10–14 minute read
Decide if fertilizer is needed
In nature, the decay of plant litter continually replaces nutrients taken up by plants, so fertilizer is not required. However, in a managed landscape we often remove plant litter and replace entire plants in annual color beds or the vegetable garden. In this case, fertilizer is needed. We may also need to fertilize if the land was bulldozed during building construction. Subsoil is often used for filling and grading and usually has poor physical properties, a low pH, and inadequate amounts of essential plant nutrients.
However, too much fertilizer can damage plants and pollute the environment. The adverse effects of overfertilization on plant health are often forgotten:
- binding of other nutrients, thus reducing their availabilty
- increased susceptibility to disease
- increased susceptibility to injury by cold, wind, and ice storms
- plant burn (dehydration)
Test your soil
Because both overfertilizing and underfertilizing can harm plants, we recommend testing your soil to determine which nutrients are needed and in what quantities. The test:
- characterizes your soil’s capacity to hold nutrients
- checks the availability of major, secondary, and some minor nutrients
- tailors recommendations for the plants you have specified
- is free except during the peak testing season
The NC Department of Agriculture provides soil testing to North Carolina residents. This service is free most of the year and available for a small fee during the peak winter months. The soil test determines the precise amounts of lime and fertilizer to add, ensuring good results. You can pick up forms and boxes at the Orange County Extension Center. Because soil can vary greatly even in a small yard, take multiple samples in each lawn or growing area. Detailed instructions are available online for sampling your soil, sending it for analysis, and interpreting the results. Your report can be accessed online.
Assess your plants
Adapted from NC CES A Gardener’s Guide to Fertilizing Trees & Shrubs
Plants use solar energy to produce carbohydrates from carbon, hydrogen, and oxygen, which are obtained from water and air. Fertilizer is needed only when the soil levels of other nutrients essential for plant metabolism are insufficient.
- Because the roots of woody plants often spread as far as 3–4 times the diameter of the leaf canopy, trees and shrubs near fertilized lawns may not require additional fertilizer.
- If a plant is growing poorly, fertilization can be helpful, but only after you correct any other problems causing poor growth.
- Because nutrient absorption is affected by plant maturity and how actively a plant is growing, fertilizing at the wrong time will provide no benefit to the plant but can have negative consequences for your soil, your groundwater and surface waters like ponds, streams, and lakes.
Before trying fertilizer as a remedy, consider common causes of poor growth:
- Site conditions
inadequate soil aeration, inadequate moisture or waterlogged soil, planting too deeply, adverse climatic conditions, improper soil pH, nutrient toxicity or deficiency
- Root damage
Recently transplanted trees and shrubs often will not resume a normal growth rate until their root systems become established. Plants disturbed by construction within the past 5–10 years may be in shock due to damaged roots and produce limited new foliage.
- Anything that inhibits sugar production in leaves
Absorbing nutrients requires energy. If the tree or shrub is under stress due to low light or extreme temperatures, it can develop nutrient deficiency problems even though adequate nutrients are available in the soil solution.
Decide which fertilizer
The timing and method of fertilizer application depend on both the target plant species and whether the fertilizer is synthetic or organic.
|Synthetic Fertilizer||Organic Fertilizer|
|refined from natural ingredients or manufactured||natural form or minimally processed|
|known chemical composition||highly variable|
|immediate nutrient availability — an advantage in situations like the vegetable garden where needs are exacting and timing must be precise to supply nutrients during critical periods of growth, bloom, and fruiting||highly variable — must break down in soil before nutrients can be used, rate depends on soil temperature, moisture, pH, and microbial composition|
|improves soil structure
benefits soil organisms
|ammonium sulfate, potassium chloride, monoammonium phosphate, processed urea||manure, cottonseed meal, cover crops, fish by-products, compost, bone meal, raw minerals|
|Adapted from the Master Gardeners of Grays Harbor & Pacific Counties, WA: Fertilizers|
Synthetic fertilizers usually act quickly and are more susceptible to leaching, so they are incorporated shallowly and applied more frequently, timed to coincide with plant growth, flowering or fruiting.
Organic fertilizers must be broken down by soil microorganisms before they can be absorbed by plants, so they are often incorporated to the depth of the bed prior to planting.
Remember, organic fertilizers are not intrinsically safer than inorganic products. They sometimes contain plant or human toxins:
Cottonseed meal can contain high amounts and should not be used on food crops
- Heavy metals
Municipal compost can contain heavy metals and should not be used on food crops.
Manures can harbor pathogens harmful to humans that can contaminate vegetable crops, especially those grown in the soil, if the manure is not handled and stored properly or is used in the wrong form or too soon before harvest.
- Herbicides toxic to plants
Some composts and manures can contain these.
Because of their relatively low nutrient content, organic fertilizers are applied at much higher rates than inorganic fertilizers. Therefore, even at the highest spreader settings you may have to make two or more passes over an area to apply the required amount of material. Avoid skips and overlap by applying ½ of the fertilizer while traveling in one direction and the remaining half while traveling in a perpendicular direction, or space piles of the material throughout the area and spread the piles out uniformly using a garden or leaf rake. To calibrate your spreader:
- Adjust it to a relatively high setting.
- Place 2–3 lbs of organic material in the hopper and spread this in a continuous straight path. Note the width of the spread path and the distance traveled.
- Calculate the rate of application. For example, if your spreader broadcasts a 5-foot-wide path and you traveled 20 feet to empty 3 lb, the rate of application would be 3 lb per 100 square feet (5 ft by 20 ft).
