What is fertilizer and why do plants need it?

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

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:

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.

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.

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.

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.

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.

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