Molasses for Plants

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Robert Pavlis

This is a hot gardening topic these days and many of the organic gardeners are promoting the idea that you should add molasses to your compost pile and to your garden. It makes the microbes grow better–they need to eat, don’t you know?

Molasses; should you eat it, or dump it onto your soil? You have come to the right place to get the facts.

Molasses for Plants
Molasses for Plants

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. Molasses contains sugar, some other carbohydrates, vitamins and a number of minerals like calcium and iron.

Molasses for Plants

You probably know that it is important to have microbes in your soil. If not, have a look at Organic Fertilizer – What is its Real Value. If having microbes is important, than it makes sense that you should feed those microbes. Feeding them will make them healthy, and make them reproduce so that you have even more microbes. Guess what? Microbes, especially the bacteria, love sugar. It’s no surprise that they also love molasses since it is mostly sugar.

Microbe Science for Gardeners Book, by Robert Pavlis

So far it all seems to make sense. Microbes are good for soil, and molasses is good for microbes, so why not add it to soil? The short answer is that there is nothing wrong with adding molasses to your garden, or to your compost pile. It will feed the microbes.

Does it Make Sense to Add Molasses?

I’ll save you the trouble of skipping to the end of this post–the answer is NO!

Understanding why the answer is no will help you understand your garden. Let’s have a look. In a normal garden, or compost pile, you have a large variety of microbes, all going about their daily lives. They find something to eat, they poop, and they die. This is a continual process that goes on a billion times a second.

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 a massive famine and most of the bacteria die.

Molasses causes a population explosion, which quickly dies down once the sugar is used up.
Molasses causes a population explosion, which quickly dies down once the sugar is used up.

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.

Is the massive population explosion good for your plants? I don’t think anyone knows, but most things in nature are better off without massive changes, and plant roots depend very much on the population of microbes around their roots. I just can’t believe a bacterial population explosion is good for the plants.

Building Natural Ponds book, by Robert Pavlis

Molasses might make your compost pile work quicker, but the first rain, or your hose, will wash the sugars out of the pile removing any benefits.

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−1in 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 reason for adding molasses is to feed the microbes, so it is important to ask, “Should the gardener 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.

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 in the garden. 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.

References:

1) Photo Source: Йоана Петрова

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

131 thoughts on “Molasses for Plants”

  1. Today’s chuckle …
    “Molasses contains a huge array of crucial minerals needed for weed plants to survive and thrive. Alongside this massive benefit, it helps bring the soil to life by feeding beneficial bacteria and fungi . This then enables plants to tap in even more nutrients already present in the soil.”

    🤦🏿‍♀️

    Reply
  2. I bought some molasses for horses, so really cheap and decided to give it a try. I chose 12 squash plants and gave 6 of them molasses, about two tables spoons in a couple of gallons of water, and this was split between the plants.
    To my utter amazement within 48 hours they looked considerably better than their neighbours. How could this happen so fast?
    So I chose another 14 zucchini and 6 received the molasses water.
    The second day after 48 hours, I went out in the evening to water them again and noticed that 8 of the plants were drooping in the heat (in the high 80sF all afternoon). But six of the plants were still upright and looking very happy. And yes they were the ones who had the dose of molasses.
    So by the next morning the 6 plants were clearly bigger and stronger than the others.
    So now with another 10 hot days forecast most of the rest have been given some molasses.
    Your arguments against molasses contain no evidence which you demand for people who support it.

    Reply
    • “So by the next morning the 6 plants were clearly bigger and stronger than the others. – that is physically impossible – plants do not grow that quickly no matter what you give them.

      Reply
  3. All the BLAHBLAH doesn’t matter, if you re removing any AG product from its growing medium then you are removing nutrients from the soil, medium etc., so unless you replace it with those elements you removed with say a carrot, then in time you will need to replace it, or get starving food, and lackluster growth, so it doesn’t matter it needs to go back in, I don’t see one post that somebody did a tissue sample to see if a plant has benefited or been stressed by Molasses. every couple of years I pay the $20 per tissue sample and feed my soil accordingly, food tastes better, keeps better and produces more, and less pests too.

    Reply
  4. Tried unsulphered blackstrap molasses and did soil tests before and after. St Augustine lawn benefited greatly from the first . But it has an acidifying effect. The lawn dropped its ph slightly from a higher pH range which was the main effect I witnessed. A similar but to lesser affect of chelated iron. I think it’s good to use when you wanto slightly lower pH and give alittle soil boost in the process. I used too much molasses the second season and found a little ag lime to spring it back from the grey wilty over acidic condition it caused from going too hard .

