A Fertilization Primer: Plant Needs, Fertilizer Choices and Application Tips
Gardeners understand that plants uptake nutrients from soil as they grow and that those soil elements must be replenished to avoid depletion over time. Nature’s method for maintaining soil health is to allow dead organic matter to decompose and re-enter soils through the action of soil organisms. For the past hundred plus years, conventional agriculture has departed from natural practices, relying on synthetic manufactured fertilizers to deliver specific elements to plants, while in many cases, neglecting soil health. Recently, organic and regenerative growers have returned to natural soil-building practices and have increased the use of natural amendments in place of synthetics. Natural fertilizer choices include raw and processed organic materials, whose source material derives from formerly living plants and organisms as well as natural minerals.
What is the best way to fertilize? Let’s take a look at soil and plant needs, fertilizer options, and application guidelines.
Plant Nutrient Requirements
What are plants made from and where do they get needed nutrients? After removing moisture, plants are:
- 80-90% carbon and oxygen and about 6% hydrogen. Plants get these elements from the atmosphere by taking in carbon dioxide during photosynthesis, and from rain absorbed from the soil through root systems.
- 3 to 12% comes from 6 macronutrients, including nitrogen, phosphorus, potassium, calcium, magnesium, and sulphur. These elements come mostly from the soil.
- About 0.5% is composed of 8 or more micronutrients whose amounts are measured in parts per million, a small but essential requirement. Micronutrients include iron, chlorine, manganese, boron, copper, molybdenum, nickel and cobalt. They are generally available in soils.
The macronutrients, most notably nitrogen, are the soil elements that need consistent replenishment.
Identifying the Needs
There are drawbacks to both over- and under-fertilizing. Too few nutrients can lead to slow growth and poor plant health. But too much nitrogen can cause excessive vegetative growth at the expense of fruiting. Excessive or poorly distributed synthetic nitrogen fertilizer can lead to plant “burning” as excessive soil salts draw moisture from plant roots, causing dehydration. Finally, synthetic nitrogen solubility makes it susceptible to leaching, a significant cause of waterway pollution. The Virginia Cooperative Extension publication Fertilizing the Vegetable Garden can help identify symptoms of macronutrient deficiencies.
In addition to nutrient content, soil pH should be managed. Excessive acidity or alkalinity affects plants’ ability to uptake needed nutrients from the soil. Most plants prefer a pH in the 6-7 range. If pH is below about 5.7, lime additions are recommended, which will help reduce acidity. If above 7, sulphur additions can reduce alkalinity.
How do we know what and how much to add to soils? The best way is to periodically perform a soil test — about every three years. Tests from the Virginia Cooperative Extension are inexpensive and easy to execute, as explained in Soil Sampling for the Home Gardener/VA Tech. The test reports provide information on soil nutrient content and pH, along with instructions for remedying any identified issues. Nitrogen content is not noted in soil tests due to its ability to quickly move through the soil, which makes snapshots of N content of limited value. Nitrogen additions can be estimated based on other soil amendments applied and the plants being grown. Direct questions about soil samples to the local VCE help desk.
What All Gardeners Should Do
Regularly following proven natural soil building practices is the best way to maintain baseline soil health. These include:
- Regularly adding organic matter to all soils. The organic matter can include purchased or home-made compost, non-pet manures, and organic mulches such as wood chips, leaf mold, sawdust, etc.
- Crop rotations, which reduce soil disease and balance nutrient demands.
- Cover cropping, especially with mixed plantings, which build structure, may add nitrogen, and provides green manures and mulch materials.
- Minimizing tilling and soil disruption to maintain soil structure and reduce carbon loss to the atmosphere.
These practices add nutrients, reducing fertilization needs while building soil structure, improving water infiltration, oxygen availability and moisture-holding capacity. They also reduce nutrient leaching and stabilize pH.
Fertilizer Math: Interpreting Package Labels
While the chemical content of raw soil amendments, like manures, are not identified, processed fertilizers, both synthetic and natural, list their contents on package labels. The headline numbers, referred to as N-P-K, list the key macronutrient elements of Nitrogen (N), Phosphorus (P) and Potassium (K).
For example, the fertilizer in the photo below is a 12-10-10, which means it is 12% nitrogen (ammonia), 10% phosphorus (phosphate) and 10% potassium (potash) by weight. This information is used to calculate how much fertilizer to use.
The soil test report pictured below recommends fertilizer application quantities for several different fertilizer products, which all calculate to the same net nitrogen addition. Note that the P and K measurements in the report are very high (VH), so the recommended additions include no P and K. In this case, the Virginia Tech Soil Lab has quantified the recommendations into “cups” of different fertilizer blends, making measurement of how much to add straightforward.
