It is said we know more about the universe than about the soil beneath our feet. Thankfully, this is gradually changing. A healthy soil is full of life, literally. Full not only of tiny living creatures such as worms and wood louse, but also of micro-organisms such as fungi and bacteria. Together with plants, they make up a complex ecosystem in which all various organisms have a function of their own and support one another. Pot plants stand alone, however, because most potting soil is sterilised and consequently contains much less soil life. Both research and practice reveal that enhancing soil life in potting soil can benefit plants in many ways.

Symbiosis between plants and soil life

Just one teaspoon of woodland soil can contain a kilometre of hyphal threads. The soil of a healthy wood is a self-sufficient, living ecosystem. The fungi in the vicinity of the roots of plants and trees are called mycorrhiza. They live in symbiosis the rhizosphere (the vicinity of plants roots together with the rhizobacteria). The plant roots cannot reach all the nutrients (water and minerals) in the soil themselves. The fungi and bacteria help the roots reach and absorb the nutrients. They are also able to make phosphate available to the plant. But they expect something in return. The plants give the micro-organisms sugars (glucose). Thus creating a symbiosis. Rhizobacteria and mycorrhiza not only help plants to take up water and minerals; they also protect the plant against pathogens, pollution and drought. This symbiosis is like a marriage of convenience. If the plant is able to extract nutrients from fertilizer, it will immediately stop the exchange with the fungi.

Soil life in potting soil

The roots of indoor plants are also in soil. However, potting soil has been heated to eliminate pathogenic bacteria. With the result that most benign bacteria and fungi, which usually help and protect the plant, will also have been removed. What are the potential benefits of adding mycorrhiza fungi and rhizoacteria to potting soil?

Research into the benefits to plants of added micro-organisms

At Wageningen Plant Research, in 2014, Dr Joeke Postma researched the benefits of mycorrhiza for pot-grown strawberries. Strawberries are susceptible to root rot (Phytophthora cactorum). Dr Postma researched whether adding a certain mycorrhiza fungus (Rhizophagus irregularis) could protect the plant from this disease. She discovered this to be the case. This species of fungus proved able to reduce root rot infection by some 50%.

Postma concludes that healthy soil life is essential to healthy plant growth. “Particularly when plants are growing not in open ground (but in potting soil, rock wool or hydroponics, for example), there will be a lack of plant-specific micro-organisms in the rhizosphere. Adding these useful bacteria or fungi can enhance plant growth and protect plants against infection.” NB: the effectiveness will vary. Not all fungi or bacteria are equally useful for all plants.

In another, similar study Postma also researched the benefits of adding certain bacteria to tomato plants growing in potting compost. She selected to use the bacterium Pseudomonas chlororaphis strain 4.4.1, on account of its capacity to inhibit fungal growth and to enhance  phosphorus uptake by the plant. The number of healthy plants rose by 30 to 105% compared to the control group in three independent greenhouse experiments.

Pius Floris: “We should not think of mycorrhiza as pesticides”

Pius Floris is a soil specialist. He is familiar with Postma’s research into the effect of mycorrhiza on strawberries. “It is wonderful that a specific species of mycorrhiza can inhibit root rot in strawberries. However, we must not think of mycorrhiza fungi and bacteria as pesticides; they are not against anything”, he says during a telephone interview. “Mycorrhiza are just as much part of the plant as the foliage. There is a natural relationship between fungi, bacteria and the plant. The problem with scientific research is that it often produce an answer to a very limited research question. Studies do not often consider the entire plant ecosystem. I see mycorrhiza as a natural stress manager. Indoor plants are sometimes too wet, sometimes too dry. If the symbiosis between fungi, bacteria and plants is in balance, the plant will be much better able to survive too much or too little water. And anywhere that mycorrhiza live, no other pathogenic fungus can grow.”

“Fertilizers kill the soil and cause disease in plants”

Floris’ thesis is clear: a healthy soil produces healthy plants. The same applies to potting soil for indoor plants. Together with his team from Plant Health Cure he develops soil bacteria and soil fungi for sustainable crop growth in agriculture and horticulture and for nurseries. Floris: “In the cultivation of plants, indoor plants are often grown in hydroponics: clay pellets with water and fertilizer. That is supposedly convenient as it avoids the need to change the potting soil every year. But fertilizer has a very high salt content and salinization is not good for plants. Put simply: fertilizers kill the soil and cause disease in plants. If you add mycorrhiza and bacteria to the potting soil you need use less pesticide, or none at all, and less water.”

The application is very simple, Floris explains: “Mycorrhiza need only be added to the potting soil once. You simply make holes in the potting soil, dilute the mycorrhiza and bacteria mixture with water and pour it into the holes. Your plant will be more resilient and you will never need to change the potting soil. Unfortunately you will still have to fertilize pot plants, simply because they use up the minerals in the pot during growth. As opposed to garden plants which have much more growing space and thus more minerals available. So fertilizing remains essential, but do use organic liquid fertilizer. The bacteria and mycorrhiza ensure optimal conversion and absorption.”

The benefits of healthy soil life in potting soil:

  • Plants will be ‘in balance’ thanks to the natural symbiois with mycorrhiza fungi and bacteria. As a result, they will be more resistant to disease, drought and excessive water.
  • Micro-organisms in the potting soil make nutrients available which the plant would otherwise not be able to absorb.
  • Synthetic fertilizers and pesticides are superfluous. Organic fertilizing is still essential.
  • The potting soil does not need to be changed.

It is sustainable and environmentally friendly.

Guest author: Marjolein Bezemer

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