showed no mycorhizzal growth. There was also some mycorhizzal infection of the control plants. This was not surprising as fungal spores are practically everywhere and will germinate when stimulated by “needy” plants. NOTE: No arbusculae were identified, possibly due to inexperience with the staining technique. Arbusculae are also short-lived compared to the vesicles and therefore less likely to be seen. They form, exchange materials with the plants, then deteriorate. Vesicles store food for the fungi and are therefore in existence for much longer.
NOTE: These are 3 slides out of many, all of which showed consistent results
Control: Vesicles (blue spherical structures) in a few cells
Synthetic: No vesicles
Control: Vesicles in multiple cells
NOTE: The fungi shown are NOT mycorhizzae. Mycorhizzal fungi cannot be cultured as they are obligate symbionts and will only grow in association with roots. We were simply curious about any differences in soil fungi other than mycorhizzae. This was just a side study as it is also extremely difficult to identify soil fungal species. Research groups rely on DNA testing.
Interpretation of these results is pure speculation. We did notice that there was less diversity of colony types with the organically treated soil than with the control or synthetically treated soil.
Published research indicates that organically treated soil promotes the growth of beneficial decomposers by providing food for them. This allows them to out-compete other fungi, including plant pathogens. We could hypothesize that the organic plates are dominated by beneficial fungi. That would make sense considering that MicroLife contains a lot of food sources for decomposers. However, we absolutely cannot say whether the fungi on any of the plates are beneficial or pathogenic.
- Fertilizing with organic fertilizer resulted in the rapid establishment of endomycorhizzae within plants roots.
- Fertilizing with organic fertilizer resulted in healthier, more pest-resistant plants than plants fertilized with synthetic fertilizers.
Here is additional information on organic vs. synthetic fertilizer, as well as about mycorhizzae.
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- It is better to use NO fertilizer than to use synthetic products. There are fungal spores everywhere so mycorhizzae will grow if plants attract them (which they won’t when synthetics are applied). Organic fertilizers contain spores of mycorhizzal fungi as well as compost-like food that they can break down as a source of nutrients that are shared with plants. Therefore, more mycorhizzae develop much faster than in untreated soil. Once a healthy mycorhizzal population is established, compost or organic fertilizers should still be used seasonally to provide food/nutrients to the soil ecosystem. At this point, it is not critical that the fertilizer contain mycorhizzal spores although most good soils and mulches contain them.
- It is better to use organic fertilizer than no fertilizer. Research has repeatedly and consistently shown that mycorhizzal associations enhance health, drought tolerance, and productivity. Here are a couple of examples.
- Not all organics are equal. I haven’t found a decent one at Home Depot or Lowe’s. The main organic ingredient in the ones they do have is chicken poop that is obtained from large chicken ranches that use hormones and arsenic-containing feed. MicroLife and Nature’s Way (both based in the Houston area) are great products and there are many others at good nurseries.
- Organic fertilizer is more economical in the long run in spite of being more expensive up front. You will spend less on water and pesticides /herbicides since healthier plants are more pest-resistant and more competitive with “weeds”. You will also need far fewer organic fertilizer applications than when using synthetic fertilizers – because you will no longer have co-dependent plants.
- Organic fertilizers do not present health or environmental risks. Your dog or child could eat them with no ill effects. Obviously, the same cannot be said for synthetics, particularly “weed & feeds” that contain herbicides. Because it is a slow release soil amendment, run-off from organically fertilized land does not input large amounts of NPK to rivers and bays, thereby avoiding the detrimental effect of synthetics (e.g., algal blooms like red tide).
| Synthetic | Organic | |
| Man-made salts of N, P, K and other nutrients | Spores of mycorhizzae + molasses, bat guano, wheat middlins, etc. (more like compost) | |
| *Energy intensive to produce | Far less energy intensive | |
| Ultimately non-sustainable due to limited deposits of N, P, K sources | Sustainable | |
| Usually fast release | Always slow release | |
| Runoff of dissolved ions contributes to harmful algal blooms (e.g., red tide) | Non-polluting |
*The high temperatures and very high pressures needed to transform N2 to NH3 are energy intensive. About one percent of the world’s annual energy consumption is used to produce ammonia, most of which becomes nitrogen fertilizer. That’s about 80 million metric tons (or roughly one percent) of annual global CO2 emissions — a significant carbon footprint.
Over 90% of plants form associations with soil fungi called mycorhizzae. (The only group of flowering plants that does not is the Cruciferae – so if you’re growing cabbage, broccoli, turnips, wasabi, etc., these fungi won’t help you but for all other crops, flowering plants, and grasses, they will.)
The relationship is a mutualistic symbiosis – both organisms benefit. The plant provides sugars (made by photosynthesis) to the fungus. The fungus provides water and nutrients (nitrogen, phosphorous, potassium, etc.). Because the tiny fungal filaments invade huge areas of soil, they greatly extend the area from which the plant can access water and nutrients. Mycorhizzae and beneficial fungal decomposers in soil break down complex organic molecules (complex carbs including cellulose and lignin, proteins, lipids, nucleic acids).
Most mycorhizzae are endomycorhizzae (also called arbuscular mycorhizzae). Endomycorhizzal filaments (hyphae) actually grow into the roots and enter the root cells. Inside the root cells, they form food storage chambers called vesicles as well as frilly growths called arbusculae (“tree-like”) where exchange of nutrients occurs with the plant.