What is important to understand is that 1.33 lbs. of 16-0-0 fertilizer means that 16% of 1.33 lbs. or about 0.21 lbs. of N per 100 square feet is the recommended N addition. The quantity of about 0.2 lbs. of nitrogen per 100 sq. ft. is a standard addition in the absence of better guidance.
Another good rule of thumb is that pelletized fertilizers typically weigh about ½ lb. per cup.
Suppose you have not had a recent soil test, and want to add the standard recommended amount of nitrogen to your garden — 0.2 lbs. of nitrogen per 100 sq. ft. Let’s figure out how much 14-0-0 Blood Meal to add to a 4’x8’ raised bed at 0.2 pounds of N per 100 sq. ft.:
- Dividing the 0.2 lbs. Nitrogen we want to add by 0.14, the percentage of blood meal that is Nitrogen, tells us we need to add 1.43 lbs. of blood meal per 100 sq. ft.
- Since our 4×8 foot raised bed is 32 sq. ft. (32% of the 100 sq. ft. in the base recommendation), multiply the 1.43 lbs. x 0.32 to determine that we want to add 0.46 lbs. of blood meal to the bed.
- Since we know that pellet fertilizers weigh about 0.5 lbs. per cup, we need to add slightly less than 1 cup of the blood meal to the bed.
- Remember, it’s best to round down in quantity, especially if you regularly add compost or other organic matter to your soil.
In the absence of a soil test, the usual recommendation is to use a balanced fertilizer like a 5-10-5 or 10-20-10 at the rate of 2-3 lbs. (4-6 cups) per 100 sq. ft. But for the soil analyzed in the soil test report pictured above, this would be a bad idea because of the high P and K levels in the soil. Excessive P and K can restrict plant access to other essential nutrients. Hence the advisability of regular testing.
When mechanized farming took hold in the early 1900s, demand for nitrogen fertilizers quickly outstripped organic fertilizer supply. This led to development of a process that extracted nitrogen from the atmosphere to create ammonia (NH3-) that could be further processed into a soluble form of nitrogen that was plant usable. This combination increased agricultural production dramatically, at the same time the world population nearly quadrupled from 2 billion in 1900 to nearly 8 billion people today. That is the positive side.
The negative side is that synthetic fertilizers provide plants the nutrients they need, but neglect soil health. They don’t replenish the organic matter that sustains soil organisms, potentially leading to a long term breakdown of soil properties. When combined with careless land management, disasters like the Dust Bowl result. Synthetic fertilizers certainly have their place, but complete reliance on them without good soil management leads to trouble.
Because synthetics are water soluble as applied, they can be fast acting. This may be an advantage for new plants that would benefit from a quick application of nitrogen. But their immediate solubility makes them subject to leaching, which removes the nutrient from the soil and can pollute ground and surface water. Being careful to add appropriate amounts of amendment and using slow-release products where possible can reduce the risk of runoff.
Synthetic fertilizers are available in a variety of forms:
- Granules: the most-used type. Easy to apply. Care is required not to over-apply.
- Spikes: convenient but materials tend to concentrate around the spike. Expensive.
- Liquids and mixable powders: Good for houseplants and as a starter for transplants.
- Foliar: Powders mixed with water and sprayed on plants. Provides a quick growth response, but benefits are as short as a couple of days. Overuse can harm plants.
Don’t use lawn fertilizers in gardens. The herbicides they may contain can kill desirable plants.
“Organic“ and “natural” fertilizers are derived from materials from formerly-living plants or organisms and from natural minerals. They differ from synthetics in that they must be decomposed into their inorganic chemical form before they are accessible to plants. They feed soil organisms and help improve soil texture and structure, improving water infiltration as well as moisture- and nutrient-holding capacity. They are not instantly soluble. It may take a couple of weeks for soil organisms to break down a natural fertilizer into a plant-available form. Natural fertilizers typically offer a fraction of their nutrient content to plants in the first year and continue to benefit plants in future years. Using natural fertilizers is not a once and done action. It is a component of a longer term commitment to building and maintaining healthy soils.
This publication from the Oregon State Extension lists the N-P-K content of some common organic soil amendments.
While synthetic fertilizers are manufactured with a planned mix of nutrients, organic materials are as nature makes them. This may require blending multiple amendments to reach a balanced nutrient mix.
Unprocessed or raw organic amendment content varies by age, whether they were composted, subject to leaching, and other factors. An important but unmeasured component is organic matter, an additional benefit to soil. Using composted raw products and understanding their source is a good way to minimize concerns about pathogens and other undesirable content.
