How to identify low organic matter

Things to look for when walking and working in the field

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Did your fields show a lot of crusting this planting season? Was there standing water where there shouldn’t be?

These and other indicators, say provincial soil health experts, mean your soil organic matter (SOM) levels could use some attention.

Why it matters: SOM improves soil health and helps reduce crop susceptibility to extreme conditions. While SOM levels remain low across the province, they can be rebuilt.

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Crusting is normal – but should it be?

“Crusting is definitely a great indicator your organic matter may be low. That’s one of the indicators on the top of my list,” say Jake Munroe, soil management specialist for field crops with the Ontario Ministry of Agriculture, Food and Rural Affairs.

Anne Verhallen, the ministry’s horticulture soil management specialist, agrees, saying crusting is one of the easiest things growers — specifically those working heavier soils — can see. This often indicates low SOM levels, as well as overall soil structure issues.

“Organic matter is usually connected to structural problems that may be influenced by other aspects of management,” she says, citing a combination of tillage, timing, pounding rain, and organic matter as interlinked factors.

“It’s so connected. If we don’t have enough SOM we likely have other issues. Coming out of winter, those freeze-thaws do a lot to the surface. The surface looks good, but that layer of freeze-thaw isn’t stable and only lasts until the first big rain. Organic matter lets you keep that structure longer.”

Munroe adds crusting alone doesn’t provide a complete picture.

“Bare soil, even if it has reasonable SOM levels, is going to be more susceptible to crusting than a no-till or minimum-till soil with residue cover. Residue helps break the impact of rainfall and reduce risk of crusting,” he says.

Water, water everywhere

Standing water is another easily identifiable indicator, though one often seen in low parts of the field. Munroe and Verhallen say it’s important to note whether water is accumulating in areas where it’s not usual.

“If your field is not draining water well, that’s an indication soil structure isn’t what it should be and organic matter is likely something that needs to be addressed,” says Munroe.

Consulting farm records, or just remembering what happened in the previous few years, can also reveal if compaction might be the specific culprit.

“You don’t need a penetrometer. It’s valuable to get out with a tile probe when conditions are not too wet or too dry,” says Munroe. “Going out in the field and even to an area where compaction is suspected […] you can feel that resistance and identify the depth.”

Is it hard to work up?

Another SOM indicator can be the amount of work required to get the ground ready for planting.

“It can be an issue if the soil doesn’t work up well when conditions are good for tillage, particularly if it needs to be worked multiple times because it’s not giving you that nice granular seed bed,” says Munroe.

Verhallen expresses a similar sentiment. As with crusting, she says ground that’s already hard but not yet “bone-dry” indicates a significant problem.

“I can walk on a field that’s well rotated and step onto another field that hasn’t been — it feels different. It doesn’t have the give,” she says.

Low fertility in sand

Coarse soils naturally retain lower SOM levels, and while crusting issues may not be as applicable, both specialists say consistent low fertility is itself a SOM indicator. Soil sampling, as well as correlations between high input rates and lower-than-expected yields, can help reveal these problems.

Erosion and soil movement is another factor for both clay and sandy soils.

“Especially in coarse sand there is issues holding on to fertility,” says Verhallen. “If there’s soil moving in winter, from wind, or after rain, you need to do something.”

Improvement is possible

Different soil types will naturally have different SOM levels. The optimum SOM level is often far higher that what currently exists.

“The numbers for organic matter on heavier clays are pretty daunting. Seven or eight per cent organic matter achieves the maximum, but we’re usually operating in the three to five range,” says Verhallen. She added that clay soil in particular doesn’t change quickly, though improvement is possible.

“It doesn’t always have to be this way. The soils had beautiful structure once,” she says.

Munroe reiterates OMAFRA’s Agronomy Guide for Field Crops identifies SOM guidelines for all soil types.

“The fundamental thing that should not be overlooked is — do you have recent soil tests, and what were the soil organic matter levels?” he says. “Have those fields tested and find out where you stand.”

About the author

Contributor

Matt McIntosh

Matt is a freelance writer based between Essex County and Chatham-Kent. He is interested in all things scientific, as well as rock n' roll, hunting and history. He also works with his parents on their sixth-generation family farm.

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