Why Container Plants Need More Fertilizer Than Garden Beds

Why a pot runs out of nutrients

Container plants need more fertilizer than plants in a garden bed for three reasons. Their roots are confined to the pot, potting mix holds nutrients poorly, and frequent watering washes nutrients out through the drainage holes. A workable schedule from Penn State Extension is a slow-release fertilizer at planting time plus a soluble fertilizer every two to four weeks.

The first reason is the simplest. Penn State explains that plants in pots only have access to the nutrients available in their pot, while in-ground plants can expand their root systems to reach nutrients in the surrounding soil. A plant in the ground that uses up what is near it grows toward more. A plant in a pot cannot.

The second reason is the mix. Garden soil holds nutrients on its particles and releases them to roots over time, a property measured as cation exchange capacity, or CEC. Iowa State University Extension describes CEC as a magnet that can hold nutrients, and states that potting media has very little of it and so is very poor at holding nutrients that plants can take up. What you add stays available only for a short time.

Watering washes fertilizer out

The third reason is water. University of Illinois Extension notes that frequently watering containers results in the leaching of nutrients, which then need to be replaced on a regular basis. Iowa State gives the same two causes for the decline in potting mixes: plant uptake and leaching during watering.

How fast this happens follows how often you water. Iowa State puts the range at once or twice a day for a small container in hot, windy weather, down to once or twice a week for a large container in cool weather. A small pot in July is therefore flushed many times more often than a large one in May, and it needs feeding more often to match.

The leaching has a useful side. University of New Hampshire Extension points out that frequent watering washes salts from the soil mix and keeps it cool, which prevents root injury. The answer is to replace what is lost, since watering less would trade a nutrient problem for a salt problem.

How long the fertilizer in the bag lasts

Many commercial potting mixes contain a slow-release fertilizer, and many gardeners add more at planting. These products are water-soluble fertilizer encased in a semi-permeable resin coating, as Illinois describes them, and small amounts of nutrients are released when they come in contact with water. Illinois puts the supply from products meant for annuals at 3 to 4 months, depending on moisture and temperature.

Temperature is the part that catches people out. Iowa State gives an example for controlled-release fertilizer: a product labeled for 6 months may last only 4 or 5 months at 80°F, and 3 or 4 months at 90°F. By those figures, a product that promises six months on the bag can come up one to three months short in a hot summer.

For that reason Iowa State cautions that slow-release fertilizers usually do not last the entire growing season. Its advice for a mix that contains one is to begin fertilizing when plant growth slows or the foliage color fades, which is typically about mid-season. New Hampshire takes a more relaxed view for vegetables and says a slow-release fertilizer applied at the recommended rate need be applied only once. The two fit together if you treat the single application as a base and watch the plants from midsummer on.

A feeding schedule for containers

The extension sources agree on the approach and differ on the interval. All of them use a water-soluble fertilizer at regular, fairly short intervals, because the nutrients do not stay in the mix.

Source	What to apply	How often
Penn State	Slow-release at planting, then a soluble fertilizer	Every two to four weeks
Iowa State	Water-soluble fertilizer	Every 1 to 3 weeks through the summer
Illinois	Liquid fertilizer	Every two weeks or so
Illinois, alternative	Half strength solution	With every watering
New Hampshire, vegetables	Liquid or water-soluble fertilizer at dilute concentration	Once a week, after 3 to 4 weeks of growth

Put together, the schedule looks like this:

1. At planting, mix in a slow-release fertilizer at the label rate. New Hampshire also suggests adding up to 50 percent compost to the potting mix before planting to supply some nutrients early, and Penn State notes that compost raises the water-holding capacity of the mix as well.
2. Let the plants grow for 3 to 4 weeks on that base.
3. Begin a water-soluble fertilizer. Pick one interval from the table and keep to it: weekly at a dilute rate, or every two to four weeks at the label rate.
4. Now and then, water with plain water only so that it runs through and leaches the pot, as Illinois advises.

Small, frequent doses suit a pot better than large, rare ones. Iowa State stresses low doses because the nutrients will mostly leach out of the potting media with watering, and notes that commercial greenhouses feed most plants with low doses of water-soluble fertilizer at every watering. For a dilute weekly feed, New Hampshire gives two example rates: 1 ounce of 20-20-20 per 4 gallons of water, or 1 to 2 tablespoons of fish emulsion per gallon of water.

Choosing the numbers on the label

A fertilizer label gives the proportions of nitrogen, phosphorus and potassium, the order Iowa State uses when it describes a ratio. Illinois gives a simple way to choose. High nitrogen sources suit plants grown for their foliage, while flowering and vegetable crops prefer lower nitrogen and higher phosphorus types.

Iowa State is more specific for container plantings and suggests looking for a complete fertilizer with a 1:2:1 ratio of nitrogen, phosphorus and potassium to keep plants healthy and blooming well. New Hampshire’s vegetable example uses an even 20-20-20. Either works as a general choice. The interval matters more than a small difference in ratio, because a pot that is fed too seldom runs short whatever the label says. For crops that suit pots, see the list of vegetables for container gardening.

What the leaves tell you

A calendar gets the feeding roughly right, and the plant shows when to adjust. Iowa State describes the two most common shortages by where they appear.

Nitrogen deficiency looks like a general yellowing of the foliage and is usually more pronounced on the old leaves. In a pot in midsummer, that pattern suggests the feedings are too far apart.

Iron deficiency shows up as yellowing between the veins, called interveinal chlorosis, on the young leaves. The cause is often the pH of the mix and not a lack of iron in it. At a high pH, Iowa State explains, nutrients like iron are converted into forms that plants cannot use. Adding more of a general fertilizer does not solve that.

Too much fertilizer leaves its own mark. The University of Georgia’s bulletin on indoor plants describes excess soluble salts as a whitish crust on the surface of the growing medium or near the rim of the container. Its remedy is to apply large quantities of water to leach the salts out, after discarding any water left standing in the saucer. Iowa State admits that without measuring the salt content of the mix, it is hard to know exactly how much fertilizer is available, so the crust and the leaf color are the practical guides.

Mix that has been through a season has lost nutrients to uptake and leaching, and may carry fertilizer salts. If you plan to use it again, the steps for reusing old potting soil cover how to refresh it, and houseplant growers who water from below can find the matching flush routine in the guide to bottom watering.