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Going underground to the root of it all

Roots, the essential ‘engine room’ of a plant, is the determining factor of health and growth. But roots may not be where you expect. David Whyte delves down into how our trees behave underground.
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My science teacher dad used to talk about ‘kiddy science’ – the ‘science’ we learned as kids. It made amusing cartoons (character when landing on a trampoline will always bounce back higher than when they started) and a simplistic understanding of things. However, one must be careful not to fall into the trap of not examining these beliefs as we get older.

One such belief is around how tree roots behave under the ground. Most folks, when drawing a tree with roots, will end up with the roots mirroring the tree top, and ending at the drip line, where the leaves end. Producing something like the clip art image here on the right.

Roots are hard to study since they are hidden from sight, and hard to excavate whole. However, think of the times you have seen a tree fallen over/toppled by a storm. What do the roots look like? Do they go down into the soil reflecting the tree’s top, or do they spread out in a relatively shallow way? In the photo left, there is no evidence of roots going straight down into the soil as shown in the illustration / ‘kiddy science’ version on the right.

Where the roots are

If the role of roots is to harvest nutrients from the soil, which are provided via fungi and the soil food web, is there any point in having roots that are not in the top layer where the soil life lives? Apart from searching for deep water, there is no point in having roots beyond the topsoil. Deep soil doesn’t have oxygen, so doesn’t support life. Hence the overwhelming majority of any tree’s roots exist in the topsoil, with a few roots going deeper to mine water in the dry seasons.

Fortunately, work was done 100 years ago where folks actually excavated plant root systems and produced drawings. The example drawings (on the opposite page) of a stone fruit (Prunus padus) and crab apple (Malus sylvestris) are from this work.

The work concluded a number of things:

  • The overwhelming majority of roots are in the topsoil.
  • Some roots may go down to reach moisture. It makes sense that trees being more drought resistant than grasses will have some roots that go deep into the soil searching for moisture.
  • A rule of thumb is that when young, plant roots spread out sideways nearly twice as far as the tree is tall. And when mature, the roots go out sideways more than the tree’s total height – assuming all things are equal. If a barrier is reached, like a road, compressed soil, another tree, or rocky ground, etc., the roots will not travel through these difficult areas, or will not extend as far as they otherwise might. This is just a rule of thumb and individual species will have different results.

Stability

How does the tree stop from falling over? There is a ‘ruler and paper experiment’ in which a ruler is placed sticking out from a bench and a large sheet of paper is placed over the end of the ruler that is on the bench. Giving the ruler a sharp wack will result in the ruler not moving or tipping off the bench as you would expect. The force of air pressure acting over the large area of the paper holds the ruler in place. A similar effect occurs with the large spreading root mass. It is the large area of dirt that holds the tree in place.

Feeding the roots

Spreading nutrients, or watering on the dripline, was supposed to target where the roots were small at their ends so they could easily absorb them. This turns out to be incorrect. The simple solution is not to apply fertilisers but build up overall soil fertility. However, if specific products are to be applied, do it from the dripline out, and thinly spread over a wide area.

Increasing soil fertility

Increasing the soil humus, carbon content, microbiology, etc., will result in increased tree growth, health, productivity, etc. Since the majority of the roots are in the topsoil, improving this soil condition will result in benefits to the tree system.

From adding compost to growing green crops, there are numerous ways to increase soil fertility. A simple rule of thumb is that if the topsoil is increasing in depth/height, then things are going in the right direction.

Tap roots

The primary root from a seed is a taproot, which branches off to secondary roots, which in turn branch to form tertiary roots, and so on, causing the development of a fibrous root system. For most plant species, the primary root dies one to a few years after germination and the main root system changes to a wide-spreading fibrous root system with mainly horizontal-growing surface roots and only a few vertical roots.

Plants that have persistent tap roots include carrot, radish, beets, etc., where the tap root is a storage organ, and dandelion, parsley, some oaks, elms, pines, and firs.

Further reading

  • More about root interactions, and how they determine friend or foe, will be in the upcoming March/April issue of OrganicNZ.
  • Digital images of work done excavating tree roots: images.wur.nl/digital/collection/coll13
  • Book using many illustrations from original root research: Roots Demystified: Change Your Gardening Habits to Help Roots Thrive by Robert Kourik.

David Whyte, B.Sc., M.Sc.(Tech)Hons., is a member of Soil & Health NZ, president of NZ Tree Crops Association and Huntly Ward councillor. He has a near-organic/permaculture citrus orchard on the banks of the Waikato River.

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