Molasses for Plants

This is a hot gardening topic these days and many of organic gardeners are promoting the idea that you should add molasses to your compost pile, to compost rea or even spray plants with it. In tea and on soil it makes microbes grow and on plants it can prevent pests.

Molasses; should you eat it, or use it in the garden?

Molasses, What is it?

Molasses is a by-product produced during the manufacture of sugar. Sugar cane or sugar beets are processed so that the sugar can be extracted. The material that is left after most of the sugar is removed is a black sticky material called molasses. It contains sugar, some other carbohydrates, vitamins and a number of minerals like calcium and iron. It may or may not contain sulfur.

Is Molasses Good for Microbes?

Microbes are opportunistic in that their populations will increase and decrease as the conditions change. Let’s assume you have not been doing too much in the garden so conditions are not changing. In that case the microbe populations remain steady. Things are chugging along at a normal pace and everybody is happy.

Now you dump a lot of molasses on the garden. Instantly, microbes sense the extra food and they start to multiply. Bacteria in a lab can divide (ie double the population) every 20 minutes. The population explodes very quickly. All those bacteria need to eat and they quickly consume the molasses you added. As the food source runs out there is massive famine and most of the bacteria die.

What has the molasses accomplished?

Not much. It is true that all of the dead bacteria go on to feed other microbes, and they help build soil structure. The minerals in the molasses stay in the soil and plants can use them, but your soil probably had enough calcium and iron before you added the molasses. The vitamins in molasses are of no value to plants.

The burst of microbe growth is short lived and adds very little value to the garden and it won’t increase the microbe population except for a very short period of time.

You might be thinking that molasses is organic and therefore it must be good for the garden. Organic fertilizers are good for the garden because they decompose slowly, feeding both microbes and plants over a long period of tine. Adding sugar does not do this.

Should Molasses Be Added to a Compost Pile?

It does not add much in terms of nutrients but people like adding it to grow microbes. As noted above, it does not really work since the sugar is used up too quickly. Sugar is also easily washed out of the pile with the first rain.

Will Molasses Keep Pests Away From Plants?

There might some benefit here if the molasses contains sulfur. Sulfur compounds are known to repel insects.

One problem with this solution is that sugar attracts insects who use it as a food source. Sugar can also lead to black mold growing on leaves which can harm the plant and attract insects.

Even if the sulfur repels insects, there are better options. Sprays of garlic or onion juice would be much more effective and they don’t contain as much sugar.

Molasses in Compost Tea

Molasses is a common additive when making compost tea. Gardeners believe that it results in higher microbe numbers and they are right. The sugar in molasses is candy for microbes and they gorge on it. I don’t think molasses is any better for this job than white sugar, but maybe?

There have however been some interesting studies that show molasses does grow more pathogens, especially in compost tea. “Salmonella populations increased from 1 to over 1000 CFU ml⁻¹ in dairy manure compost tea with 1% molasses, and from 1 to over 350,000 CFU ml⁻¹ in chicken manure compost tea by 72 h. E. coli populations increased from 1 to approximately 1000 CFU ml⁻¹ in both types of tea by 72 h. Pathogen regrowth did not occur when molasses was eliminated or kept below 0.2%.”

Do You Need to Feed the Microbes?

The main reason for adding molasses is to feed the microbes, so it is important to ask, “is it important to feed the microbes?” The answer is a resounding YES! However, there are many ways to do this. Adding compost, wood chips or other organic matter as a mulch is the best way. This provides a slow, steady release of food for the microbes.

You also don’t need to add purchased microbes.

Molasses is a product that we can use to feed people and animals. I’d rather eat gingerbread cookies than compost and wood chips. From an environmental point of view it makes more sense to put non-edible organic matter in the garden and keep the food in the fridge.

There is no “magic” in molasses. It’s just another source of organic matter that will be decomposed very quickly. All organic matter contains carbohydrates, sugars, minerals and vitamins, just like molasses. Don’t believe me …… consider the fact that molasses is made from plants; sugar cane or sugar beets.