Plant Fertilizer 101: Kick Your Garden into the Next Gear with the Right Nutrients at the Right Time
Master your garden's nutrition with our ultimate fertilizer guide. Identify nutrient deficiencies, get the perfect timing, and decode those mysterious NPK numbers.
Just like all living things, plants need nutrients to survive. But it's not often that soil alone can provide all the necessary macro and micronutrients, as well as trace minerals and other elements. These nutrients drive growth, fruit and flower production and healthy root development.
Not all plants need the same nutrients, and even the same plant needs different nutrients at different stages of its development. In this guide we'll look at types of fertilizers, rates of application, and how to best apply it to improve garden soil. Let's dive in!
What is Fertilizer?
Fertilizer is a concentration of nutrients, minerals, and elements that can be applied to plants to help them grow. All fertilizers should have three numbers listed on their packaging, such as 10-5-10. This is called NPK, and it's the ratio of nitrogen, phosphorus and potassium, in that order, that's contained in that particular fertilizer mix. Other trace elements and secondary nutrients will also be contained in varying amounts.
Article continues belowThere are many forms of fertilizer available to give to your plants, shrubs and lawns. They can be organic fertilizer types or conventional synthetic feeds.
A fertilizer may come as a spray, powder, or granules, like the blue ones in the picture below.
Regardless of which type you have, those three NPK numbers are always in the same order.
Why We Need to Use Fertilizer
Plants need around 17 key elements. Of these, carbon, hydrogen, and oxygen are absorbed through the air and water, so they're pretty much taken care of by nature.
The remaining 14 elements are found primarily in the soil. When soil is depleted or low in an essential element, it's up to you to add it back in with fertilizer. Plants that have been fertilized don't just grow better – they're also stronger and better able to combat pests and disease.
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Decoding Fertilizer Numbers
All commercial fertilizer should list three numbers, separated by hyphens. These numbers represent the amount of nitrogen, phosphorus, and potassium, always in that order. Hence the term NPK. (Confusingly enough, the elemental symbol for potassium is K). These are the three macronutrients most needed by plants.
Each number represents the percentage of a given nutrient that is in a fertilizer mix.
For instance, a 100-pound 10-10-10 bag contains 10 pounds of each macronutrient. The same size bag but with a ratio of 10-20-15 will contain 10 pounds of nitrogen, 20 pounds of phosphorus, and 15 pounds of potassium. If all three numbers are the same, you're dealing with what's called a balanced fertilizer.
Nitrogen is primarily important for vegetative growth, so plants that are grown mainly for their leaves need a higher nitrogen number. This is why we use a high-nitrogen fertilizer on lawns.
A plant grown primarily for its flowers may need a formula with a higher phosphorus number.
Potassium helps move water, nutrients, and carbohydrates through the plant.
In addition to the NPK nutrients, a side panel will generally list any secondary nutrients as well as minerals. Secondary nutrients are calcium, magnesium and sulfur. Plants don’t need as much of these nutrients. Micro-nutrients are copper, molybdenum, nickel, chlorine, zinc, iron, manganese and boron.
Different Types of Fertilizer
Plant fertilizers may be organic or synthetically sourced. You’ll see different modes of application (ie slow release or fast) as well as different ratios of nitrogen, phosphorus and potassium. The most simple and ancient fertilizers are well-rotted manure or compost, which can be very effective.
However, it can be difficult to pinpoint the exact amount of nutrients in each of these. On average, garden compost is fairly balanced in the three macronutrients, but it contains a smaller amounts than commercial fertilizer.
Manure is especially high in nitrogen. Compost is more of a soil conditioner, while manure is often a top dress or soil amendment prior to planting.
Commercial fertilizers start with raw materials like sulfur, ammonia and phosphate rock. These ingredients are crushed, mixed and chemically treated. Organic fertilizers are mineral rocks combined with natural plant and animal derivatives.
Both organic and synthetic fertilizers come in liquid, granular, and powder form. Granular fertilizers are designed to be worked in around the root zone. Liquid forms are diluted with water and can be a soil drench or foliar spray. Powders are similar to granular fertilizers and are worked into soil at precise pound measurements per area.
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Choosing the Best Fertilizer for Your Needs
Before you apply any fertilizer, it's important to do a soil test. This test kit from Amazon will do the trick. The test will tell you which nutrients your soil is deficient in, which will help you select the right fertilizer. Soil tests will also tell the pH of the soil. The pH will affect a plant’s ability to uptake nutrients.
Vegetables like a pH of around 6.5, turfgrass needs 6-7, and most trees and shrubs prefer 5.5-5.6, while acid-loving plants like blueberries need a pH of 5. Some fertilizers will also help amend your soil's pH. You can also raise soil's acidity or make soil more alkaline with other amendments.
Once you have determined the pH is adequate for your plants, the individual nutrient needs can be addressed.
When to Fertilize Your Plants
One of the biggest mistakes people make with fertilizer is the timing. Misjudging the best times to apply fertilizer can cause more harm than good.
Most plants are dormant in winter and should not be fertilized. In cold regions, fertilizing too late in the summer can force new growth that will be sensitive to freezes in fall. Generally speaking, spring fertilizing is recommended for a lot of plants that are about to start active growth.
There are both fast and slow-release fertilizers. In cooler areas, a fast release is best because a slow-release formula requires the soil temperatures to be around 70°F (21°C) to release nutrients.
The frequency of fertilizing depends on the plant. For example, tomatoes are heavy feeders. If you're fertilizing tomatoes, bear in mind that they need more frequent feeding than most of your other plants. That being said, it's important not to over-fertilize, as this can burn plants, cause unsustainable growth spurts, and run off into waterways. Always read the label carefully to make sure you're applying the right amount.
How to Use Plant Fertilizer
Allowing for the type of plant, time of year, and even weather conditions when adding food, there are several ways to fertilize plants. One way is to broadcast prior to planting and mix into the soil to a depth of four inches (10cm). Another method is a band application, where plant food is arranged in bands a few inches below the surface of the soil to feed roots.
Foliar spray (applying a mist of fertilizer directly to the leaves) works especially well with plants that have large leaves. Starter fertilizers are gently formulated to avoid injury to young or newly transplanted plants. Side dressing (applying fertilizer to a vegetable garden that has already begun to grow) increases yields, but must be watered in thoroughly. If you have questions about fertilizing in your zone, check with your local extension service for information tailored to your area.
Frequently Asked Questions
What is the Most Important Fertilizer for Plants?
This depends on what kind of plant you're growing, and what stage of development the plant is at. Ffor instance, crops that have started to fruit will need phosphorus and potassium, whereas nitrogen is good for encouraging leaf growth.
However, as a general rule, the most important fertilizer is an all-purpose blend of 10-10-10 NPK (nitrogen, phosphorus, potassium). This can be administered in granular form (as a slow-release feed) or liquid form (added approximately once a month during active growth, or more frequently depending on the plant).
Should You Water Plants Before or After Fertilizing Them?
This question has caused plenty of debate in gardening circles, and expert opinions often differ on the running order of water and food. Again, it can depend on the plant, time of year, and type of feeding you have chosen.
It can help to water a few hours before feeding, as it can ‘prepare’ the soil or compost and reduce the risk of air pockets. However, granular feeds do their best work if you water the plant after application. Indeed, some feeds rely on water to kickstart plant absorption. If you add a lawn feed, some experts recommend waiting a day or so before watering.
