Like all living things, plants need nutrients to grow and thrive. Essential nutrients for plant growth include nitrogen, phosphorus, potassium, calcium, sulphur, magnesium, copper, boron, and others. Nitrogen, phosphorus, and potassium, represented on garden product labels as N, P, and K, are the three macronutrients that form the basis of plant growth.

How Do Plants Access Nutrients?

Nutrients in the soil originate from many sources: decomposed organic materials, weathered rocks, and even particles blown in on the wind. Soil itself is made up of particles of different sizes and textures including sand, silt, and clay, as well as aggregates of those particles, broken-down organic matter, and the essential spaces between them. Pores in the soil allow oxygen and water to travel below the surface, providing water for thirsty plants and enabling aerobic bacterial activity.

Plants use their roots to explore the soil, growing outward in the direction of water and nutrients. The large surface area of the root network readily absorbs water and nutrients, which plants use to power their growth. By building symbiotic relationships with fungal colonies in the soil, root systems extend their reach and can more efficiently uptake nutrients. This mutually beneficial relationship with mycorrhizal fungi creates stronger, more resilient plants.

What Is NPK?

Most fertilizers list three numbers on their label indicating the amounts of nitrogen (N), phosphorus (P), and potassium (K) they contain. These are the main macronutrients that contribute to plant growth and health.

Nitrogen – N

Nitrogen is a major component in chlorophyll production, making it essential for photosynthesis.  Green, leafy growth is stimulated by nitrogen, and nitrogen deficiencies will present as yellowing leaves. An excess of nitrogen can burn plant tissue, so be sparing in your application of high-nitrogen fertilizers.  Some sources of nitrogen include blood meal, feather meal, and manure.

Phosphorous – P

Phosphorous is important for the growth of both roots and stems. Strong roots improve the efficiency of water and nutrient uptake. Phosphorous also stimulates the production of blooms and fruits on plants. Plants lacking in phosphorous may display stunted and spindly growth or discoloured leaves. High amounts of phosphorous can be toxic to plants. Phosphorous has low solubility and can build up in the soil if used excessively. Sources of phosphorus include rock dust and bone meal.

Potassium – K

Potassium is a necessary component in fruit and vegetable production and increases plants’ resistance to damage from insects and disease. Potassium also plays a role in the movement of nutrients, water, and carbohydrates in plant tissue. Plants deficient in potassium will show brown leaf edges and discoloured, striped tissue. Sources of potassium include kelp meal and greensand.

What Other Macronutrients Do Plants Need?

Carbon, Hydrogen, and Oxygen

Carbon forms the basis of most plant structures and is derived from carbon dioxide in the air. Hydrogen and oxygen come primarily from water, though some oxygen is taken from the air. They contribute to growth and the transport of energy throughout the plant.

Calcium – Ca

Calcium is part of the cellular walls of plants and is important for the development of leaves and roots. It also assists in the absorption of nitrogen. Calcium deficiency in plants may appear as stunted growth with spotty leaves.

Magnesium – Mg

Magnesium is an essential ingredient in chlorophyll. It also helps with the uptake of other nutrients, especially phosphorous. Plants deficient in magnesium display lowered resistance to drought, cold, and disease.

Sulphur – S

Sulphur is needed for stress tolerance and chlorophyll production, and must be in the form of sulphate to be used by plants. Symptoms of deficiency include stunted growth and pale green leaves.

What Are Micronutrients?

Beyond NPK, many other micronutrients contribute to the health and resiliency of plants. These may be found in concentrations as low as 1%, and micronutrient deficiencies are less common but still impactful. Many organic fertilizers and composts contain a range of micronutrients. 

Boron – B

Boron plays many roles in plant health, including cell wall development, flowering and pollination, and the movement of energy throughout the plant.

Chlorine – Cl

Chlorine contributes to osmosis, disease resistance, and nutrient uptake. Too much chlorine can be deadly for plants, and most plants receive enough from the soil and water.

Copper – Cu

Copper is part of the process of photosynthesis, and copper deficiency can cause chlorosis.

Iron – Fe

Iron is essential for the generation of chlorophyll and plants lacking in iron will display chlorosis and poor growth.

Manganese – Mn

Manganese contributes to the creation of chloroplasts, pollen, and root cells. Manganese deficiency presents similarly to iron deficiency, with chlorosis and discoloured leaves.

Molybdenum – Mo

Molybdenum plays an important role in the processing of nitrogen, which plants use to create amino acids. Deficiencies in molybdenum are uncommon but may present similarly to nitrogen deficiencies.

Zinc – Zn

Zinc is a part of the creation of chlorophyll and carbohydrates and contributes to growth regulation. Zinc deficiency causes damage to new leaves in the form of distorted growth, necrotic spots, and chlorosis.

Choosing a Garden Fertilizer

Organic fertilizers are derived from organic materials and typically take longer to break down in the soil than their synthetic counterparts. As they decompose over time, organic fertilizers release nutrients, improve soil texture, and encourage the activity and growth of microorganisms. These microbes contribute to the decomposition of organic matter and the availability of nutrients in the soil. Proper application of organic fertilizers can help to strengthen the health and resiliency of your soil over time. Organic fertilizers should be applied before they’re needed to give them time to break down, and may be added to your soil in the fall or spring for the best effect. Cold soil temperatures in the late fall and early spring slow the activity of microorganisms, which in turn slows the breakdown of organic materials, but once the weather warms in late spring those microbes will be back at work.

Synthetic fertilizers are often formulated to be water soluble and can be taken up almost immediately by plants when applied. Quick-release formulations are ideal for delivering needed nutrients in a hurry when plants are lacking something essential. Synthetic fertilizers are readily available to plants even when soil temperatures are too low for decomposing microbes to be active. Because they’re so readily available, synthetic fertilizers may cause damage and burns to plant tissue when applied in excess. Some synthetic fertilizers are formulated to be slow-release to aid in balanced growth and minimize the risk of excess nutrient application.

Fertilizers come in three main forms: granular, powder, and liquid. Fertilizer spikes are also a common way to apply fertilizer to trees and shrubs, and some fertilizers can be applied as a foliar spray to the leaves of the plant. Powdered fertilizers take longer to break down and become available to the plant, while liquid fertilizers are highly water soluble and make nutrients more readily available.

Nutrients are an essential part of soil and plant health, and whatever option you choose it’s important to ensure your plants get the nutrients they need to thrive! If you’re not sure where to begin, try testing your soil to see where you might be lacking. You can find tests for NPK and other nutrients here!