You can make an organic version of 10-10-10:
Mother Earth News A Better Way to Fertilize Your Garden: Homemade Organic Fertilizer
Consider nitrogen capital
There are limits to what fertilizer can do. Your soil, mulch, and plant litter comprise a system that acts as a “bank” for nitrogen. Fertilizers, even organic fertilizers, are too mobile (they are either used, leached or volatilized) to build up the capital in this bank. For a healthy system, fertilizing should be complemented by improving the soil with compost.
- Undisturbed sites have very high total nitrogen levels, but over 95% is tied up in organic matter and not available to plants. Slowly-available nitrogen makes up 1–3% of the total nitrogen, while nitrogen available for plant uptake is less than 2% of the total nitrogen.
- Nitrogen capital is essentially removed on very disturbed sites.
- The addition of inorganic fertilizer dramatically increases available nitrogen, but does little to build nitrogen capital.
- The application of organic fertilizers raises the available and the slowly-available nitrogen, but does not add to the long-term reserves.
- In contrast, adding compost to the soil can increase available, slowly-available, and total nitrogen reserves to levels comparable to undisturbed soils.
Fertilizing best practices
Following best practices when you fertilize both improves results and is kinder to the environment.
Calibrate your spreader to ensure that you are applying the correct amount and clean your spreader over grass, not the driveway. Improper fertilizer application, even when the fertilizer is organic, can contribute to surface and ground water pollution, induce a plant nutrient deficiency or toxicity, or cause salt burn.
Use a fertilizer that contains 30% or more of the nitrogen in slow release forms. Look for words such as: “water insoluble nitrogen (WIN)”, “controlled release nitrogen”, “sulfur coated urea (SCU)”, “IBDU”, “ureaformaldehyde (UF)” or “resin-coated urea”.
Apply needed nutrients only. This also can be less expensive than applying a complete fertilizer, which contains all 3 macronutrients (nitrogen, N; phosphorus, P; and potassium, K). Generally, trees and shrubs need a ratio of 3:1:1 of N:P:K, while flowering plants need a higher amount of P than N or K. Overapplication of nitrogen in particular can be harmful, causing growth that is easily damaged by insects, disease or cold snaps.
Remember that recently transplanted trees and shrubs or those that have experienced root disturbance cannot grow until the roots recover and will not benefit from fertilizer application. Recovery requires ~ 6–12 months per inch of trunk caliper for trees, so it may be several to many years before a larger tree resumes growth.
Nitrogen need depends on the stage of plant growth:
- At planting time, nitrogen need is relatively low, but enough should be supplied to support new root generation and top growth. High rates should be avoided because they will trigger excessive top growth that cannot be supported by a limited root system.
- Maximum growth of landscape plants is usually desired during the first few growing seasons after establishment. To achieve maximum growth, constant rates of nitrogen are needed.
- As the plant begins to mature and the rapid growth rate is no longer needed or desired, lower levels of nitrogen are suffficient to maintain plant vigor.
- Plants growing in a restricted root zone should not receive large amounts of nitrogen, because restricted roots cannot support excessive top growth.
- Some plants, once established, may not need additional fertilizer to perform well. Silver maple, willow, ligustrum, and forsythia are good examples.
- Because of their fibrous root systems, some ornamentals (for example, azalea, dogwood, hemlock, and rhododendron) are easily damaged by fertilizers. Split applications of water-soluble nutrients or slow-release formulations are recommended.
Don’t use products that combine fertilizer with a pesticide or herbicide, including preemergent herbicides like crabgrass preventer. Apply these separately and only if needed. In addition to the environmental benefits, you may prevent unintended damage to your plants. For example, dogwoods, redbuds, apples, forsythia, and roses are extremely sensitive to the herbicide dicamba. Because roots can extend far beyond the drip line, damage can occur even when application is distant from the plant.
Established trees and shrubs usually do not require fertilization yearly — every 3–4 years is recommended. Mature trees and shrubs require even less. Fertilizer need also varies with soil type and rainfall. Plants in well-drained soil, especially if low in organic matter, will require more fertilizer because leaching will be greater and also because root systems will be more extensive, so they can support greater top growth. Similarly, periods of high rainfall can lead to both more leaching and greater growth, both of which increase fertilizer need.
Incorporate fertilizer into the soil where possible and avoid using granular fertilizer if heavy rain is forecast. Digging a wide planting hole and thoroughly tilling the soil results in rapid plant establishment and better root growth. Tilling ensures that physical and chemical amendments are evenly distributed in the root zone. Equally important, tilling improves soil structure so that soil water can effectively dissolve and transport the nutrients you have so carefully added. The goal is to provide the roots with moist, well-drained, well-aerated, and fertile growing environment.
Because plants absorb nutrients best when their root systems are healthy and actively growing (spring and fall), the best time to apply fertilizer is about two weeks before these periods so that the nutrients can move into the root zone in time to be available for plant use. Summer feeding is not mandatory.
Preventing runoff maximizes plant benefit and also saves money.
- Mulch plantings to prevent runoff and erosion.
- Border beds with grass or groundcovers.
- Plant cover crops to reduce erosion and runoff of fertilizer that remains after the garden has been put to bed.
- Improve your soil by aerating your lawn and adding organic matter to plantings. Good soil both absorbs more water (reducing runoff) and binds more nutrients (reducing leaching).
- Avoid drainage paths. Don’t apply fertilizer to ditches or natural drainage areas.