Ask the Scientist: Soil amendments for greenhouse tomatoes

Rodale Institute Compost Production Specialist Rick Carr talks about what is happening in our fields and yours.

Marissa asks:

I work for an organic tomato greenhouse up in Maine. We are using a soil-less mix, compost and fertilizer from a local gentleman. It has produced wonderful results, however, already at this point in the season we have depleted our mix of nutrients and our heirloom varieties are starting to show flower abortion and die off. I was wondering if you might help in figuring out how we can overcome this issue with top dressing with compost and specific amendments. Thank you for your time.

Rick says:

First, congratulations on your organic tomato operation – sounds tasty. You mentioned using compost as your media. Is it 100% compost? If so, you will need to be careful with watering. Compost can provide numerous benefits to plant production, regardless of the plant, but it can be a double-edged sword. Amending too high may damage your plants. Some causes for damage are the organic salt content of the compost as well as the water holding capacity. Too much water will drown plant roots – an effect that is visualized on the furthest growing parts.

Because not all compost is created equally, it’s not possible for me to predict what effect your material will have on plant growth. Therefore, I recommend never amending with more than 50% compost. I am currently testing three different compost materials here at Rodale Institute to demonstrate various amendment rates. The rate of compost amendments is viewed as a bell curve – the extremes are lower than the middle. I am still crunching the data but, as a general observation, growth at 100% compost is less than 50%. One compost even prevented seed germination at 100%. For two of the compost materials, the best amendment rate is somewhere between 50-75%, while the third is between 25-50%. The compost mix is a soil-less media containing compost, pine bark fines, rice hulls and coconut coir.

If we are confident that your die back is due to nutrient loss, I have a couple options for you: top-dress with compost or prepare and apply compost extracts. Here is a fact sheet on compost extracts: Compost Extracts Fact Sheet. Not knowing your compost, I will conservatively suggest that your compost-to-water mixture should be between 1:30-60 compost:water (v:v). I hope that helps. Good luck!

4 Responses to “Ask the Scientist: Soil amendments for greenhouse tomatoes”

  1. Shin Kai

    Unfortunately, Rick, you did not actually address the most important question: What is the cause for the blossom drop in Marissa’s tomatoes. I am a grower myself and typically the main cause is almost NEVER a lack of nutrients in the growing medium. Plenty of studies show that. Instead most often temperatures at both ends of the extreme and humidity are the cause of blossom drop in tomatoes.

    While a lack in N can in cause blossom drop, it is far more likely that either temperature or humidity have been at an extreme value during a period of time. Other more likely explanations are lack of water, lack of pollination, as well as insect damage or disease.

    I find it surprising why Rodale Institute avoids answering this question by rambling about an experiment with little bearing to the actual problem of blossom drop. Also, it seems that Rick mistakes compost and compost teas for a fertilizer, which they are not. While aerobic compost might solve the problem, it is far from clear whether nutrient deficiency was the cause. Please, Rick, apply the scientific method with more rigor or we growers will lose confidence in such recommendations.

    Shin Kai

  2. amanda

    Thank you, Shin, for noting that in addition to nutritional stress, environmental stress can also cause blossom drop.

    The team here at Rodale Institute tries to answer reader questions to the best of our abilities. Marissa specifically asks for compost and amendment recommendations and since we can’t actually visit her greenhouse in Maine, we have worked on the assumption she has determined lack of nutrients is her problem as she states.

    In making these Q&As public, our hope is that other growers and researchers will chime in with additional information on their experiences, observations and research results, too.

  3. Trina

    Thanks for that Compost Extracts Fact Sheet. I notice you didn’t use any unsulfured molases?! What is your take on that?
    AND…I could find no mention of what material the “bag” is made of. And please explain why using a bag is part of your procedure?

    AND… do you agree with one of the comments that compost is not fertilizer? What about vermicompost and/or vermicompost tea?

  4. Ron

    I used to grow greenhouse tomatoes. If you are losing the fifth or sixth cluster it could be the plant trying manage the fruit load. That was a common problem I had in the spring when I didn’t manage my fruit pruning properly. It could also be ethylene gas too, that sometimes happens when using CO2 generators or combustion issues. I once lost several clusters when I thought I would be clever while trying dehumidify my greenhouse by running an exhaust fan AND the furnace at the same time…big mistake!

    Best if luck to you.


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