03/11/2026
One question I get asked a lot is: What fertilizer should I use?
Most people are hoping for a quick answer like “use fish fertilizer” or “add kelp.” The truth is, without a soil test, there’s no way to know what your soil actually needs.
Here in Colorado, our soils are usually neutral to slightly alkaline. Neutral is ideal for most vegetable crops, but when soils become too alkaline they often contain higher levels of calcium carbonate. When that happens, plants can struggle to convert iron into a usable form. This means that no matter how much fertilizer you add, plants may still struggle to take up nutrients.
That’s why soil pH is one of the most important factors when planning a garden. You don’t want to spend money on fertilizer only to end up with poor plant growth.
Many of our soils are also clay-based, which means they can be more sodic and prone to salinity buildup. This can become an even bigger issue if you’re watering with household water that runs through a water softener.
Because of this, gardeners often add organic matter to improve soil structure. This helps open up pore space so roots can access water, oxygen, and nutrients more easily.
But even with better soil structure, you still need to know what nutrients are already present before choosing a fertilizer.
A lot of advice online says to just use a “balanced fertilizer.” That can work for raised beds or container gardens, where nutrients are used up and leach out more quickly.
However, in in-ground gardens, nutrients behave differently. Phosphorus and potassium tend to stay in the soil and build up over time, while nitrogen is soluble and can leach away much faster.
If your garden has been established for several years, there’s a good chance you already have plenty of phosphorus and potassium in your soil. In many cases, the nutrient that actually needs replenishing is nitrogen.
When that’s the case, a nitrogen-focused fertilizer (like urea) can be a better option, since it replaces the nutrient that is most likely to be lost from the soil.
The bottom line: before spending money on fertilizer, test your soil. It’s the easiest way to understand what your plants actually need, and what they don’t.
Learn to interpret soil tests taken from the vegetable, lawn, and new garden areas. #4250