By Steve Reiners, Professor and Chair, Horticulture Section, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic that ran Spring-Summer 2020. Read more articles from 2020 | Read later articles in this series.
June 26, 2020
Plants are a lot like us. They get hungry. Even though we may have a big breakfast, we start looking for something to eat a few hours later. As you prepared your vegetable garden this spring, you likely added compost and fertilizers to feed your plants. But your plants may need a snack too.
Some nutrients that were added in the spring may have leached away – carried by water so deep in the soil that the crop roots can’t reach them. The critical nutrient nitrogen can be lost to the air. For these reasons, we don’t recommend adding all the nutrients your crops will need first thing in spring.
Vegetables don’t take up many nutrients soon after planting. If direct-seeded, they use the resources in the seed for the first two weeks. If transplanted, they use the nutrients in the starter solution you hopefully provided at planting. As the plants begin to grow rapidly, usually a few weeks after planting, their nutritional needs increase exponentially.
Nutrients need to be present during this time of rapid growth to achieve maximum yields. That’s why we recommend sidedressing some plant food at least once during the growing season. Plus, annual vegetables tend to have a brief window when they take up most of the nutrients they need.
Take sweet corn, for example. If we don’t have nitrogen easily available in the soil at the right time, it yields poorly. That critical period is when the corn is about 8-12 inches tall. Once larger, all the fertilizer in the world won’t make a difference.
In contrast, a long-season crop like tomatoes needs nutrients throughout the growing season.
We call adding nutrients after the crop is established sidedressing. It is typically done by either sprinkling some fertilizer around individual plants or by applying it in a band near the planted crop.
Some general rules of thumb, sidedress:
- leafy vegetables about 3 to 4 weeks after planting
- root vegetables about 4 to 6 weeks after planting
- squash, cucumbers, and other vining cucurbits just as the vines begin to spread and cover the ground
- for tomatoes, peppers, and eggplants, start about 4 weeks after planting and then every 3 to 4 weeks
What should you use? You have lots of choices. Fertilizers vary in how “available” they are to the plant. In general, organic materials like compost and rock phosphate are slowly available. They require soil microbes to release the nutrients in forms that the roots can absorb.
Synthetic fertilizers, like calcium nitrate or a soluble fertilizer like Miracle-Gro are immediately available to the plant without microbial action. The quick availability comes with a price as it is easy to burn your plants if you apply too much or put it too close to your plants. Plus, they just feed the plants directly rather than feeding both the plants and the soil like organic materials do.
For an organic gardener, adding a handful of compost is fine but a more quickly available option would be cottonseed meal or blood meal. A couple of tablespoons per plant is good for widely spaced plants like tomatoes. Two tablespoons per foot of row would be fine for row crops like beets and carrots. Try to apply before a rain or irrigate immediately after. You can mix fish emulsion with water and apply it as a liquid fertilizer. Just follow the directions on the label.
For those not following organic practices, you can use synthetic nitrogen fertilizers like calcium nitrate or ammonium sulfate,. Or a complete fertilizer like 10-10-10 that provides nitrogen, phosphorus and potassium, the three major nutrients that crops need. A tablespoon per plant is fine, just don’t apply too close to the plant as injury can occur. A synthetic soluble fertilizer mixed with water is quickly available as well.
Want to see your plants green up overnight. Add a tablespoon of Epsom salts (magnesium sulfate) to a gallon of water and sprinkle on the foliage. The magnesium gives the plants a boost and increases photosynthesis.
This week in the garden:
Keep up with the watering. And don’t forget to keep your compost moist to keep those microbes active. It’s not too early to start planning for your fall garden. Broccoli, cauliflower, beets, lettuce, spinach and other cool-season crops do great with a midsummer planting and harvest after frost. Make sure you have the seeds for planting. These can follow spring crops like peas and lettuce.