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Plant Nutrients

Indeterminate (cordon) tomato vine plants growing outside in an English garden, UK

Plants require nutrients to grow and for good health. Most plant nutrients are common chemical elements. Elemental plant nutrients are generally used in one of three ways:

  • They become part of plant cells and are basic to the structure of plants.
  • They are metabolic, required for biochemical functions within the plant such as photosynthesis or reproduction.
  • They are catalysts that assist metabolic functions.

Most of the elements that plants need to grow come from the soil. Keeping the soil healthy is the quickest and least expensive way to ensure healthy and productive crops.

Nutrients in the soil and air

Plants require the macronutrients nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) from the soil. Plants also need micronutrients from the soil: iron (Fe), boron (B), copper (Cu), zinc (Zn), and manganese (Mn). From the air, plants require carbon (C), hydrogen (H), and oxygen (O).

Macronutrients

Of the macronutrients, nitrogen is essential for shoot and leaf growth especially in the early part of the growing season. Phosphorus is important in the formation of roots. Potassium aids flower and fruit formation and general plant health.

Plant nutrients to grow vegetables
Mineral elements for plant growth

Feeding the soil

The regular addition of organic material to the garden—aged compost and manure—provides almost all the minerals required to support vegetable, herb, and fruit growth. A soil’s structure, moisture-holding capacity, and drainage are improved with the addition of compost and manure and so is its store of essential plant nutrients.

Fertilizers

Additional soil additives and fertilizers are seldom required once the soil is fertile and receives light composting and manuring regularly. Additives and fertilizers added to a balanced soil are akin to seasonings added as a finishing touch to a meal. They are best used on a plant by plant basis to supplement the work of healthy soil. 

Organic soil amendments

Few gardeners have perfect soil. Almost all soils can be improved with the addition of organic amendments. An organic soil amendment is any natural material that improves soil structure. Organic amendments not only improve soil structure but also increase soil fertility by releasing nutrients as the organic matter decomposes.

The best soil amendments are finished compost, rotted manure, chopped leaves, straw, and grass clippings. Adding one inch of nearly fully decomposed organic matter to the garden once or twice a year will improve and refresh the soil almost immediately.

How amendments help the soil

Organic soil amendments increase the moisture and nutrient holding capacity of sandy soils and allow water and air to flow easily through clay soils. Adding partially decayed organic compost will increase soil fertility by releasing nutrients, particularly nitrogen, as the organic matter decomposes.

Organic matter helps stimulate the growth and reproductive capacity of soil microorganisms and worms that in turn add humus to the soil. (Keep in mind that worms and soil microorganisms work more quickly to decompose organic matter in warm weather than in cool weather.)

Soil nutrients for vegetables
Mineral elements on the soil help plants grow.

How chemical elements help plants

Here are key elements from the soil and how they help plants:

  • Nitrogen (N): essential to plant growth; part of the chlorophyll molecule, which gives plants their green color and is involved in creating food for the plant through photosynthesis.
  • Phosphorus (P): essential for plant cell division and development of new tissue; aids in complex energy transformations in the plant that help convert other nutrients into usable building blocks for growth.
  • Potassium (K): essential element for plant growth; helps plants use water and resist drought and enhances fruits and vegetables. 
  • Sulfur (S): essential for the growth and development of plants; key to formation of chlorophyll that permits photosynthesis through which plants produce starch, sugars, oils, fats, vitamins and other compounds.
  • Magnesium (Mg): essential for photosynthesis in plants; without magnesium, chlorophyll cannot capture sun energy needed for photosynthesis. 
  • Calcium (Ca): crucial for plant growth and makes plants less susceptible to diseases and pests.
  • Iron (Fe):  enzyme and chlorophyll production, nitrogen fixing, and development and metabolism are all dependent on iron.
  • Boron (B): increases flower production and retention, pollen tube elongation and germination, and seed and fruit development
  • Copper (Cu): required in the process of photosynthesis, is essential in plant respiration and assists in plant metabolism of carbohydrates and proteins.
  • Zinc (Zn): help the plant produce chlorophyll; when the soil is deficient in zinc and plant growth is stunted. 
  • Manganese (Mn): contributes to biological systems including photosynthesis, respiration, and nitrogen assimilation and also is involved in pollen germination, pollen tube growth, root cell elongation and resistance to root pathogens.

Here are key elements from the air or atmosphere and how they help plants:

  • Carbon (C): plants take carbon dioxide from the atmosphere and turn it into sugars through photosynthesis; the carbon that plants absorb from the atmosphere in photosynthesis becomes part of the soil when they die and decompose.
  • Hydrogen (H): combines with carbon during the photosynthesis process and release oxygen into the atmosphere which is used by all living beings; all living beings need oxygen for respiration. 
  • Oxygen (O): essential because it makes the process of plant cell respiration more efficient (known as aerobic respiration).

Related articles of interest:

Vegetable Plant Nutrients: Sources and Deficiencies

Your Vegetable Garden Soil

How to Improve Clay Soil

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.

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