What is fertilizer and why do plants need it?

Organic and synthetic fertilizers are responsible for so much plant growth. 
This sprawling field of corn won't grow optimally without the right fertilizer. Olga Rolenko / Getty Images

In order to understand the building blocks of lush, healthy vegetation, one word frequently emerges as a cornerstone of agricultural and gardening success: fertilizer. From backyard gardens to vast agricultural fields, fertilizers are the unsung heroes that fuel growth and vitality. But what is fertilizer, and why is it so foundational for plant life?

In this article, we'll delve into the world of fertilizers, unearthing their composition, significance, and the pivotal role they play in nurturing plant life. As we proceed, we'll also demystify fertilizers, recognizing them as an essential element of plant nourishment and soil health, rather than a mere gardening accessory.


The Building Blocks of Organic Fertilizer

In order for a plant to grow and thrive, it needs a number of different chemical elements. The most important are:

  • Carbon, hydrogen and oxygen, which are readily available from air and water
  • Nitrogen, phosphorus, and potassium (a.k.a. potash), which are the three macronutrients and elements you find in most packaged fertilizers
  • Sulfur, calcium, and magnesium, which are secondary nutrients
  • Boron, cobalt, copper, iron, manganese, molybdenum, and zinc, which are micronutrients


Plant Nutrients for Soil Fertility

The most important of these (the ones that are needed in the largest quantity by a plant) are nitrogen, phosphorus, and potassium. If you have read the articles How Cells Work and How Food Works, you have heard about things like amino acids, cell membranes and ATP. Nitrogen, phosphorus and potassium are important because they are necessary for these basic building blocks. For example:

  • Every amino acid contains nitrogen.
  • Every molecule making up every cell's membrane contains phosphorous (the membrane molecules are called phospholipids), and so does every molecule of ATP (the main energy source of all cells).
  • Potassium makes up 1 percent to 2 percent of the weight of any plant and, as an ion in cells, is essential to metabolism.

Without nitrogen, phosphorus and potassium, the plant simply cannot grow because it cannot make the pieces it needs. It's like a car factory running out of steel or a road crew running out of asphalt. Unsurprisingly, this chemical makeup explains why nitrogen fertilizers, phosphate fertilizers, and potassium fertilizers are some of the most common options on the market.


Dead Matter Makes For Great Organic Fertilizers

If any of the macronutrients are missing or hard to obtain from the soil, this will limit the growth rate for the plant. In nature, the nitrogen, phosphorous and potassium often come from the decay of plants that have died. In the case of nitrogen, the recycling of nitrogen from dead to living plants is often the only source of nitrogen in the soil.

To make plants grow faster, what you need to do is supply the elements that the plants need in readily available forms. That is the goal of fertilizer. Most fertilizers supply just nitrogen, phosphorus and potassium because the other chemicals are needed in much lower quantities and are generally available in most soils. Nitrogen, phosphorus and potassium availability is the big limit to growth.


Understanding Commercial Fertilizers

Ever look for a bag of nitrogen containing fertilizers, only to discover it's always mixed in with potash fertilizers? A simple nitrogen fertilizer, as it turns out, will only supply a fraction of the essential nutrients needed for organic matter, which in turn limits plant growth and crop yield.

The numbers on a bag of fertilizer tell you the percentages of available nitrogen, phosphorus and potassium found in the bag. So 12-8-10 fertilizer has 12-percent nitrogen, 8-percent phosphorous and 10-percent potassium. In a 100-pound bag, therefore, 12 pounds is nitrogen, 8 pounds is phosphorous and 10 pounds is potassium. The other 70 pounds is known as ballast and has no value to the plants.


Other Types of Fertilizers

We should point out that there are other options as well. Synthetic fertilizers, engineered through chemical processes, offer a precise balance of nutrients, tailored to meet specific plant needs. These granular or powdered formulations are celebrated for their concentrated nutrient content and quick-release properties, providing an immediate boost to plant growth.

On the other hand, liquid fertilizers, either synthetic or organic, deliver nutrients in a soluble form, ensuring rapid absorption by plant roots. Their ease of application, through methods like foliar feeding or drip irrigation, makes them a favored choice for both indoor plants and organic farming operations. While synthetic fertilizers promise efficiency and targeted nutrition, liquid fertilizers excel in their quick action and versatility.


Innovative Approaches to Fertilizer

Fertilizers play a multifaceted role in agricultural sciences, intertwining natural processes with human innovation. The integration of green manure crops, for instance, showcases a method where plants are grown and plowed back into the soil, enriching it with organic matter and nutrients.

Meanwhile, anhydrous ammonia, a potent source of nitrogen, exemplifies the chemical ingenuity in harnessing atmospheric nitrogen, vital for plant growth, in a form readily usable by crops. This process, intriguingly, contributes to the global carbon dioxide cycle, underlining the interplay between agriculture and environmental dynamics.


Finally, the utilization of sewage sludge as a fertilizer highlights the recycling of waste into valuable nutrients, although it raises concerns about potential contaminants. These diverse approaches to fertilization, from the organic to the synthetic, reflect our ongoing quest to sustainably meet the demands of a growing population while respecting our planet's ecosystems.

How Soil Structure Gives Us Life

So why don't people need fertilizer to grow? Because we get everything we need from the plants we eat or from the meat of animals that ate plants. Plants are factories that do all of the work to process the basic elements of life and make them available to us.

To get more information on fertilizer and other related topics, check out the links on the next page.


This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.