Fertilizers: FAQ From Your Local Garden Center

Published On: November 3rd, 2021


If you have rich, loamy soil with a high amount of organic matter you may not need to fertilize your plantings – particularly if they are primarily ornamental trees, shrubs, and perennials. However, if you have depleted soils or grow heavy feeders like blooming annuals or vegetable plants, fertilizers can be the difference between successful and unsuccessful plants.

Identifying the N-P-K Ratio of Your Fertilizer

Nitrogen (N)

Nitrogen assists in the development of the vegetative growth of plants. It’s necessary for healthy leaves and stems. Minimal plant growth or pale leaf colors are common indicators of a lack of nitrogen. Too much nitrogen may encourage excess vegetative growth, at the expense of blooms or fruit.

Phosphorus (P)

Phosphorus assists in root development, flowering, and fruiting. It also aids in the plant’s ability to resist disease. A phosphorus deficiency increases the likelihood of plant disease.

Potassium (K)

Potassium is a necessary macronutrient for the proper functioning of all living cells. In plants, it’s a critical component of photosynthesis. Low plant vigor and discoloration or curling leaf tips are symptoms of potassium deficiency. You often see these symptoms on older leaves because plants will deliver accessible potassium to newer growth first.

You typically find an N-P-K ratio on the front of fertilizer packaging. The three numbers represent the percent of each nutrient in the fertilizer, by weight. These numbers alone will not tell you whether it’s quick-release, slow-release, or organicExpect to see higher numbers on synthetic fertilizers than on organic fertilizers. More on that later.

what do the the npk nurtrient levels on fertilizers mean

Secondary & Micronutrients

secondary and micronutrients found on the backside of your fertilizers package

In addition to these three primary nutrients, plants also need secondary and micronutrients to thrive.

  • Secondary nutrients are Calcium, Magnesium, and Sulfur.
  • Micronutrients, or trace elements, include Iron, Boron, Copper, Zinc, Molybdenum, Nickel, Manganese, and Chlorine.

Fertilizers may or may not contain all of these secondary and micronutrients, as they are necessary for very small quantities and may not need to be added to your soil. Conducting a soil test is the best way to determine which specific nutrients are lacking in your soil.

Where Can You Test Your Soil?

  1. Pick up a basic soil test kit here at Heyden’s Gardens.
  2. The UW-Madison Soil & Forage Lab offers comprehensive Soil Testing for Homeowners for a small fee.
  3. The Milwaukee Health Department offers Soil Screening Tests. The cost is $15 for nutrient analysis and $10 for lead screening for each bagged sample.

What are Mycorrhizae?

Plant scientists have only relatively recently come to appreciate the importance of mycorrhizae in plant health. Mycorrhizae are living organisms (fungi) with a symbiotic or mutually beneficial relationship with other plants. These fungi colonize the roots of plants and exponentially increase the root’s ability to access and utilize moisture and nutrients. Since they are unable to photosynthesize themselves, these fungi depend on the plant to provide the carbohydrates they need to survive. In this way, both organisms benefit.

Many of the potting mixes and fertilizers we carry contain beneficial mycorrhizal.

The Difference Between Organic and Synthetic Fertilizer

Organic fertilizer is like a healthy, home-cooked meal, and synthetic fertilizer is like a quick-energy candy bar. 😊

  1. Organic fertilizers come from animal matter (manure) or vegetable matter (compost). Organic fertilizers break down over time and provide slow-release fertilization while at the same time improving soil quality.
  2. Synthetic fertilizers come from commercial manufacturers and are mineral-based. Synthetic fertilizers are fast-acting as they don’t need to decompose to supply nutrients to the plant. Nutrients from synthetic fertilizers don’t remain in the soil, so repeat applications are necessary to maintain fertility.

When and How Do You Fertilize?

  • In-ground plantings: We recommend an annual application of granular slow-release organic fertilizer. If plants show signs of nutrient deficiencies, you can apply once more mid-season. We don’t recommend fertilizing in late summer or fall because this can promote late-season new growth. Freezing weather damages new growth that doesn’t have adequate time to harden off.
  • Annuals: We recommend frequent feeding with soluble fertilizer. Our growing season is short, so we want our annual plants to grow quickly and bloom prolifically. A weekly or bi-weekly fertilization will encourage rapid growth and abundant blossoms. Our favorite soluble fertilizer is Jacks (link here to Jack’s blog feature).

How Does pH Affect Nutrient Availability?

Our soils in southeastern Wisconsin are quite alkaline because we’re built on a foundation of glacial limestone. High pH soils can inhibit a plant’s ability to utilize certain nutrients. You usually see this in our area as iron chlorosis.

Plants suffering from iron chlorosis develop yellow leaves in contrast with a network of prominent dark green veins. Several plants –including certain Maples and some Oaks – are especially prone to this condition in our native soils. Adding iron to the soil doesn’t remedy the problem since the plants can’t utilize the iron due to high soil pH. Therefore, we lower the soil’s pH so the plant can use existing soil nutrients.

garden fertilizers help prevent plant and leaf chlorosis yellowing leaves with green veins

How Do You Lower Soil pH?

  1. Slow-release elemental sulfur
  2. Faster-release aluminum sulfate

Adding sulfur to the soil remedies chlorosis and provides more beneficial growing conditions for specific Maples, Oaks, and acid-loving plants like Blueberries, Azaleas, and some Hydrangeas.

Repeat applications will be necessary to maintain a lower pH level. For repeated applications, we prefer the use of elemental sulfur over aluminum sulfate as it is possible for aluminum to build up to toxic levels in the soil over time. You can find both slow-release elemental sulfur and faster-release aluminum sulfate here at Heyden’s Gardens.

Shop Heyden’s Gardens for Garden Fertilizer Supplies

Granular Garden Fertilizers

happy frog organic garden fertilizer for sale at heyden's gardens in cedarburg

Liquid Plant Food

liquid plant food garden fertilizer for sale at heyden's gardens in cedarburg

Indoor/Outdoor Potting Soil

potting soil for indoor outdoor plants with garden fertilizer for sale at heyden's gardens in cedarburg