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The Rose-Phosphate-Mycorrhizae Connection

by Rita PerwichConsulting Rosarian, San Diego Rose Society.

Rosarians generally spend a lot of time tending to their roses. Plain and simple: a beautiful rose garden requires dedication and care. What if you find out that some of the practices you have been dutifully following may not be good for your roses? I know that thought turns me inside out and upside down! Which aspects of rose care should be re-examined? Are there any that should be retired from practice? Are we wasting our time and our money, and worse yet inadvertently harming our soil and our roses with some of our good intentions? The use of phosphorus fertilizer and myccorhizal activity in our soils are two topics that bear on each other and need to be examined.

(ABOVE: 'Julia Child' by Rita Perwich.)


A handful to a cupful of phosphate fertilizer placed in the bottom of the planting hole by rosarians is common practice because phosphorus is not mobile and rosarians want to ensure the rose has adequate phosphorus for root development and beautiful blooms. In San Diego, we continue to add this macronutrient to our fertilizing regimen which commences in early spring and ends in mid-October.


A few years ago many of us had not heard of mycorrhizae. Now we all want to have it in our soil. Mycorrhizal fungi in the soil have been symbiotic ‘plant partners’ for millions of years. Roots of plants under mild nutritional stress release chemical cues that stimulate mycorrhizal growth. The mycorrhizal hyphae penetrate cell walls of these receptive roots creating passageways between the partners. Large networks of fine filaments or hyphae extend beyond the root mass helping plants access soil water and mineral nutrients from otherwise inaccessible areas. In return, since mycorrhizae are non-photosnythetic and unable to produce their own food, the plant transfers back nutrients through the hyphae to its symbiotic partner.

Benefits to Plants Colonized by Mycorrhizae:

  1. The mycorrhizal network increases root mass which helps provide plants with a more consistent water supply enabling the plant to keep its stomata open longer and thereby assisting the plant in the process of photosynthesis;

  2. One of the main benefits of mycorrhizae is the mobilization and uptake of phosphorus which is especially important in alkaline or nutritionally deficient soils. Mycorrhizae can easily solubilize rock phosphate which is difficult for plant roots to mobilize. When plant roots perceive a lack of available phosphate, they are receptive to mycorrhizal infection and subsequent uptake of this phosphate source;

  3. Colonization of plants by mycorrhizae provides more resistance to pathogens such as Verticulum, and pests, including nematodes, and limits the available space for colonization by other pathogens;

  4. Mycorrhizae can protect uptake by the roots of toxic minerals such as aluminum, chromium and lead;

  5. Mycorrhizae release enzymes that free nutrients from the soil for plant use reducing the need for additional fertilization;

  6. Mycorrhizae can ameliorate salt stress.

Mycorrhizae can be Damaged

Mycorrhizae can be damaged and unable to function when there is soil disruption, contaminants in the soil including fungicides, and excessive fertilizer application, especially fertilizers containing soluble forms of phosphate. The reason for this is that in nutrient-rich, and also adequately watered soils, plants are less dependent on mycorrhizae causing the fungi to retreat and remain inactive.

The Rose-Phosphorus-Mycorrhizae Connection

In her article entitled The Myth of Phosphate, Part II Dr. Linda Chalker-Scott, associate professor and extension urban horticulturist at Washington State University, states that although there is plenty of rose literature advocating phosphorus fertilizer for root and flower growth, she finds no scientific evidence that roses need high levels of phosphate. In fact, studies show that excess phosphorus in the soil is detrimental to plants and can result in leaf chlorosis. This is because excess amounts of phosphorus in the soil limit the uptake by the plant of other essential nutrients such as iron, manganese and zinc. Chalker-Scott writes that like most other perennial landscape plants, roses are rarely deficient in any nutrient other than nitrogen. “Addition of any non-deficient nutrient to a landscape is a waste of time and money, and can injure soil organisms.” She adds that this is particularly true of the mycorrhizal associations that occur between various fungi and plant roots. In another article, Mycorrhizae. So, What the Heck Are they Anyway? Chalker-Scott states that numerous studies have shown that when a plant senses that its tissues or the soil contain enough phosphate, it no longer needs its mycorrhizal partner and so becomes less receptive to infection by mycorrhizal spores. She stresses plainly that phosphate fertilizer is deadly to mycorrhizal associations, “To encourage these hard-working and beneficial fungi in your plant community, you’ve got to cut down the junk (plant) food -- stop using phosphate fertilizer.”

Encouraging Mycorrhizal Colonization

To encourage mycorrhizal colonization, Chalker-Scott stresses the avoidance of over-watering and over-fertilization. She continues that conservative additions of nitrogen or organic material increase mycorrhizal infectivity; warm temperatures favor mycorrhizal colonization; and mild drought, nutrient deficiency, the reduction of pesticide use and tilling, and an increase in the diversity of plant material promote increased numbers and biodiversity of mycorrhizal species.

Mycorrhizal Amendments

Mycorrhizal amendments are marketed and available for sale in various liquid and powdered forms in addition to being present in some fertilizer blends. Should we spend our money buying these products, and our time in applying them? Chalker-Scott reports that scientific studies have found no significant value in the addition of packaged mycorrhizae to healthy soils so their application is a waste of money and resources. She advocates instead that the best way to cultivate beneficial microbes is the addition of organic matter and “thoughtful, sustainable horticultural practices.”

Over-application of fertilizers, pesticides and fungicides detrimentally impact our soil system. The ‘less is more’ method of adding judicious amounts of nutrients to our soil only when needed, and gardening practices based on scientific studies are the best way to improve our soil and facilitate beneficial mycorrhizal activity. My research has convinced me to get that long over-due soil test.


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