Best Fertilizer for Vegetables by Garden Type

The three numbers on the front of the bag are the percentages by weight of nitrogen (N), phosphate (P2O5) and potash (K2O), always in that order. A 100-pound bag labeled 10-10-10 therefore holds 10 pounds of each.

Each of the three does a different job in the plant, which is why one bag does not suit every crop.

A bag adds all three whether the soil needs them or not, and extra phosphorus is not harmless. Too much can aggravate iron and zinc deficiencies and raise the salt content of the soil.

A soil test shows which nutrients the soil already holds, which makes it the best first step in deciding what to apply. Where phosphorus and potassium test very high, a nitrogen-only fertilizer is an appropriate choice.

Both kinds grow good vegetables, because roots take up the same form of each nutrient whichever bag it came from. What differs is how quickly the nutrient arrives and what the fertilizer does to the soil around it.

Organic fertilizers such as blood meal, cottonseed meal and manure release over a fairly long period, since soil organisms have to break them down first. That makes them work best in moist, warm soil, and it means they can fall short of a crop's nitrogen need early in the season.

To mix your own from manure, bone meal and wood ash, see how to make homemade vegetable fertilizer.

Synthetic fertilizers generally act more quickly, but they do nothing for soil structure. Over-applying a nitrogen fertilizer such as ammonium sulfate burns roots and can stunt or kill the plants.

Specialty blends labeled for roses, azaleas or vegetables are generally an expensive alternative to a standard blend applied at an adequate rate.

What you harvest decides how much nitrogen a crop wants, and the University of Nevada, Reno groups vegetables on that basis.

Nitrogen has its greatest effect for three to four weeks after it goes on, so heavy feeding, or a side-dress before fruit set, can leave tomatoes all vine and no fruit, and in Virginia Tech's example a heavy feeding on June 1 may delay flowers until July 1 and ripening until late August.

Heavy manure or slow-release fertilizer on fruit crops keeps leaf and vine growth going into late summer, so the fruit develops very late in the season.

Root crops need less nitrogen than leafy ones. Carrots and beets also react to fresh manure, which can give misshapen roots.

In a bed, raised or in the ground, the organic matter in the soil decides how much nitrogen you have to add, because it releases some on its own as it breaks down. Colorado State's yearly amounts for a home vegetable garden shrink as organic matter rises and reach zero at 4% or higher.

To turn pounds of nitrogen into pounds of fertilizer, divide by the percent nitrogen on the label. A 10-10-10 bag is 10% nitrogen, so 0.2 pound of nitrogen takes 2 pounds of that fertilizer per 100 square feet.

Sandy soil needs nitrogen more often than clay. Organic matter helps there because it holds over ten times more water and nutrients than sand does.

Pots and planters

Frequent watering drains nutrients out of a container, so plants in pots need fertilizer regularly whatever the potting mix. Slow-release fertilizer can supply nutrients for 2 to 3 months, liquid feeds have to go on more often, and long-season crops such as cucumbers, tomatoes, eggplant and peppers may need a light feeding every two weeks to keep producing.

A potting mix with fertilizer already in it supplies plants for eight to ten weeks, and a water-soluble feed every two to three weeks follows. A pot has no deep soil to cushion a heavy hand, so a dose above the label rate burns roots and can kill the plant.

Clemson, writing for South Carolina, puts all the recommended phosphorus and potassium on when you prep the bed, along with 30 to 40% of the nitrogen. The rest of the nitrogen goes on 3 to 4 weeks after planting, broadcast or side-dressed.

On sandy soil that second dose is split in two, 3 weeks after planting and again 3 to 4 weeks later.

Pale green or yellow growth and a slower growth rate about four to five weeks after planting mean vegetables may need more nitrogen, though peas and tomatoes are left out of that extra dose.

Tomatoes are where the sources disagree. Colorado State side-dresses them when the first fruits reach 1 inch, while Oregon State and Nevada advise against extra mid-season nitrogen for them. A soil test that shows high nitrogen at the start of the season may mean none is needed that year for any crop.

The vegetable garden fertilizer chart turns these rates into pounds of fertilizer for a bed of your size. The full schedule, from bed prep to each crop's side-dress, is in when to apply vegetable fertilizer.

Fertilizer burns plants when its salts sit against young roots or leaves, so where it lands matters. Fertilizer placed in or too near the seed row exposes young roots to excess salts and causes root burn, which is why it is banded to the side of the seeds and below them.

Fertilizer goes on in one of three ways.

Once the fertilizer is down, water moves it to the roots. Nitrogen in chemical fertilizers dissolves readily and travels with irrigation or rain, so you need to water the bed yourself if no rain falls within a day. The same watering washes stray granules off the leaves, which avoids scorch after broadcasting.

Where a symptom starts on the plant narrows down which nutrient is short, and the pattern is worth matching against the list below before any more fertilizer goes on.

A foliar spray can correct a deficiency fast, since absorption begins within minutes and for most nutrients finishes within 1 to 2 days. It supplements soil feeding at a critical moment because leaves cannot absorb as much as the plant needs. Applying it early in the morning lowers the risk of burn, which rises as the air warms, and a spray stronger than the label burns foliage quickly.