    Reply
  5. There is a body of literature on the use of molasses in agriculture. Clear to me that soil composition is critical, as well as crop selection.
    I just took a slice from one study:

    A. Pyakurel et al. (2019) Int. J. Appl. Sci. Biotechnol. Vol 7(1): 49-53
    Cleasby TG (1957) Use Of Molasses On The Land. Tongaat:…

    Tongaat Sugar company Ltd.
    Dahal BR and Bhandari S (2019) Biofertilizer: A next generation
    fertilizer for sustainable rice production. International Journal of Graduate Research and Review 5 (1):1-5.

    (2015) Effect of organic fertilizers on soil chemical properties on vineyard calcareous soil. Agriculturae ConspectusScientificus 80(2):79-84.
    Kavithal V and Ramdas V (2013) Nutritional composition of raw fresh and shade dried form of spinach leaf. An international Journal 1 (8): 767-770.

    Samavat S and Samavat S (2014) Effects of fulvic acid and sugarcane molasses on yeild and qualities of tomato. International Resaerch Journal of Applied and Basic Sciences 8(3):266-268.
    Sanli A, Karadogan T and Tosun B (2015) The effects of sugar beet molasses application on root yield and sugar content of sugar beet (Beta vulgaris L.). Tarla Bitkileri Merkez Arastirma Enstitusu Dergisi 24 (2):103-108.

    Treatments (5×5)

    Yield was found maximum when organic fertilizer was applied along with molasses (3.08) and least yield was obtained in control plot (1.22) as shown in Table 4. The difference is significant at 1% level of significance. Molasses increased root and shoot length, and also root and shoot dry weight (Suliasih and Widawati, 2017). Sugarcane molasses showed better result in terms of shoot and root length, fresh and dry weight of tomato plant than ash or other source of nutrients (Vawdrey and Stirling, 1997). Application of molasses increase yield of leafy vegetables (Chandraju et al., 2…..

    Reply
      • This is from research 2020 using Black Strap Molasses, Epsom Salts and Food Wastes. They had very promising results. Your point that adding the molasses to the compost is unnatural and it is but no more unnatural than hot composting. In growing our food, it is also unnatural but in trying to stay within parameters and growing with the least amount of harm maybe the new motto. I am thinking we have to develop new methods and practices. The ones where there is no money to be made if basically supplied by natural then it is left to the small farmer and gardeners to develop.

        https://jestec.taylors.edu.my/Vol%2015%20issue%202%20April%202020/15_2_10.pdf

        Reply
        • “adding the molasses to the compost is unnatural” – I don’t think I ever said that.
          I don’t understand your point. molasses and Epsom salts do contain nutrients plants will use – so what? What is the environmental cost of making and shipping the molasses? You need to include that in your evaluation. Then add in the fact that molasses can be used as a food source instead of applying it to soil – is that not better use in a world with starving people?

          Not sure why you listed the reference. The research shows that when you mix molasses, Epsom salts and food scraps you produce a fertilizer product that contains the nutrients plants need. So what? You get the same thing from just food scraps.

          Reply
  6. Anecdotal evidence is often the basis for studies. I think you’re unnecessarily combative without actually reading what you’re arguing with. I thought the scientific method began with a hypothesis. The whole point of science is to prove or disprove theories and build a foundation of knowledge based on proof. All that your reader was saying above was that the anecdotal evidence and widespread use of molasses as an alleged beneficial fertilizer might warrant properly controlled scientific study.

    Reply
    • In general what you is correct. Anecdotal information (it is not evidence) can lead to studies. But in a case where we understand how fertilizer works quite well, and we know the chemical components in molasses, we don’t really need to do any testing. We already know what any nutrients in molasses will do in soil.

      Reply
  7. I am going to NMSU for sustainable agriculture I have 20 years commercial experien .One thing you tend to leave out ,I am talking about the Author , Certain species of plants are better accustomed to different terroirs ,some thrive in acidic soil ,some cannot live at all in acidic mediums,my personal experience that I have detailed in control studies and documented with grow journals with photographs I can show you the whole life span vegetative and flowering phases to harvest and total biomass with the non and added and control plants and my results proved Unsulphured molasses does make a difference actually . Especially in cannabis crops.One thing you fail to realize in Botany is there is not a one size fits all approach to all species,Certain plants prefer certain environments even phenotypic expression within certain genotypes is affected .So I think you should look into UC Davis studies and academic papers that have tested your theory .I thought molasses was malarchy and then I did the test ,I was wrong.So you can find published journals on it but what you fail to mention is differences in sulphured and Unsulphured because sulphur is actually bad for microbes and fungi and bacteria

    Reply
    • You did a study. You say there are studies showing molasses works. Why have you not listed a signal link to such a study?

      “One thing you fail to realize in Botany is there is not a one size fits all approach to all species” – of course I understand this.

      Reply

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