Roots and Shoots: What to Feed the Food?

Making sense of fertilizer

By Pamela Doan

As an organic gardener, I’m suspicious and baffled by the options in the fertilizer aisle. My approach has been to do nothing. Prior to planting, I add compost and organic matter to build up healthy, rich soil in my raised beds and nature does the rest. But I get mixed results. I turned for advice to Jarret Nelson, who oversees 6 acres of organic vegetables at Glynwood and feeds hundreds of people through its CSA.

If you start with healthy soil, do vegetables need fertilizer?

It depends how much compost you start with. The primary nutrient requirement for plants is nitrogen, and it can be released slowly or quickly. Compost, like most organic sources, releases a steady amount as it breaks down. If you started with a pound per square foot (roughly a 2-inch top dressing), you’re probably fine. If you didn’t, you might want to add organic fertilizer.

Do all vegetables need the same amount?

There can be significant differences. A lettuce head that will be there for 50 to 60 days needs a lot less than a tomato plant that’s in for 100 days. Some crops are heavy feeders: tomatoes, potatoes, cabbage, Brussels sprouts, eggplant, peppers, cauliflower, broccoli. If you have kale or chard and want to keep them going for a long time, you need more fertilizer.

The label on fertilizers says N/P/K, for Nitrogen, Phosphorus/Phospate and Potassium/Potash, giving the amounts that are included. How much of each do you need?

Phosphorus and potassium are less important than nitrogen. Nitrogen is soluble and leaches out of soil quickly. That’s why you need to have things continuously breaking down. Phosphorus is not very soluble and if you have high levels of it, you don’t need to add any.

Potassium leaches out but not as much as nitrogen and you may not need to add more. Soil tests are a good idea to know what’s going on.

Have Your Soil Tested

Master Gardeners from the Cornell Cooperative Extension of Putnam County will have a booth at the Cold Spring Farmers’ Market at Boscobel from 9:30 a.m. to 1:30 p.m. on Saturday, July 8, to answer gardening questions. They will also offer soil pH tests for $5.

Can you do harm by adding fertilizer?

One issue is runoff and too much nitrogen will also attract certain pests like aphids. It changes something about the biochemistry of the plant.

Phosphorus stays in the soil and you can turn your soil infertile if you continuously add too much. You’d have to be putting in a fair amount every year, though. I’m not sure if you can overdo potassium, but going crazy dumping stuff isn’t a good idea.

If I check pH balance for acidity and alkalinity, does that indicate the nutrient level?

That tells you how easily plants can uptake nutrients. Adding nutrients won’t help if pH is too low or too high. You want to be in the 6-to-8 range.

What does “organic” fertilizer mean when you see it on a label?

If you see OMRI, that means it’s Organic Materials Review Institute certified. That’s the only claim that means something, which is that there are no chemicals. Most OMRI fertilizers are made from products such as soybeans, fish, seaweed, blood, feather or bone, peanuts, alfalfa.

Adding compost before you plant vegetables ensures a steady diet during the season. (Photo by Filippo Giuliani/Shutterstock)

How are they different from conventional fertilizers?

A lot of conventional fertilizers are ammonia based, created in a lab, and made out of natural gas. That’s part of why it’s an environmental issue. Also, because it’s based on a finite resource, it’s not sustainable. They are a lot more powerful. A common organic fertilizer will be 5/3/4 (N/P/K) while a common conventional one will be 30/30/30.

Should you add manure to compost or directly?

Manure is a good nutrient source. There are some food safety concerns, though. Either apply it in the fall before planting in the spring or add it to compost. I wouldn’t recommend it as a mid-year fertilizer. In general, liquid fertilizers are more quick acting and work well later in the season for a boost. Solids are better before you plant. If I were a gardener, I would just put a lot of compost down.

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