(News article from September 11, 2025)

When to add fertilizer, which fertilizer to use, and how to apply it are questions I get asked regularly. If you are new to gardening, the world of fertilizers and soil amendments can be confusing. With so many options available, how do you know which one to select and how much to use? To understand fertilizers, we need a basic understanding of soil fertility and plant nutrient needs.

Plants, like us, require different nutrients or minerals for healthy growth. We obtain these from the foods that we eat. Plants take up the nutrients they need through their root systems from the soil or media they are growing in. When we are not getting enough of the required nutrients and minerals from the food we eat, we may need to take supplements in the form of vitamins to maintain a healthy body. When plants aren’t getting the necessary amount of nutrients for optimum growth, we need to fertilize them.

There are 16 essential plant nutrients. These are grouped by the amount of nutrient needed. The first grouping is called primary nutrients since plants need these in the highest concentrations. These are oxygen, hydrogen, carbon, nitrogen, phosphorus, and potassium. Oxygen, hydrogen, and carbon are supplied by air and water. Nitrogen, phosphorus, and potassium are often the most limiting plant nutrients because insufficient amounts are supplied by the soil. These are abbreviated as N-P-K.

When you see a bag of fertilizer, the three numbers you see on the bag refer to the concentration of N-P-K in that fertilizer. For example, a bag of 13-13-13 has 13 pounds each of nitrogen, phosphorus, and potassium per 100 lbs. of that fertilizer. We call a fertilizer that has all three nutrients a complete fertilizer. When nutrient concentrations are equal, we call this a balanced, complete fertilizer.

The second group of essential plant nutrients are referred to as secondary. These are just as necessary as the primary nutrients but are needed in smaller amounts. Secondary nutrients are calcium, magnesium, and sulfur.

The last grouping of essential plant nutrients is called trace or micronutrients. These are needed in the smallest concentrations by plants. The trace nutrients are boron, chlorine, iron, copper, manganese, molybdenum, and zinc. A very small number of plants may also need either cobalt, nickel, silicon, sodium, and/or vanadium.

In a healthy, nutrient-rich soil, all the essential elements are typically present. However, not all soils are created equally, and there are many factors that can influence nutrient concentrations and availability. One of these factors is the pH of the soil – how acidic or basic it is. Most plants prefer soil on the slightly acidic side, between a pH value of 6.0-7.0. At this value, all the required minerals necessary for healthy growth are available to plant roots. We can adjust the soil pH by adding either sulfur to lower it or lime to increase it. The quantity of sulfur or lime added should be based on the recommendations from a soil testing facility.

Other factors that influence nutrient availability include soil texture – the amount of sand, silt, and clay in a soil. Soils with a good balance of these components are referred to as loamy soils. Soil organic matter content, microbial activity, moisture content, and structure also affect nutrient availability. Soils with rich organic matter and microbial activity, good water flow and holding capacity, and good structure – not too loose or too compact – are the ideal.

History of how the soil has been treated will also influence soil fertility. Growing the same plants in the same location repeatedly, like in our vegetable garden, will deplete the soil of nutrients quicker than if we rotate crops. Soils with heavy traffic will be more compact and less fertile. Also, intensive tillage and erosion negatively impact soil fertility.