How To Apply Nitrogen To Garden

The Source of the Plant Problem

No more spotty turf, skimpy blooms, or stingy vegetable plants. If that's your mission, you're not alone: Homeowners spent over $6 billion on fertilizers in 2013. But randomly adding fertilizers won't produce a beautiful bounty; both plants and soil require a nutrient-rich diet targeted to their needs. Ahead, we answer your questions, troubleshoot problems, and guide you toward the ideal mix for your lawn, flowers, veggies, shrubs, and trees. You'll learn what to sprinkle, spray, mix in, and spread around for a garden that's the envy of the neighbors.

Fertilizers vs. Soil Amendments

Q: How do fertilizers and soil amendments differ?

A: Fertilizers, which can be organic (e.g., bone meal) or chemical (such as superphosphate), provide nutrients directly to plants, especially nitrogen (N) to fuel leaf and stem growth, phosphorus (P) to stimulate root growth and seed formation, and potassium (K) to promote flowering and fruiting. Amendments, such as peat moss, animal manures, and compost, indirectly impact plants by improving the structure, texture, and microbial activity of soil. A top layer of organic mulch, such as pine bark, to conserve moisture and deter weeds, also feeds the soil as it decomposes. Used together, these can all help deliver the right nutrient balance to your garden.

Stop the Struggle

Q: All my plants seem to struggle. What should I add?

A: First, feed your soil. Top-dressing beds with several inches of well-aged compost in spring and fall will increase soil's ability to hold nutrients and water, make it "fluffier," and stimulate microbial action. Once you've improved the soil, fertilizers—as indicated by a soil test—will fill in any deficiencies and give your plants a shot in the arm.

Soil Tests

Q: Do I really need a soil test?

A: Don't guess, test. At about $15, it's the smartest garden investment you can make. Obtain a kit through a university extension service (such as soiltest.umass.edu), and get results in about a week. The analysis will provide exact recommendations for nutrients. Retest every five years or so.

Analyzing the Fertilizer Label

Q: What do the numbers on the fertilizer label mean?

A: The set of numbers on the label, known as the fertilizer grade, guarantees the actual percentage, by weight, of nitrogen, phosphorus (a.k.a. phosphate), and potassium (a.k.a. potash), collectively referred to as N-P-K. For example, a label reading 21-7-14 means the product is 21 percent nitrogen, 7 percent phosphorus, and 14 percent potassium; the other 58 percent is filler, like clay, that helps with even distribution and prevents chemical burn. The N-P-K numbers will help you pick the best combination of nutrients to address deficiencies. See Wrangling your ratio, to find the fertilizer grade that your soil-test report recommends.

Synthetic vs. Organic

Q: Does it matter if I buy synthetic or organic nutrients?

A: Yes, depending on whether you need a quick fix for a failing plant or hope to enrich your soil long term. Synthetic fertilizers are primarily water soluble and fast acting. They can green up a lawn or revive a severely slumping plant in as little as a few days but will do little to improve soil texture or fertility for future growing seasons. Organic fertilizers generally break down over a period of months or even years to stimulate beneficial soil microorganisms and improve soil structure. "Bottom line, building healthy soil with organic fertilizers and compost is the better choice for the long-term health of your garden," says horticulturist Lee Reich, Ph.D., author of Grow Fruit Naturally.

The Do-it-Right Garden Diet

Vegetables and annuals are particularly vulnerable to nutrient deficiency, while trees and shrubs fare better on less food. Here, the whats, whens, and hows of getting your plants to their peak.

Perennials

Eschew high-nitrogen fertilizers, because they promote excess foliage at the expense of flowers and roots, cause floppy stems, and can increase disease susceptibility. Annual or semi-annual applications of aged manure or finished compost will restore trace elements while improving soil texture and water retention.

What: Low-nitrogen, high-phosphorus, slow-release fertilizer (like Garden Food by Espoma, about $7 for a 6.75-lb bag; MidlandHardware).

When: With proper soil prep, many perennials will be happy with one early-spring application of fertilizer.

How: Broadcast throughout beds or side-dress in the general root zone of each plant.

Garden Diet: Turf

Manufacturers have made it pretty mistake-proof, so a product labeled lawn fertilizer ought to have the right mix of ingredients. Bulk up nutrients by raking a few inches of compost into turf in the fall. "This will nourish roots and make a lawn stronger and greener," says This Old House landscape contractor Roger Cook. "Aerate the lawn first to get the compost down where it will do the most good."

What: An all-purpose fertilizer with a 3-1-2 ratio or a specialty lawn food (like Ringer Lawn Restore, about $32 for a 25-lb bag; MidlandHardware).

When: Twice a year, once in late spring and once in early fall.

How: Use a broadcast spreader to prevent patchy spots and odd growth patterns.

