Benefits of Molasses for the Garden? Soil Priming! [Commentary]

Not scientific? Master Gardener? *Alarm bells*

Glucose, a six-carbon sugar is one of the sugars in molasses. It can prime carbon in the soil, increasing soil organic matter decomposition by bacteria that make it more plant available as dissolved organic matter.

Priming or a “Priming Effect” is said to occur when something that is added to soil or compost affects the rate of decomposition occurring on the soil organic matter (SOM), either positively or negatively. Organic matter is made up mostly of carbon and nitrogen, so adding a substrate containing certain ratios of these nutrients to soil may affect the microbes that are mineralizing SOM. Fertilizers, plant litter, detritus, and carbohydrate exhudates from living roots, can potentially positively or negatively prime SOM decomposition

Look at fig (3c) for organic soil here:

The red indicates positive carbon priming when glucose is added.
Notice how different the glucose priming effects are for wood (a), leaf (b), organic soil (c) and mineral soils (d) in each of the squares. Think about why that might be, and where the carbon-based lifeforms are most abundant. The organic soil!

Priming is how organic soils are formed when plants exude carbon compounds from their roots to feed soil microbes.

Unfortunately not all soils provide enough nutrients to the plants to create enough exudates to prime soil carbon and maintain nutrient cycling, and soil carbon is mostly determined by rainfall (78%) and how land is managed, which is why we tend to augment soils with fertilizers that perform this priming.

As shown in fig (3c), priming soils and composts with nitrogen can also aid this process, an example of priming compost are these “dreadlock” roots formed when using “Next Gen” compost that adds nitrogen and other minerals.

Priming, however it’s done, generally results in higher dissolved organic matter and microbial abundance, diversity and nutrient cycling resulting in more plant available nutrients.

Soil carbon to nitrogen ratios can determine whether carbon or nitrogen is the best choice to prime organic matter and will depend on the soil and the optimum range that soil microbes like fungi that knit the soil food web together like to feed on. Typically that’s the C:N of 30:1 that microbes are made of. Probably why there’s a tipping point at 3% soil carbon. Many bare and underperforming soils are well below this and crave carbon. Priming soil organic carbon is how biochar works so long as you add enough

The type of carbon or nitrogen source when priming is important too, as it may alter microbial communities. The more complex the carbon source the more potential there may be for enzymatic pathways that the microbes can express to create compounds that change their environment.

Different species of plants change their own environments by exuding different exudates that host different microbes that build the environment for them.

Doing the work of the plant by amending soils ourselves may benefit or hinder these microbes.

It’s important to note that many types of molasses are heavily processed and end up with sugars but very few minerals in them, and this may change the microbial community detrimentally.

What gets added to white sugar to make it brown? Molasses.

You can see it does contain and add some minerals.


In general, the less processed something is, the more minerals it contains, and the more diversity it will support, thereby allowing the plant to feed and select for the microbes it wants through its exudates rather than what will eat what we amend the soils with.

Cultivate those soil microbes with carbon where appropriate.


2 thoughts on “Benefits of Molasses for the Garden? Soil Priming! [Commentary]

  1. Feeding the plants might make them lazy. So they will give less sugar away to the microbes, (giving need for even more sugar added as fertiliser) Could be the wrong way to go? David C Johnson (Johnson Su compost system) has exellent results on biological only-farming. (they add molasses to the compost when they spread it)


    1. Totally agree that feeding plants some nutrients might make them lazy. There are studies showing exactly that with nitrogen and phosphorous. They’re linked on the blog somewhere. In the former researchers found soil amended with urea (nitrogen) resulted in less carbon being exuded by plants and therefore less made available for symbiotic fungi which then reduced in numbers, which in turn shared less with the rest of the nitrifying ecosystem that would normally supply the plant with nitrogen. As a result, when there’s a short supply of carbon to go around microbes go to war for those resources. An example of that is a study on predatory nematodes proliferating when there are fewer fungi that would normally prey on the nematodes to mediate their numbers.
      In the case of molasses and especially the store bought refined and heated variety, it contains mostly carbon compounds, which plants normally exude to feed soil organisms when there is excess up to about 7% soil organic carbon. David C Johnson’s work already cited on the blog has shown this. Adding phosphorous is the other I’ve seen that makes plants selfish and not share their carbon with fungi that would otherwise mine phosphorous for them.
      Carbon is the backbone of all life, it is dominant in DNA. Most soils David restores lack soluble carbon to build life forms in the beginning, which makes molasses a great kickstarter to prime those microbes along with the fungi and other microbes already in his high carbon compost.
      There’s also an article on here somewhere where I attempt to improve on his composter with living roots in order to select for endo and ectomycorrhizal fungi that form the symbiotic relationships with plants.

      Liked by 1 person

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