For citrus, Texas A&M fruit specialist Monte Nesbitt recommends giving your citrus trees 60% of their annual nitrogen on Valentine’s Day, 20% on Mother’s Day, and the remaining 20% on Father’s Day. However, this schedule assumes you’re using ammonium sulfate (21-0-0), a “fast-release” chemical fertilizer. How should this schedule be adjusted for other fertilizers?
To figure this out, I first needed to learn the rate at which the nitrogen in cottonseed meal becomes available to plants. I found the answer in a 2013 study (see here: https://journals.ashs.org/hortsci/view/journals/hortsci/48/7/article-p891.xml), which determined for a residential soil, plant-available nitrogen from cottonseed meal peaks at approximately 30 days after application, and then decreases to approximately 50% of that maximum over the next 30 days. This suggests that to follow a schedule similar to Mr. Nesbitt’s using cottonseed meal, the fertilizer would need to be applied approximately one month earlier! In other words, if I want my citrus trees to have peak available nitrogen on Valentine’s Day, I need to start fertilizing now in mid-January.
I also started to wonder about nutrient timing, the unpredictability of weather and citrus growth flushes, and what the nutrient availability would be like if I fertilized on a different schedule. Fertilizing three times a year based on arbitrary dates seems to come with the risk of fertilizing when the trees aren’t growing due to unseasonable weather, or not fertilizing when they are. I think a better approach for the home grower may be to aim for a more consistent level of nutrient availability over the growing season, by applying cottonseed meal on a monthly schedule (see chart below). For 2020 I’m going to try giving my trees 30% of their annual fertilizer allotment in January, and then giving them another 10% each month through August. I think that following such a schedule will mean that my trees will always have what they need, when they need it. I’ll let you know how it goes!