The Truth About Phosphates and Mycorrhizal Fungi

One of the most well-documented benefits from mycorrhizal fungi is the increase in the uptake of phosphates by the host plant. Mycorrhizal fungi increase the amounts of phosphate appearing in host plant tissue, and radio tracer studies have confirmed that this phosphate is being provided via the mycorrhizal fungi.

High Available Phosphate Reduces Root Colonization

Interestingly, research has also shown that, when soils contain high levels of available phosphate, the degree of root colonization by mycorrhizal fungi diminishes. The higher the level of available phosphate in soils, the lower the level of root colonization by mycorrhizal fungi. This effect is documented both for ecto- and endomycorrhizal plants. Note that this effect only applies to available phosphates. Insoluble phosphates are unavailable, and do not affect mycorrhizal development.

Phosphate Does Not Kill Mycorrhizal Fungi

Oddly, many people have erroneously concluded that phosphate kills mycorrhizal fungi. In fact, there is no killing effect going on here. Instead, in soils having high available phosphate, the host plant apparently opts to restrict fungal colonization. The fungi and their spores are still alive. They are simply experiencing a higher level of restrictions from the host plant. In fact, there is ample evidence to show that the host plant has significant control over when and where mycorrhizal fungi may enter root tissue. Theoretically, under conditions of high available phosphate, the restrictions are increased. So the levels of soluble phosphate in the soil can affect whether the host plant opens the door to the root widely or narrowly. But the mycorrhizal fungi are not killed by phosphates. This misunderstanding has been promoted by various companies who attempt to blend and sell mycorrhizal products with no real grasp of their biology.

So let’s correct the record regarding phosphates. Here is the real story:

  • Phosphates, both available and unavailable forms, do not kill mycorrhizal fungi.
  • High levels of available soil phosphate (soluble phosphates) result in reduced root colonization by mycorrhizal fungi.
  • Insoluble phosphate does not affect mycorrhizal development. The insoluble phosphate in natural soils can represent as much as 99% of the soil’s total phosphate content.

What Does this Mean in Practice?

From a practical standpoint, this phosphate effect has significant ramifications. When applying mycorrhizal fungi inoculants, it is important to avoid applying high levels of soluble phosphate. High soluble phosphate can effectively close the door (partway) to root colonization by these fungi. This effect can be turned into a benefit, because you can save money by using less fertilizer. Mycorrhizal fungi can effectively increase phosphate uptake by converting insoluble phosphates to soluble, available forms, thereby requiring much less fertilizer application.

There are various options at your disposal. If your soils contain significant amounts of unavailable phosphates (like insoluble rock phosphate), then you will not need to add much soluble phosphates, since mycorrhizal fungi will gradually mobilize (i.e., solubilize) this phosphate source. Mycorrhizal fungi will take advantage of the rock phosphates that would otherwise have been unavailable to your plants. So you can save on your fertilizer budget.

If your soils have low levels of total phosphates (available and unavailable), then you will want to apply some phosphates. But you will have to avoid applying high levels of soluble phosphate at the same time you apply the mycorrhizal fungi inoculant. The best approach would be to apply only low levels of phosphates initially, and then spread out your phosphate applications, so you are not dumping a high level on the soil all at once. That is a bad practice anyway, since soluble fertilizers will readily leach away with each rainfall. One of the main reasons for using mycorrhizal fungi inoculants is to reduce your dependence on high fertilizer applications. So don’t shoot yourself in the foot by combining high amounts of soluble phosphates with a mycorrhizal fungi inoculation program.

How Much Phosphate is Too High?

The amount of soluble phosphate needed to inhibit mycorrhizal fungi colonization is quite variable. It is different with different plants, with different fungi, with different soils, and it is affected by environmental factors such as light intensity, soil moisture and soil pH. But we have some numerical data from various agricultural studies. Here is the data from one study on wheat, barley and rye.

Baon et al. 1992. Phosphorus efficiencies of three cereals as related to indigenous mycorrhizal infection. Australian J. Agricultural Research. 43: 479-491. (Columns A & C only).

Amount of P in Soil Mycorrhiza Fertilizer Needed to Achieve Column A or B Levels
A B C D E F G
P (Elemental) mg/Kg P2O5 Equivalent PPM Amount of Root Colonization P2O5 Lb/Acre Equivalent DAP* Lb/Acre Equivalent ArborGreen‡ Lb/Acre Equivalent ArborGreen‡ Lb/1000 sq ft
5 12 Normal (40%) 22 65 220 5
10 <23/td> Marked Reduction 45 130 450 10
30 69 Almost None 135 400 1350 30

*Diammonium phosphate 10-34-0 -- Davey Tree fertilizer 30-10-7

Columns E, and F, and G show the amount of fertilizers (as phosphate equivalent; as DAP, used in agriculture; or as ArborGreen, used in horticulture) needed to supply the amount of elemental phosphorus shown in column A, or soluble phosphate shown in column B. (This assumes the soil starts with zero phosphate.) Because high phosphate fertilizers are common in agriculture, farmers need to be much more careful with phosphate applications when using mycorrhizal fungi or mycorrhizal stimulants, like Myconate.

Summary

In summary, mycorrhizal fungi and their host plants both need phosphates, and both work hard to accumulate this important mineral nutrient. But when soluble phosphates are abundant, the host plants tend to increase their restrictions to mycorrhizal fungi, resulting in lower levels of root colonization. So avoid applying high levels of soluble phosphates at the same time that you apply mycorrhizal fungi. Instead, consider reducing your total phosphate fertility, and spread your phosphate applications out more broadly.