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.
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!