Plants “eat” sunlight to create energy through a process called photosynthesis—the roots take up water from the soil, the leaves absorb carbon dioxide from the air, and the chlorophyll (substance that makes plants green) consumes energy from sunlight. The light energy is used to convert the water and carbon dioxide into oxygen and nutrients (sugars); some of the sugars are used for growth, while excess sugars are stored in the roots.

Essential Elements for Plant Growth

Macronutrients are used more regularly and in a higher volume for general plant growth than micronutrients. Deficiencies (especially of macronutrients) can adversely affect plant growth, causing stunted or slow growth, chlorosis (yellow leaves), or even cell death.

Macro/primary nutrients: Carbon (C), Hydrogen (H), Oxygen (O), Nirtogen (N), Phosphorous (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulfur (S)

Micro/secondary nutrients: Iron (Fe), Manganese (Mn), Boron (B), Molybdenum (Mo), Copper (Cu), Zinc (Zn), Chlorine (Cl), Nickel (Ni), Cobalt (Co), Sodium (Na), Silicon (Si)

Symbiotic Microbial Partnerships (Probiotics)

Plants rely on fungi to facilitate the uptake of nutrients and minerals from soil. Fungi that form symbiotic relationships with plant roots are called mycorrhizae (Mycor = fungus, Rhizae = root). Fungi are physically incorporated into the root structure and colonize living root tissue during active plant growth, helping plants obtain phosphate and other minerals like zinc from soil. The fungus benefits by eating excess sugars and starch produced by the plant. Mycorrhizae help increase the root system by extending hyphae far out into surrounding soil, allowing access to minerals otherwise out of reach. Mycorrhizae can also function as a physical barrier to pathogens and act to boost general defense mechanisms.

What is fertilizer?

Fertilizer is a material added to soil that promotes plant growth; it may be applied in granular or liquid form. Most fertilizers contain nitrogen (N), phosphorus (P), and potassium (K) and have percentages of these nutrients listed on the label, represented by three numbers that are always in the same order (N-P-K). Fertilizers may contain either synthetic or organic ingredients. Minerals such as phosphorous, sulfur, and potash are mined from the environment and crushed or chemically treated for application. Organic inputs such as manure, seaweed. and compost are made from animal feces and plant or animal decomposed matter. Industrial or synthetic fertilizers (such as ammonium phosphate, urea, ammonium nitrate) are produced by humans through industrial chemical reactions.

What is the importance of the numbers?

Based on the percentage or ratio of nutrients, one fertilizer may have a different effect on plant growth than another. Knowledge of the basic role each primary nutrient preforms for plant growth will help you select the proper fertilizer for your needs. When in doubt, simply use a balanced or “all purpose” fertilizer for all your needs (all three numbers are the same, or very close).

  1. Nitrogen (first number) provides plants the ability to produce more chlorophyll, which allows them to grow more quickly; it is primarily responsible for leafy-green growth.
  2. Phosphorous/Phosphate (second number) aids in root development and increases flowering ability, bloom size, and fruit and seed production; often emphasized in starter fertilizers and those formulated for flowers or vegetables
  3. Potassium/Potash (third number) has many functions—it guards against diseases, aids in drought protection and cold tolerance, improves root development, and assists with photosynthesis; Potassium also helps to regulate the other nutrients available to the plant.

Granular vs. Liquid

Liquid fertilizers contain water-soluble nutrients that are taken up quickly by plant roots for rapid, short-term results; they may be found as a powder or liquid concentrate that is mixed with water and given to plants in liquid form.

Granular fertilizers are generally slower to release their nutrients and feed over a longer period of time; they can be applied by mixing into the soil when planting or by scattering over the top of the soil around the dripline or root zone and watering into the soil. Some granular fertilizers are coated in a material that is designed to be a time-release of material over a period of several months (e.g. Osmocote).

Never fertilize a plant that is drought-stressed or very dry without rehydrating it first. Always measure accurately and use the recommended amount or less to avoid foliage burn.

How often should fertilizer be applied?

The length of time between applications will be found on your fertilizer label, along with other helpful information. Most synthetic fertilizers are faster to become active when compared to organics. Depending on soil temperature and other conditions, granular G&B organic fertilizers take up to 3 weeks to become available as plant nutrients and generally last about 6 to 8 weeks.

Soil pH & Effect on Available Nutrients

Basic soil chemistry (pH) plays a role in which nutrients are available or unavailable to plants. For example, in acidic soils, many of the primary nutrients begin to slow their availability while minerals such as iron become more soluble. Most plants prefer to grow in soil between a pH of 6.5 and 7.5, where most nutrients are readily available.

Soil biology affects plant communities by being either bacteria- or fungal-dominate and also by the diverse microbial species within each genera. Certain species of fungi prefer to associate with the roots of specific plants, forming a predictable relationship over time.

Take a soil test to determine your site-specific nutrient levels. Use a home test kit or a local laboratory such as A&L Western Agricultural Laboratories.

Which plants should be fertilized more often?

It is usually wise to fertilize new plantings and feed regularly until they become established (2 to 3 years). Immature plants have undeveloped root systems that do not yet contain the resources to be completely self-sufficient for ideal growth. Annual bedding and vegetable plants need to be high performers with constant blooms and fruit production, but only have a one-year root system, so they do best when given regular fertilizer during the growing season.

A few other landscape plants are notoriously needy in the fertilizer department. These “heavy feeders” include roses and gardenias. We recommend feeding roses with Portland Rose Society fertilizer (15-10-10 synthetic or 5-4-4 organic), while gardenias prefer a more acidic blend, such as G&B Rhododendron, Azalea & Camellia food or liquid fertilizer made for acid-loving plants.

Depending on other maintenance and cultural practices, mature landscape plants may seldom need to be fertilized.

When is the best time to fertilize?

Most plants benefit from fertilizing during their active growth season. For our area, this is usually late March or early April through September. Indoor plants are usually fertilized from March through September and not fed during winter months.

What about fertilizing lawns?

Yes, most lawns require a constant replacement of nitrogen unless the grass clippings are allowed to fall back onto the lawn to break down as fertilizer. The most critical time to feed lawns is during the fall to help maintain through winter.

Will fertilizer help my “sick” plant?

It is best to get a diagnosis for your specific plant problem before assuming that a dose of fertilizer is what it needs.

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