Tree fruit growers are increasingly learning about the role organic matter plays in improving overall soil health. One important revelation: not all organic matter is the same.

Soil scientists now understand that while total soil organic matter is important, a critical regulator of water and nutrient holding capacity, one fraction provides the fuel for the soil food web.

This fresh organic matter or “active carbon” made up of partially decomposed plant material, root exudates and living and dead microbes is dynamic.

This material changes more rapidly than total soil organic matter and tends to be a great indicator of the direction soil organic matter is moving, as well as predicting biological activity.

Active carbon is a small fraction of soil. Total organic matter is about 2 percent of the soil in an orchard. Active carbon is about 10 percent of that 2 percent.

But this tiny fraction is important for tracking change and for catalyzing the critical nutrient cycling, soil structure forming, and root protection functions the soil biota do for us.

“When we think about soil organic matter, you have to think about not one homogeneous thing, called soil organic matter, that doesn’t change very much, but that it’s very dynamic,” said Tom Forge, research scientist with Agriculture and Agri-Food Canada in Summerland, British Columbia. “Orchard management practices such as tillage, pesticides and irrigation all influence the soil food web and impinge on soil health, but organic matter management has the largest influence of all management practices.”

“We know that by enhancing soil organic matter input and enhancing overall soil biological activity, we can effect certain microbial interactions right around the root, that have direct influences on root growth,” he said.

For example, organic mulches create a system-level response. Bacteria produce glues that help to hold soil together in little clay particles. Fungi help to enmesh those particles and hold together larger aggregates.

Earthworms come along and make larger pores. In short, all of the organisms in the soil are involved in this aggregation process, which is really important for enhancing overall soil quality and health.

Growers have a number of ways to enhance root-zone soil organic matter, including:

—Amendments, namely compost and manures, generally provide organic matter and nutrients, though there are limited opportunities to incorporate them into the soil beneath the soil surface.

—Mulches, which also provide a physical protection to the soil, moderating soil temperature and moisture fluctuations and, because they are also surface applied, take longer to affect the organic profile down into the soil.

—Mow and blow of alley vegetation onto the tree row.

Overall, these practices help to enhance the water holding capacity of the soil, the cation exchange capacity or nutrient holding capacity, and the soil’s ability to fuel soil food web activity, which in turn leads to many other benefits, including suppression of pests and pathogens.

How those practices affect organic matter and overall health of the soil over time is the subject of Forge’s ongoing research in British Columbia.

One cherry orchard

To better understand the role organic matter plays in a replant situation, Forge initiated a study on a cherry block — Crystalline and Skeena cherries on Mazzard rootstock — that had previously been the subject of a 10-year irrigation study and included plots that had been mulched with bark and wood chips continuously for 10 years.

In spring of 2015, the old trees were removed, and the soil within each row was prepared for replanting while keeping track of where the old mulch plots were.

Half of the rows were fumigated, and then compost was applied at 50 tonnes per hectare (roughly 20 tons per acre) to plots within each row overlapping the old mulch plots, resulting in four types of plots within each row: a history of organic mulch, compost amendment, both sources of extra organic matter, and no additional organic matter.

The use of mulch for 10 years prior to replanting resulted in enhanced soil organic matter carbon contents, whereas incorporation of compost immediately before replanting had little effect on soil organic matter carbon, but did increase soil organic nitrogen contents.

Forge also examined populations of the parasitic root-lesion nematodes in the soil, both the overall population (nematodes per liter of soil) and per gram of root, which showed how heavily parasitized those trees were at that point of time.

As expected, he found a strong effect of fumigation, resulting in lower root-lesion nematode populations.

Adding the compost suppressed overall root-lesion nematode populations, in both fumigated and nonfumigated soil.

When the root-lesion nematode suppression is combined with the enhanced nutrient availability and populations of beneficial soil organisms, the compost treatment means “happier plants growing faster,” he said.

The compost is helping to protect the roots from nematodes, either by suppressing their numbers out in the soil or by affecting bacteria and fungi that grow on the surface of the root, he said.

While fumigation had a major effect on tree growth, illustrating the importance of the replant disease complex, the experiment showed no obvious benefit of compost to tree growth by the end of the second season.

This is in contrast to an earlier experiment in which Forge and his co-workers, including graduate student Tristan Watson, observed that tree growth in compost-amended plots exceeded untreated plots and caught up with fumigated plots by the end of the third growing season.

Thinking of these results from the previous experiment, Forge hypothesizes that they may still see improved tree performance in future years.

“There are several ways compost can be influencing a reduction in parasitic nematodes,” he said. “The end result is this is good field data demonstrating it.”

Forge and a graduate student are continuing the research this year. •

– by Shannon Dininny