Less is More: How to Apply Compost in Your Vegetable Garden

When compost is over-applied, excess nitrogen and phosphorus can make their way to nearby bodies of water through leaching and runoff and harm the environment.

Farmers avoid this by testing their compost, much like they test their soil. Then, they use the compost's nutrient content to calculate the amount to apply based on the needs of the crops they grow. You can use the same approach in your vegetable garden.

To do this, you'll need:

1. A soil test report to understand the amounts of nutrients already in your soil, and
2. A compost test report to know what nutrients your compost will contribute.

Most soil testing labs also analyze compost. For example, the Penn State Agricultural Analytical Services Laboratory offers compost testing services. You can learn more and submit a sample by visiting compost testing.

Compost is commonly applied based on the plant's nitrogen or phosphorus needs. But phosphorus is often over-applied when compost rates are based on nitrogen. We'll focus on the amount of compost to apply based on the plant's phosphorus needs. This approach helps keep your plants productive while protecting the environment.

Step 1: Find Out How Much Phosphate is in One Pound of Compost

Start by locating the percentage of phosphate in the "as is" weight basis column of your compost report. The report below shows 0.19% phosphate.

To convert this to pounds of phosphate per pound of compost, divide the percentage by 100. 0.19% ÷ 100 = 0.0019 lb phosphate in 1 lb compost.

Phosphate in compost exists in organic and inorganic forms. The organic portion must be mineralized, or converted into inorganic forms, before it can be used by plants. For simplicity and to avoid over-application and environmental issues, we’ll assume that all the phosphate in compost is available to plants for our calculation.

Step 2: Calculate How Much Compost to Apply

To determine the amount of compost to apply, divide the recommended phosphate rate by the phosphate content in one pound of compost. You can find the recommended phosphate rate in your soil test report, though it may need interpretation first. This article has step-by-step instructions on how to do that: How to tailor fertilizer recommendations to the fertilizers you have.

For example, let’s say the recommendation is 0.58 lbs of phosphate per 100 square feet, and we determined that our compost has 0.0019 lbs of phosphate per pound. 0.58 ÷ 0.0019 = 305. So, you would apply 305 lb of compost per 100 square feet of garden area.

Less is More

This photo shows plots from an experiment where we tested different compost types and application rates. Each square represents a different compost treatment. We grew bell peppers in these plots, and the plants had all the nutrients they needed.

Notice how thin the compost layer is; you can see the soil beneath it. You don't need much compost. If you don't have a compost analysis report, a practical approach is to apply compost no more than about ¼ inch thick, and only every few years.

This article benefited from editorial assistance provided by Microsoft Copilot.