Garden Diet: Trees and Shrubs

To encourage lateral root growth, avoid fertilizing trees or shrubs in their first growing season. Established shade trees and shrubs may get the nutrients they need from the soil, decomposing leaves, or lawn fertilizers applied around them. As long as a tree or shrub is healthy and vigorous, there's probably no need for additional fertilizers. However, competing trees and shrubs that put out little new growth but are otherwise healthy can benefit from a once-yearly dose of fertilizer to promote root growth.

What: A slow-release granular nitrogen fertilizer (like Nursery Plus by Shultz, about $10 for a 3.5-lb bag; Amazon).

When: Early spring or autumn.

How: Broadcast within a tree's or shrub's root zone (the area from the trunk outward past the outermost branches).

Garden Diet: Annuals and Vegetables

Since we ask these plants to pump out lush flowers and premium produce throughout a very short season, they gobble up nutrients at a rapid rate; even fertile soil will likely need a boost. Begin with a soil test, as many vegetables require specific nutrients; correct deficiencies per test recommendations; and add several inches of amendments, such as compost, to soil.

What: A balanced fertilizer (try Osmocote Flower and Vegetable 14-14-14, about $14 for a 5-lb box; Lowe's) or Hi-Yield Bone & Blood Meal (about $8 for a 3-lb bag; Newegg).

When: Six weeks after the last spring frost, for both annuals and vegetables.

How: If planting in rows, mix in fertilizer while preparing soil. For individual plants, add fertilizer to the bottom of the planting hole.

Garden Diet: Container Plants

Plants in pots need regular fertilizing because available nutrients are quickly utilized or leached out by frequent watering.

What: Fresh potting soil, which provides adequate nutrients but should be supplemented during the growing season with a balanced slow-release fertilizer (like All-Purpose Fertilizer, about $7 for a 1-lb can; gardeners.com).

When: Every four to six weeks during the growing season.

How: Side-dress; scratch in with a trowel.

Garden Diet: Roses

The secret to healthy roses is great soil, 4 inches of mulch in the spring, and even, regular watering. "More roses die from too much or too little water than from over- or underfertilizing," says Tom Carruth, who oversees some 1,200 rose varieties at the Huntington Library Botanical Gardens, in San Marino, California. Roses feed heavily on nitrogen, which must be replenished, even in rich, fertile soil. Carruth also advises spraying liquid kelp emulsion on leaves between feedings. Planting roses in a berm (a rounded mound) promotes warmer soil and better drainage.

What: Rather than traditional rose food, Carruth uses nitrogen-rich Scotts Natural Lawn Food (11-2-2), a slow-release organic formula ($23 for a 29-lb bag; Lowe's).

When: Once new leaves emerge after spring pruning, and again after the first flush of flowers. All fertilizing in Zones 1-7 should stop in late July; in warmer zones, the last feeding is early September.

How: Broadcast dry, as if you were feeding chickens; spray liquid emulsion onto the whole plant, including leaves.

What's Plaguing Your Plants?

Identify nutrient deficiencies in your garden and fight back with appropriate fixes

Bottom leaves turn yellow

Other symptoms: Newest leaves are pale green; yellowish, spindly stems; slow growth.

Deficiency: Nitrogen—it depletes faster than other nutrients.

Solution: Apply high-nitrogen fertilizers, such as cottonseed meal or ammonium sulfate. Add organic aged compost or well-aged manure for a steady release of nitrogen.

Leaves Turn Reddish-Purple

Other symptoms: Small, stunted leaves; foliage may appear burned at the tips; older leaves may become dark green. Reduced fruit or seed production.

Deficiency: Phosphorus

Solution: Dig in fertilizers such as greensand (a mineral, also called glauconite) or superphosphate. The ideal time to add high-phosphorus fertilizers, critical for seedling root development and early growth, is before planting as indicated by a soil test. If plants show symptoms midseason, side-dress (apply fertilizer around the base of plants) with additional fertilizer.

Scorched and/or Wilted Leaf Edges

Other symptoms: Yellowing between the leaf veins (called interveinal chlorosis); slow growth; poor flowering or fruiting.

Deficiency: Potassium

Solution: If symptoms appear during the growing season, spray all leaves and stems weekly with a liquid fertilizer, like fish emulsion, until plant recovers. Potassium, unlike nitrogen, isn't mobile in the soil. If a soil test indicates a deficiency, address the upper levels of soil by side-dressing with manure or greensand in fall.

Wrangling Your Ratio

So you've had a soil test and received a report on how to correct nutrient deficiencies. Now you need to convert the recommendation, which is given in pounds per area, to the ratio of nitrogen, phosphorus, and potassium that will best suit your needs. To calculate the ratio, divide the weight of each of the three nutrients (N-P-K) by the nutrient with the lowest weight. For example, if your recommendation is for fertilizer composed of 1.5 pounds of nitrogen, 0.5 pound of phosphorus, and 1 pound of potassium, divide all those numbers by 0.5. The result? A ratio of 3-1-2. Purchase fertilizer with a label that most closely corresponds to that ratio, such as 9-3-6.