Container Gardening · Garden Musings · Perennials · Shrubs · Trees · Vegetable Gardening

Understanding Fertilizers: What do those numbers mean?

It won’t be long before plants should be growing like gangbusters in our gardens again. What can we do if a few of them seem less than enthusiastic to wake up or are struggling to get their green on?

First, commit to building the health of your soil over the upcoming season. “Feed your soil and the soil will feed your plants,” the saying goes. Mulch the garden with compost, and mix compost with soil when planting annuals and perennials.

Plants need nutrients to thrive and they take what they need from the soil, or try to. Soils differ in the amount of nutrients they offer. The nutrient levels in my soil are different than the amount of nutrients in yours.

Veteran gardeners may be able to diagnose nutrient deficiencies by observing their plants. If leaves begin to turn yellow, nitrogen might be the solution. Foliage turning purple may signal the need for phosphorus. The best way to determine the amount of nutrients in your soil is to do a soil test. Basic test kits are inexpensive and available at local garden centers. Call your local extension office for information on labs that perform detailed, comprehensive soil analyses for a fee.

Results of soil tests report nutrient levels and may give recommendations to improve them. Fertilizers and soil amendments help abate deficiencies, but which ones should we purchase?

First, let’s look at the difference between fertilizers and soil amendments. Amendments are added to the soil to improve the soil structure and increase microorganism activity. They may or may not offer nutrients to plants.

Soil amendments are organic or inorganic. Examples of organic amendments include worm castings, composted manure and compost. In addition to improving soil composition, they possess small amounts of nutrients. Inorganic amendments, like gypsum, vermiculite and perlite, improve soil drainage but do not contribute any nutrients.

Soil amendments, mixed in the soil, make life easier for plants. Roots grow stronger and deeper; foliage is healthier; and stems are stronger in amendment-rich soil.

Fertilizers, on the other hand, supply nutrients directly to plants but do not improve the soil structure. Fertilizers can be synthetic or organic.

Synthetic fertilizers are manufactured from chemical sources. They are available in both granular and liquid forms. They provide nutrients to plants quickly. Most synthetic fertilizers do not contain micronutrients or secondary nutrients.

Notice all the secondary nutrients and micronutrients in Dr. Earth’s Tomato & Vegetable fertilizer. This is an organic fertilizer.

Organic fertilizers come from the remains of living things. They release their nutrients slowly as they are broken down by bacteria in the soil. Organic fertilizers often contain secondary nutrients and micronutrients. Bone meal, blood meal and fish emulsion are examples of organic fertilizers.

Now to the numbers…what do those numbers on fertilizer packages mean?

Any product labeled as a fertilizer must include an analysis of its contents on the label. It is always three numbers – 10-10-10, 10-20-10, 20-0-0. These numbers report the percentage, by weight, of Nitrogen (N), Phosphorus (P), and Potassium (K) in the package. Mathematicians will notice that in all three examples given, the three numbers do not add up to 100. They never will. The remaining ingredients are called fillers and are included in the fertilizer to help us apply them evenly.

Nitrogen, Phosphorous and Potassium are the three main nutrients plants need to thrive. Each of them influences plant growth differently.

Blood meal is an organic source of nitrogen.

Nitrogen (N) helps plants produce more chlorophyll. Nitrogen promotes lush, deep green leaves and tall stems. It also affects the overall health of the plant.

This synthetic fertilizer is high in phosphorous and boosts blooms.

Phosphorus (P) is responsible for healthy root systems and flower, fruit, and seed production. It also boosts a plant’s immune system, increasing its ability to withstand environmental stress.

This synthetic fertilizer has something to offer all parts of a shrub and extra nitrogen for all those leaves.

Potassium (K), also known as potash, is critical to the overall health of the plant. It helps plants tolerate drought and temperature extremes and improves disease resistance.

Too much of a good thing is not a good thing when it comes to fertilizers. Always follow the directions on the label. Too much fertilizer can burn plants. Plants fed with fertilizers too high in nitrogen may result in delayed or reduced flowering. Plants fed with fertilizers too high potassium may have weak stems.

Many plants growing in healthy soil don’t need fertilizers. The soil provides all the nutrients they require. If plants show signs of nutrient deficiency, consider the type of plant and what its needs might be. Then head off to your local garden center confident to purchase the right fertilizer.

With the mysteries of fertilizers revealed, get ready to garden with me!

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