Atlantic Forestry March 2023

It’s the pits

Most woodlot owners will tell you that they value their land not just on the basis of current standing timber volumes, but also for the land’s productive potential decades from now, even extending to future generations. And yet, a great many of us have made important decisions about harvesting – and some of us have even purchased acreage – with virtually no knowledge of what lies below the duff. This may be changing. Until quite recently, digging a pit in the woods was something you only did when you were building a privy at the camp. Now it has become a standard part of forest management planning, to assess the properties and the capacities of soils on a given site. 

Pits had been dug in advance for the forest soils field day held this past fall on Jim Crooker’s woodlot in Queens County, N.S., organized by the Mersey Tobeatic Research Institute (MTRI). Attendees didn’t have to break a sweat, but they did have to get their hands dirty, because the first thing Kevin Keys asked them to do was take a palmful of dirt and wet it just enough to form a squishy ball. (Better to go easy on the water, he said, and add a little spit if necessary.) Pretty soon everyone was engrossed in kneading their muddy hands, like toddlers playing in a puddle.

This was meant to be a quick-and-dirty lesson on soil composition. Keys, a senior research forester and soils specialist with the Nova Scotia Department of Natural Resources and Renewables, demonstrated how to assess the texture by rolling a bit of paste between your thumb and forefinger. “If you can feel any grit, that’s sand,” he said. “The other fraction is silt and clay combined. Only clay is sticky. Silt is greasy or smooth.”

Keys showed how to slowly pull thumb and forefinger apart to evaluate adherence. Is the sensation “distinctly” sticky, or just “slightly” sticky? Clay helps to hold nutrients, he pointed out, but high clay content is associated with poor drainage. He had everyone wash their hands and start over with a different soil sample, this one noticeably grittier, even to an amateur. Clearly a person could, with practice, learn to make reasonably accurate estimates.

In one of the soil pits, Keys pointed out evidence that the site was formerly cultivated for agriculture. Crooker, whose family has worked this land for six generations, confirmed that it was farmed until about 80 years ago, like many of the drumlins in that area. “A soil profile is like the last chapter, and you have to write the book,” Keys said.  He listed five factors that contribute to that final chapter: parent material, topography, organisms, climate, and the passage of time.

At another site on the woodlot, it wasn’t really possible to dig a pit, with only about five centimetres of soil over the bedrock. There were mature and seemingly healthy spruce and pine growing all around, but Keys said he would recommend zero harvesting in a stand with such shallow soil.

As for nutrients, Keys said standard laboratory analysis only provides concentrations, but a proper inventory must take into account soil depth and bulk density. He demonstrated the use of a soil corer to take samples. He said that assessing site capacity involves calculating kilograms of calcium per hectare, for example.

HEALTHY SOILS

The field day went hand-in-hand with an MTRI webinar titled “Healthy Soils, Healthy Woodlands,” which featured Keys as the lead presenter. He cited the American soil scientist Charles Kellogg, who wrote: “Each soil has had its own history. Like a river, a mountain, a forest, or any natural thing, its present condition is due to the influences of many things and events of the past.” (The Soils that Support Us, 1956)

Keys emphasized not only the historical influences, but also changes that occur due to ongoing natural or anthropogenic factors. “Soils are not static,” he said. “They’re living systems. They’re ecosystems unto themselves, essentially. They’re dynamic.”

This is why we now talk about “soil health,” which can be degraded or improved. Keys said it’s a function of chemistry (pH, nutrient availability, and organic matter), physical properties (porosity, water holding capacity, nutrient holding capacity), and biology (microbiome, meso- and macro-fauna, and mycorrhizae). We know the most about soil chemistry, he said, and we also know a fair bit about soils’ physical properties. “It’s the biology side that we have the least knowledge about.”

One of the limiting factors for soils in Nova Scotia is low pH. “Our soils are naturally acidic, to a certain degree, and typically that increases naturally over time, not at a fast rate but at a steady rate,” Keys said. “But the trouble in this part of the world is that this natural acidity has been augmented by decades of acid rain.”

Over the past 20 or 30 years, clean-air legislation in Canada and the U.S. has significantly reduced the sulphate and nitrate emissions that cause acid precipitation, but Nova Scotia soils have not recovered because much of the base cations – calcium, magnesium, and potassium – have been leached out, reducing the soils’ capacity to buffer or neutralize acidity. Low pH restricts nutrient availability, so soils in many parts of Nova Scotia remain in rough shape.

This is why it’s particularly important that the province is now using the Nutrient Budget Model (NBM) that has been developed during the past decade – calibrated with Nova Scotia data, and factoring in acid rain impact. The NBM is being integrated into Crown forest management, and it is also being made available to private landowners. “With the harvest rates, you’re going to be removing nutrients,” Keys said, “and you don’t want to remove nutrients beyond the capacity of the ecosystem to supply it long-term.”

