Marl

Writing yesterday’s post on soil horizons, I ran across a couple of unfamiliar words. Marl was one of them. Here’s what I’ve learned about marl.

Marl (Tashkoskim) CC BY-SA 4.0

The word marl comes to us from the late 19th century. It is short for marbled. Rather than being as hard as your favorite cat’s eye or aggie marble, marl is a porous, relatively soft mudstone. And it contains many of the nutrients your plants love.

The lower cliffs of Dover and much of New Jersey contain marl.

White Cliffs of Dover (Immanuel Giel) CC BY-SA 3.0

Marl formation

Marl forms as carbonate-rich clay mud and silt are blended with algae that loves alkaline waters. The carbonate is from the dead shells of water-dwelling organisms and the calcified algae. Originally the term marl only referred to freshwater formations. We now know that marl occurs in saltwater too.

Coastal marl formation, southern Sicily (Carnby) CC BY 3.0

Types of marl

Marl contains 65–35% carbonate and 35–65% clay, depending on the type. It can be blue, green, sand-colored, or red. Marl contains calcium, iron, phosphorus, potash, silicic acid, and sulfur. It also contains magnesium, but only a little. Magnesium deficiencies are common in fields treated with marl.

Depending on how it formed and its mineral composition, marl, which crumbles easily, can become marlstone, which does not.​

​​Marl in the garden

In the 1800s, marl was mined for use in gardens and agriculture as a soil conditioner. In 1877, more than 15,000 tons of the stuff was mined from a single county in New Jersey. It is still used today to neutralize acidic soil and provide plants with nutrients needed to grow and thrive, though not nearly as often as in the past.

​Marl is also used to make cement. Now we know.

Marl deposits inside a discarded paint can (Eric Charles Laska) CC BY-SA 4.0