Healthy soils and Happy trees

Most soils in our neck of the woods are nutritionally poor. They have been weathered, eroded, washed away, uplifted, turned over, salted, and inundated by millions of years of geological and climatic processes. Over this time, many plant species have gone the way of the dinosaurs only to be seen again in fossilised deposits across the planet. Other plant species and communities have evolved to cope with the changes in the climate and soil conditions they find themselves growing in.

When you consider the native plants we have growing around us (and thus are using in our reveg projects), many of these have formed special relationships with soil organisms to survive our harsh environmental conditions and poor soils. I'd like to briefly touch on some of these relationships and what makes them so special. The eucalypts would have to be our most iconic genus of plants. They grow in just about every nook and cranny on this wide brown land. They have evolved to survive many environmental conditions and their ability to survive fire is very well known.

However, when we dig a little deeper and look at the other plants we use for reveg projects, we start to uncover some interesting facts about how they have learnt to survive in extreme environmental conditions.

1. Many native plants have a symbiotic relationship with fungi in the soil. A symbiotic relationship means both organisms benefit. The plant uses the fungi's extensive web of mycelium (fungi roots) to extract water and nutrient from greater distances than its own roots can reach. The fungi extracts carbohydrates and vitamins from the plant. These fungi are usually termed myccorhizal fungi and there are three types:
• Ectotrophic – attach themselves to the outside of the plant roots like a glove. Usually found in association with trees.
• Endotrophic- enter the plant roots and develop there. Usually found in association with herbaceous plants especially orchids and heathland species.
• Vesicular-arbuscular (VAM)- enter the plant roots and develop there. Usually found in association with grasses and crop plants. Some of you will have heard of the importance of VAM in discussions about Biological Farming techniques.
It has been proven that plants will grow better when planted in soil with myccorhizal fungi already existing. Soils that have been ploughed and laid fallow for an extended period may have depleted fungi stocks and will need myccorhizal fungi added at time of planting.

2. Proteoid roots are special cluster roots that grow on members of the Protea family (and others). They are pretty amazing really! These roots form on plants growing in soils with low phosphorus content. The proteoid roots actually modify the surrounding soil by exuding acids into the soil breaking down soil particles (specifically Fe, Al, and Ca) releasing phosphorus to the plant. It is interesting to note that the formation of these roots will be inhibited in soils of mineral abundance.
Almost all of the Australian members of the Protea family live in soils with poorly available nutrients and most do not form a myccorhizal relationship with soil fungi. Many of these are often pioneer plants living in heathlands. Members of the protea family include:
• Banksia • Hakea • Grevillea • Macadamia • Lomatia • Telopea

3. Nitrogen Fixing is the term used when plants are able to utilise N from the soil. There are thousands of plant species that can fix nitrogen (legumes) but they can only do this in a symbiotic relationship with certain soil bacteria. When farmers sow a new pasture which includes clover or lucerne, the seed of these plants is treated with the appropriate bacteria before it is sown. Enzymes within the bacteria help to change molecular N (unusable by plant) into a useable form of N. Most, but not all, of our native legume plants utilise rhizobial bacteria to fix N.
These include the following:
• Wattles. • Egg and Bacon /Pea plants (pultanaea, indigo, dillwynia, viminaria, goodia)

But there is always a spanner in the works. The she oaks (allocasuarina, casuarina) are also able to fix nitrogen but they do this using another bacteria called frankia. Once again, if there is an adequate amount of usable N already in the soil, the bacteria will not release the enzymes to fix nitrogen for the plants.

In the nursery, we have to keep this information in mind when growing the above plant species. Many potting mixes these days are "soil-less" and thus contain no living fungi for the above purposes. Many fertilisers contain high amounts of N and P which would inhibit the formation of appropriate bacteria in the potting mix. Rhizobia and Frankia bacteria can be added to the potting mix in the nursery or in the seed drill if you are direct drilling tree seed on site. Members of the protea family should be grown in potting mix with low P amounts so they can grow and produce proteoid roots.

Isn't it amazing that these plant/fungi relationships exist when the assumption is often made that natives will grow just about anywhere...

1. McLean, Cassandra, Burnley Horticulture College. "Investigation of the importance of myccorhizae on propagation of some members of the family Epacridaceae."
2. BiofarmAgriculturalwebsite,Rockhampton,Qld,Myccorhizaeassociationwithpasturesand crops.
3. LawrieCowebsite,Wingfield,S.A.,Vesicular-ArbuscularMyccorhizaenotes. 4. Nemoy, Philip, Introduction to Proteoid Roots. www.tau.ac.il 5. Dictionary of Botany, Penguin Pub. Blackmore editor, 1984.

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