The province is also working on ground disturbance guidelines for Crown land. “Not all disturbance is damage,” Keys stressed. “There are types and levels of disturbance that you can consider damaging, but that depends on the soil type, so you can’t just use a blanket value for everything. We’ve related disturbance or damage thresholds to particular soil types.”

Damage to the physical structure of soils is mainly related to loss of aeration porosity, but loss of organic matter is also a significant factor. The new guidelines will minimize both, based on modelled impacts on site productivity.

QUANTIFICATION

Keys is a member of Dalhousie University’s Centre for Sustainable Soil Management (CSSM) – and he intends to maintain this relationship when he retires from his government job this spring and takes on the role of project scientist with the non-profit Family Forest Network. The webinar featured four other CSSM members – starting with David Burton, the group’s founding director, who is a professor in the Department of Plant, Food, and Environmental Sciences. Burton talked about the growing recognition that soils are critical not only for food and fibre production, but also for maintaining water quality, biodiversity, and climate regulation. One of the fundamental ways of improving soil health is to increase soil organic matter, which is 50 percent carbon. He said that increasing soil carbon by a tenth of a percent will quadruple CO2 capture in the top 15 centimetres of soil.

A key part of Burton’s work involves finding better ways to quantify the self-sustaining capacity and functionality of soils. “One of the things we’re looking at is spectral analysis of soil, to measure a wide range of properties,” he said, noting that his colleagues Derek Lynch and Gordon Price are involved in survey work to build a “spectral library” for soils in Eastern Canada. “This technology uses the really rich data a full-spectrum analysis gives, as to the physical, chemical, and biological content of the soil.” He said another important research stream involves digital mapping of soils, using “machine learning” to extract information from large data sets. The webinar also featured Dalhousie microbiologist John Rohde, who talked about the potential use of genetic sequencing to characterize bacteria and fungi in soils.

The remaining presentations focused on amending forest soils with limestone, as a remedy for acidification (a topic examined by Zack Metcalfe in his article “Liming the woods” in the May 2021 issue of AFR). The practice has not been adopted widely, though it has been used with some success in Sugar maple stands in Quebec. Eddie Halfyard, research scientist with the Nova Scotia Salmon Association (NSSA), provided an update on trials conducted at the Otter Ponds Demonstration Forest in Mooseland, N.S. He said the treatment increased base cation concentrations, except in the deepest soil horizons (possibly because the effects hadn’t penetrated that far yet), and tree tissue analysis showed rapid uptake of calcium and magnesium by softwoods and hardwoods, except for Red maple.

Helicopter liming is expensive, Halfyard acknowledged, but a cost-benefit analysis indicated that it can be economically viable on the basis of increased sustainable timber volume – even without ascribing value to other benefits such as carbon capture. The NSSA is now investigating ground-based application, and has worked to develop a spreader that can be mounted on a forestry forwarder.

Shannon Sterling, an associate professor in Dalhousie’s Department of Earth and Environmental Sciences, is also involved in the project at Otter Ponds. She spoke about plans for a new trial using crushed basalt instead of lime. Basalt is a volcanic rock rather than a carbonate rock, so it weathers more slowly, providing a gentler pH increase. Sterling said it has been trialled on cropland in Scotland, and on a softwood plantation on former agricultural land in Wales, but Nova Scotia will be the first jurisdiction to spread basalt on forest sites.

INTENSIVE MANAGEMENT

What this boils down to, perhaps, is a growing recognition that soil cannot be taken for granted. We live on planet Earth, and we still have much to learn about this natural resource that underpins our survival here.

In Nova Scotia, advances in soil science are coinciding with a large experiment in intensive forest management. The province has allocated 10 percent of Crown land to “high production” forestry. (Currently that amounts to 185,000 hectares; the area may increase as additional Crown land is acquired, but it will not exceed 10 percent.) This land will be devoted to softwood plantations managed on rotations of 30-50 years. Thus far, 9,395 hectares of potentially suitable Crown blocks have been identified, and assessments are ongoing. The Department of Natural Resources and Renewables estimates that about 5,000 hectares will be added to the high-production zone each year for the next 35 years, and that this land base will eventually sustain an annual harvest exceeding one million green metric tonnes of high-quality spruce timber.

Crown blocks considered for high-production forestry will not include designated or pending protected areas, old-growth, sensitive forest ecosystems or habitats, special management zones for wildlife, or rich ecosites that commonly support tolerant hardwood forests. The chosen sites must have “inherent soil fertility and drainage characteristics” to support short rotations of fast-growing softwoods, but the province recognizes that added fertility will be required. (Herbicide use will be allowed, but not publicly funded, and licensees are being encouraged to develop microsite application practices, ultimately eliminating the need for broadcast applications.)

As part of the “triad” model for Crown land, which features more ecologically sensitive management on the remainder of the working forest, this new approach has broad support. However, there are significant risks associated with such intensive (and expensive) methods of growing trees, especially in our era of rapid climate change. For soil researchers, it’s a tremendous opportunity. DL