Tag: soils

Rooting around – the differences between taproots and mature roots

A seedling with green cotyledons and emerging radical

Most of us have witnessed dicot seed germination at some point in our lives – watching the coytledons transform from seed halves to green, photosynthetic structures, while the radicle developed into the seedling root system. This seedling root – or taproot – is important to seedling survival as it buries itself in the soil to provide structural support and to give rise to fine roots for water and nutrient absorption. But that’s where much of our visual experience ends – because we don’t see what’s happening underground. Without additional visual information we imagine the taproot to continue growing deep into the soil. And while this perception is borne out when we pull up carrots, dandelions, and other plants without woody root systems, the fact is that woody plants do not have persistent taproots – they are strictly juvenile structures. Understanding the reality of woody root systems is critical in learning how to protect and encourage their growth and establishment.

Mature carrots have taproots. Mature trees do not. Photo courtesy of Pixnio.

Trees, shrubs, and other woody perennials all have juvenile taproots just like their herbaceous counterparts. But these long-lived plants develop different morphologies over time, which are primarily determined by their soil environment. Water, nutrients, and oxygen are all requirements for sustained root growth. Gardeners always remember the first two of these needs, but often forget the third. And it’s oxygen availability that often has the biggest effect on how deeply root systems can grow.

Roots grow where they can. Sometimes that zone can be very shallow, as this coastal forest photo shows.

Whole-plant physiologists have known for a long time that “roots grow where they can” (Plant Physiology, Salisbury and Ross, 1992). But this knowledge has become less shared over time, as whole-plant physiologists at universities have been largely replaced by those who focus on cellular, molecular, and genetic influences (and can bring in large grants to support their institution). Sadly, many of these researchers seem to have little understanding about how whole plants function. Simply looking at the current standard plant physiology textbook (Plant Physiology and Development, Taiz et al., 2014) reveals as much. (To be fair, there is now a stripped-down version of this text called Fundamentals of Plant Physiology, [Taiz et al., 2018] but even this text has little to do with whole plants in their natural environment.) If academics don’t understand how plants function in their environment, their students won’t learn either.

The Table of Contents for Plant Physiology and Development. You won’t find a discussion of woody root ecophysiology in here.

Well. Time to move on from my soapbox moment on the state of higher education.

Roots grow where oxygen is plentiful. It becomes a limiting factor as soil depth increases. Photo courtesy of Wikimedia.

Let’s look at what happens with a young tree as it develops. The taproot grows as deeply as it can, but eventually runs out of oxygen so vertical growth stops. At the same time, lateral root growth increases, because the levels of oxygen closer to the soil surface are higher. These lateral roots, and their associated fine roots, develop into the adult root system, continuing to grow outwards like spokes on a wheel. When pockets of oxygen are found, roots dive down to exploit resources. These are called sinker roots and they can help stabilize trees as well as contribute to water and nutrient uptake.

Gardeners and others who work with trees and other woody species would do well to remember that woody root systems, by and large, resemble pancakes rather than carrots. These pancakes can extend far beyond the diameter of the crown – so this means protecting the soils outside as well as inside the dripline.

Typical root structure of a mature tree in its natural environment. No taproot here!

The complicated issue of heavy metals in residential soils, part 1: What are toxic heavy metals, and where do they come from?

The popularity of home gardens is exploding as we wait out the COVID pandemic

So many of us are growing our own vegetables – either as experienced home gardeners or as COVID19-isolated novices. There is a lot of effort in figuring out garden beds, vegetable choices, and growing medium – but one of the issues rarely considered is whether there are heavy metals present in the local soil and/or growing medium. We can’t see heavy metals, or smell them, so we need to have a way of assessing their presence before we plant edibles.

In the next few months, I’ll tackle the complicated science behind this invisible threat. Today, let’s look at the heavy metals that are commonly found in garden soils and where they might come from.

Arsenic, chromium, and lead products

What heavy metals do gardeners need to monitor in their soils?

Heavy metals are exactly that – they are dense elements that have certain chemical properties that define them as metals. In fact, most known elements are considered to be heavy metals. Fortunately, there are only a handful of heavy metals that are commonly found in residential soils. Some of these heavy metals are necessary for life – iron, manganese, and zinc, for example – but others have no known biological function. Arsenic and lead, for instance, can interfere with enzymatic activity and effectively poison biochemical pathways. There is no “safe” level of heavy metals that are not essential nutrients.

Here’s a table of the most common toxic heavy metals that might be found in your soil, and possible anthropogenic sources:

Heavy metal Sources of contamination
Aluminum* Smelting
Arsenic Pesticides, smelting, treated timbers (old)
Cadmium Paint
Chromium Fly ash, metals industry, paint, leather tanning, treated timbers (old)
Lead Gasoline (leaded), paint, pesticides, plumbing, smelting, solder
Nickel Plumbing, smelting

*Aluminum is a light metal, not a heavy metal, but has similar biochemical poisoning activity as toxic heavy metals

Some of these sources of contamination are not relevant to where I live – why do I need to test my soil?

Gardeners may be tempted to look at the chart above and feel relieved, because pesticides and paint no longer contain heavy metals, they don’t use old treated timbers, and they know that leaded gasoline is a thing of the past. What many don’t consider, however, is that heavy metals are elemental – they don’t break down, though they may change their chemical form. They are a permanent part of soil chemistry unless they are removed by physical or biological means.

  • Old garden pesticides were very effective – this one contained both lead and arsenic
  • Old landscape timbers release copper, chromium and arsenic into the soil

The underlying soil in housing developments built on old agricultural land often contains high levels of arsenic – because that was the active ingredient in pesticides many decades ago. If the topsoil was removed during construction, it may have been taken to a commercial soil facility where it would have been used to create landscape fill mixes for new landscapes elsewhere. The same is true for land near older roadways where lead from gasoline was released from vehicles over many decades. Not only are lead, arsenic, and other heavy metals in the soil, they also end up in the air when soil is disturbed by erosion or tilling.

Nearly all soils contain some level of some heavy metals. They are naturally occurring, after all, so their presence is not necessarily from anthropogenic activities. Regardless of the source, it’s important to know whether any of these harmful elements are in your garden soils, especially if you are growing edibles. A soil test is the only way to find out.

Here is a soil test of my own raised bed system. While my nutrient levels are optimum, and lead is very low, the aluminum level is quite high. What should I do?

Why aren’t there guidelines on heavy metal uptake in vegetable gardens?

It would be ideal if there was a list of “safe” and “dangerous” vegetables to plant when heavy metals are present. Unfortunately, real life rarely fits into lists and there are numerous sources of variability. Next month I’ll discuss the complications that arise when we consider plant species, heavy metals, and environmental variables.