There are commercially available blends of organic fertilizers that provide balanced nutrients. The product in the photo above contains multiple ingredients to reach its 5-3-3 N–P-K analysis.
There are a couple of organic amendments that deserve special mention:
- Biochar is a charcoal-like product that comes from burning materials like wood, crop residue, and manure in a low-oxygen, high-temperature environment. Contents vary based on feedstock and combustion process. All are high in carbon and when added to soil, increase soil porosity, aiding water infiltration and storage, oxygen availability, nutrient retention, and microbial colonization. It does not provide nitrogen and can even cause a nitrogen deficiency immediately after addition due to microbial proliferation. It increases soil alkalinity. Agricultural productivity studies show variable results for biochar, depending on source material, soil condition, and crops grown. It is being studied as a way to sequester large amounts of carbon in soil.
Adding small amounts of commercially available biochar to garden soil is unlikely to do harm and does help certain soil characteristics. It is unclear whether it positively impacts plant health, but is a useful component of a thoughtful soil building program. It is not a balanced fertilizer.
- Bio-Solids are the generic name for processed sewage sludge that is used as a soil amendment. Processing includes either heat drying at temperatures of 176°F to 1000°F or hot composting at greater than 131°F to kill pathogens. The EPA classifies biosolids as Class A or B, with Class A meeting the most stringent pollutant, pathogen and vector attraction requirements. Class B biosolids require a permit for field application. Class A products may be applied to farm fields without permitting. The cleanest products are called Class A: EQ (Exceptional Quality) and may be bagged and sold to homeowners without restriction.
Biosolids add nitrogen, phosphorus and organic matter to soil. They improve soil structure and conserve landfill space. When applied, they may have a distinctive odor which typically fades away fairly quickly and is not in any way harmful.
Milorganite is an example of a commercial biosolids product that is marketed to home gardeners. It is a heat dried product that is produced by the Milwaukee Metro Sewer district. There are multiple products in the Milorganite portfolio, but a typical analysis is 6-4-0.
There is still controversy over the agricultural use of biosolids. They are disallowed in the USDA organic certification program. EPA is currently studying their contents and environmental impact. While they contain many elements that build soil health and provide a productive way to dispose of human wastes, some maintain that they pose a risk of contaminating and further degrading our farmland. The debate continues.
Tips on applying fertilizers include:
- Read and follow package directions.
- For raised beds and intensive planting patterns, broadcast pellets, taking care not to concentrate it in holes or bands. Incorporate it into the top few inches of soil.
- For row plantings use a side dressing technique. Fertilize along both sides of rows, a few inches from stems to avoid burning plant roots. Allow roots to reach for the nutrients.
- Synthetic fertilizers are water soluble and become plant-accessible quickly when wet. This also means that they can leach away in a heavy rain. Better to make multiple small applications than one big one.
- Organic fertilizers are made plant-available as they are broken down by soil organisms. Most are activated over multiple years rather than immediately. The organisms require air and water to do their work. Moistening incorporated fertilizer gets the process started.
- It can make sense to use a small dose of a synthetic for immediate impact with a larger application of an organic material for steady longer term benefit.
To Sum Up…
The key things to remember about smart fertilization include:
- Start with good soil management practices to maximize plant health and minimize needed fertilizer additions.
- Get a soil test every few years to be certain you are adding what your plants need.
- Organic or natural fertilizers add to soil health while feeding plants. They are a good component of a long term soil management effort.
- Synthetics are quick acting and can provide plants what they need, but in the absence of a good soil management approach, can lead to soil degradation.
- In the absence of a soil test, follow fertilizer package directions regarding quantities and application techniques. Err on the low side.
- Since fertilization intends to replace nutrients taken from the soil by plants, multiple plantings per year, spring and fall for example, require fresh applications.
North Carolina Extension Gardener Handbook: https://content.ces.ncsu.edu/extension-gardener-handbook/1-soils-and-plant-nutrients
“How To Convert an Inorganic Fertilizer Recommendation to an Organic One,” Univ.of Ga Extension Pub.No.C853
“Here’s the Scoop on Chemical and Organic Fertilizers,” Oregon St. Ext. oregonstate.edu
“Using Biochar in the Garden,” Univ.Arizona Ext. Backyard Gardener (2020)
“Using Biosolids in Gardens and Landscapes,” https://pubs.extension.wsu.edu/using-biosolids-in-gardens-and-landscapes-home-garden-series
“Basic Information about Biosolids,” U.S. Environmental Protection Agency, https://www.epa.gov/biosolids/basic-information-about-biosolids
“Explanation of Soil Tests,” VCE, Va.Coop.Ext. Pub. No. 452-701
Soil Management in Home Gardens and Landscapes, Penn State Extension, (September 2017).