Vegetable Garden Fertilizer Chart With Amounts by Crop and Bed Size

Crops differ most in how much nitrogen they use over a season. Utah State Extension sorts common vegetables into three groups.

Most of a mixed garden sits in the middle row. The high group is where the extra nitrogen goes, and Utah State does not put it all on at once. Its guidance for high-demand vegetables is to apply the intermediate rate before planting, then band or broadcast the additional nitrogen once plants are well established. Sweet corn, onions and potatoes therefore get their extra share as side-dresses.

Legumes are where sources part ways. Utah State lists 1 to 2 pounds of nitrogen per 1,000 square feet for peas and beans, while Colorado State advises avoiding nitrogen where legumes are planted, because it can reduce their yield. Both agree that peas and beans need the least, and in a bed with a history of good bean crops the Colorado answer of little or none is the safer starting point.

Soil organic matter lowers the number for every group. Colorado State's standard for home vegetable gardens is 0.2 pound of actual nitrogen per 100 square feet when organic matter is below 2 percent, half that at 2 to 3 percent, and none at 4 percent or more. A soil test report that lists organic matter is therefore already telling the gardener which row of that scale applies.

The three numbers on a bag are percentages by weight. Clemson Extension explains that a 100-pound bag of 18-4-10 holds 18 pounds of nitrogen, 4 pounds of phosphate and 10 pounds of potash, and the rest of the weight is carrier material that helps spread it.

Penn State's conversion follows from that. The recommended amount of a nutrient is divided by its percentage written as a decimal, and the answer is the weight of product to apply. Iowa State makes the same point from the other direction, noting that the amount of nutrient to apply does not equal the amount of fertilizer to apply.

Penn State bases the calculation on nitrogen first and phosphate second, because an excess of either one causes environmental problems. When a bag supplies more phosphorus or potassium than the test asked for, matching the nitrogen and accepting the extra is the safer error than matching the phosphorus and overshooting the nitrogen.

Organic products follow the same arithmetic with lower percentages, so the weight of product goes up. Utah State lists blood meal at about 12 percent nitrogen and composts at 1 to 3 percent, so meeting a bed's nitrogen need from compost alone takes a large volume of it. Organic materials and their numbers are covered in the homemade vegetable fertilizer guide, and the fertilizer calculator runs the division for any label.

A garden with no soil test can still use a measured rate. Penn State's general recommendation is 1.75 pounds of 10-10-10 per 100 square feet for the season, and University of Minnesota's lettuce guidance is 2 pounds of 10-10-10 per 100 square feet. Either one supplies close to 0.2 pound of nitrogen per 100 square feet, which matches Colorado State's standard for soil low in organic matter.

The weakness of a balanced bag is the phosphorus and potassium that come with the nitrogen. Minnesota notes that many of its soils already have enough phosphorus and advises a low- or no-phosphorus fertilizer unless a soil test specifically calls for more. Utah State's version of the no-test approach is to apply only the phosphorus and potassium amounts listed for soils already testing adequate, which it calls less accurate but conservative.

Rates per 100 square feet only become useful once they are scaled to a real bed. The bed's area divided by 100 gives the share of the rate it takes, so a 4 by 8 foot bed of 32 square feet takes 0.32 of it.

The cup figures use Clemson's rule that a pint of most fertilizers weighs about a pound, and Colorado State's note that a pound of 21-0-0 is about 2 cups. Granule sizes differ between products, so a kitchen scale gives a closer answer for a small bed than a measuring cup does.

The two columns are not added together. The first is a season's balanced feed for a bed without a test. The second is a nitrogen-only amount for a bed whose test shows phosphorus and potassium are already adequate. Either one is split between the pre-plant feed and the side-dresses rather than applied in a single pass.

Side-dress rates are usually written per length of row, because the fertilizer goes in a band beside the plants. The last column scales each published rate to 10 feet of row, which suits a raised bed better than 100 feet does.

The table shows how far apart crops sit. An onion row takes several times the nitrogen of a carrot row the same length, which matches Utah State placing onions in its high-demand group and carrots in the middle one.

Tomatoes and corn share a rate in Minnesota's guidance, but Colorado State does not recommend urea or ammonium nitrate as nitrogen sources for tomatoes, so a tomato side-dress is better drawn from another product. Clemson places side-dress fertilizer 4 to 6 inches from the plants, and Minnesota scratches it into the soil surface before watering.

Every number above is a ceiling for that crop, not a floor to exceed for a bigger harvest. Minnesota Extension warns that too much nitrogen gives bushy, leafy tomatoes and peppers that are slow to bear fruit, and that in carrots it produces leaves at the expense of roots.

Salt is the other cost. Maryland Extension notes that too much fertilizer can burn plants and build excess salt in the soil.

Compost can overshoot too. Utah State cautions that repeated compost applications sized to meet nitrogen needs alone can build up excess phosphorus and salt in the soil over time. A bed fed generous compost every year is the bed most likely to test high in phosphorus, and the one where a nitrogen-only side-dress makes the most sense.

A bed that grows more than one crop in a year does not get one season total. University of Minnesota's guidance is to apply nitrogen immediately before or after planting each annual crop, so a fall planting of broccoli after spring lettuce gets its own nitrogen at its own planting.

Phosphorus and potassium follow the soil test, not the planting count. Clemson suggests a new soil sample each year, and a bed that grows two or three crops a year is the one where that yearly test earns its cost.

Containers use their own schedule rather than any of these rows. Penn State pairs a slow-release fertilizer at planting with a soluble fertilizer every two to four weeks, and Iowa State notes that slow-release products seldom last the whole season in a pot.