Plants need several different nutrients, some of which are supplied through the application of fertilizer. The most important ones are nitrogen, phosphorus and potassium. These are the three that are found in most fertilizers, although some fertilizers contain only one or two of them and others contain additional nutrients. 

Proper fertilization has numerous benefits, including keeping plants healthy, maximizing yields, protecting the environment and saving money. Doing so boils down to applying only the needed nutrients in the needed amounts at the right time. 

However, even if the nutrients are supplied in adequate amounts, plants may not be able to use them if the soil acidity is out of range. Acidity is indicated by the pH scale which goes from zero (highest level of acidity) to 14 (lowest level of acidity). Most plants perform best at pH levels near the halfway point, around 6 or 7, although some prefer it lower. Azaleas and blueberries, for example, do best at a soil pH around 5. That might seem like a small difference, but the pH scale is logarithmic, which means that a pH of 5 is ten times more acidic than a pH of 6. 

For any given garden, lawn or landscape planting, the starting point is to determine the existing nutrient levels (especially phosphorus and potassium), as well as the soil pH. By far the best way to do so is by collecting and submitting soil samples for laboratory analysis. Once the baseline values are known, a fertilization plan tailored to the particular plants at a particular site can be developed. Followup sampling on a 2 or 3 year cycle is recommended to keep things on track. 

The most successful approach is to establish good soil fertility at or before planting, and then to apply nutrients at a frequency and rate that will maintain that fertility for the life of the plant(s).

The basic principles of managing soil fertility in Piedmont North Carolina lawns and gardens is covered below, followed by general guidance for specific plant types.

ADJUSTING SOIL pH

Soils in North Carolina naturally tend to have a pH that is lower than most garden plants prefer. Gardeners can raise the pH (i.e. lower the acidity) by applying lime. The effect usually lasts for a couple of years, but the pH will gradually drop again over time. 

The soil analysis will indicate how much lime is needed to raise the pH to the optimum level. In rare instances, the pH will need to be lowered, in which case a sulfur application will be indicated. Once soil pH has been adjusted by a lime application, it will gradually begin to fall again. Thus, after three or four years another soil sample should be taken to see if supplemental lime is needed. For vegetable gardens, annual flower beds and new planting areas, the lime should be broadcast evenly over the site and then mixed into the rooting zone through tillage prior to plant installation. 

PROVIDING NITROGEN, PHOSPHORUS AND POTASSIUM

• Nitrogen is subject to rapid leaching and volatilization (turning into vapor). Thus, nitrogen applications don’t last more than a few weeks. While the soil analysis report from NCDACS will include a nitrogen recommendation, it’s only the starting point. Plants that are heavy feeders on nitrogen (e.g. vegetables, most lawn grasses, annual flowers, fruit crops) will usually need one or several additional applications each growing season. This can be mitigated to some extent by using organic or slow release fertilizers, and by maintaining high levels of organic matter in the soil.
• Phosphorus tends to stay put. In soils that have a deficiency, the soil analysis report will indicate an application to boost it up to optimum levels. From that point forward, occasional light applications are sufficient to keep it in the sufficiency zone.
• Potassium dissipates more quickly than phosphorus, but not as quickly as nitrogen. Maintenance applications will be at moderate levels. 

The percentages of these nutrients found in a given bag of fertilizer is indicated on the label by three numbers, separated by a hyphen (e.g. 10-10-10 or 8-0-24). The numbers are always listed in the same order: nitrogen then phosphate (to supply phosphorus) then potash (to supply potassium). Since the three major nutrients behave differently, it’s normally unnecessary and even counterproductive to use something like 8-8-8, 10-10-10 or 20-20-20 for every application. A general rule of thumb for the yearly maintenance applications is to apply 4 pounds of nitrogen, 1 pound of phosphate and 2 pounds of potash per 1000 square feet.

It can be challenging to find a fertilizer that has the appropriate ratio of the different nutrients. Thus, gardeners may want to apply the nutrients separately using fertilizers that emphasize only one of the three. Here are some examples:

• All or mostly Nitrogen
  • Blood meal
  • Cottonseed meal
  • Soybean meal
  • Feather meal
  • Nitrate of soda (a.k.a. sodium nitrate)
  • Urea
  • Calcium nitrate 
  • Ammonium sulfate
• All or mostly Phosphorus
  • Bone meal
  • Triple super phosphate
• All or mostly potassium
  • Muriate of potash
  • Kelp

WHAT ABOUT OTHER NUTRIENTS?

There are over a dozen additional nutrients that plants need, but gardeners normally don’t need to apply them as the soil usually has sufficient quantities already. Additionally, compost and manures contain some of these other nutrients, as do natural or organic fertilizers.

GENERAL GUIDELINES FOR SPECIFIC PLANT TYPES

Note that the guidance below is for maintenance fertility and assumes proper pH and nutrient levels were established at or prior to planting.

LAWNS

The timing, rate and number of fertilizer applications varies significantly depending on the turf species (e.g. tall fescue, bermuda, zoysia, etc.). Application timing will coincide with the corresponding growing season, i.e. fall and spring for cool season turf, spring and summer for warm season turf. Slow release turf fertilizers with high nitrogen, low phosphate and medium potash levels are generally best. Lime applications will be needed about every third year, and rate should be based on soil analysis. 

For more details, visit the NC State Extension TurfFiles portal.

VEGETABLE GARDENS AND ANNUAL FLOWER BEDS

Each year before planting, a fertilizer application can be broadcast over the area and then mixed into the root zone. For that pre-season application, aim for about 4 pounds of nitrogen, 1 pound of phosphate, and 2 pounds of potash per 1000 square feet. Organic or slow release fertilizer is generally best. 

One or two additional nitrogen applications spaced out by a few weeks will be beneficial for long season crops such as corn, tomatoes and annual flowers (e.g. marigolds, petunias, etc.). In vegetable gardens, this is known as a sidedress application. Choose a fertilizer that contains only or mostly nitrogen, preferably slow release. 

Lime applications will be needed about every third year, and rate should be based on soil analysis. 

Here are additional details on fertilizing bedding plants (a.k.a. annual flowers beds). 

Here are the recommended sidedressing rates for vegetable gardens, from the following NCDACS publication: Fertilization of Lawns, Gardens and Ornamentals.

Supplemental nitrogen for vegetables 

FRUIT PLANTINGS

Since fruit plantings are expected to last several years, soil pH and phosphorus levels cannot be easily corrected after the plants are installed. Installing any new planting without a pre-installation soil analysis can be a costly mistake, but especially so with fruit crops. Small fruits (grapes, strawberries, blackberries, blueberries) may benefit from an annual maintenance application in early spring. Tree fruits (apples, peaches, etc.) may not need annual fertilization. Crop specific production guides from NC State Extension provide details. Soil pH should be checked every two or three years and adjusted as indicated on the sample report.

Here are links to crop specific information:

Raising small fruits

Raising tree fruits

TREES AND SHRUBS

If soil pH and phosphorus levels are adjusted prior to planting, then additional lime and fertilizer applications will rarely be needed for trees and shrubs. They can be monitored through periodic visual inspection. A healthy flush of spring growth is a positive indicator.

For more information, see A Gardener’s Guide to Fertilizing Trees and Shrubs.

FINE TUNING THE PROGRAM

Over time, gardeners who use practices such as cover cropping, slow release fertilizer, organic fertilizer, and organic mulches will likely find that reduced fertilizer application rates provide excellent results. These practices can help soil store nutrients more effectively.

Ultimately, gardeners can use their experience and direct observation to further refine the ongoing fertility program. Monitoring growth rates, foliage color, plant health and production will provide vital insight.