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Interpreting soil tests for fruit and vegetable crops

After you receive a soil test report, the next step is to interpret it and determine what, if any, amendments to add. You may need to add nutrients, organic matter, or change the soil pH, depending on the results and on the crop being planted.

The same basic steps apply regardless of the source of nutrients (manure, compost, synthetic fertilizer, etc.). Each material contains a certain concentration of nutrients and knowing this is important for figuring out how much actual product to use. Commercially available amendments list these concentrations on the label as a percent of total nutrients.

You might have seen numbers like 8-2-4, 10-26-26, or something similar. Those numbers refer to the amount of nitrogen (N), phosphorus (P) as P2O5, and potassium (K) as K2O, present in the given material.

If using manure or compost from your farm or another farm, it is important to estimate or determine the nutrient concentrations before making the application. This will assure the correct amount is applied and will minimize potential environmental problems from applying too much.

Many soil testing labs also offer manure and compost testing services. Compost sellers may also be able to provide you with an analysis of their products.

Before you begin: Get to know fertilizer labels

Fertilizers containing N, P (phosphate), and K (potash) list these concentrations as N-P-K on the label. For example:

  • A fertilizer labeled 8-2-4 is 8% nitrogen (N), 2% phosphorus (P2O5) and 4% potassium (K2O).
  • An ammonium phosphate fertilizer labeled 16-20-0 contains 16% N, 20% P, and 0% K.
  • A non-NPK fertilizer for other nutrients like calcium or boron will also list the percentage of that nutrient on the label.

For simplicity, P2O5 is referred to as P and K2O is referred to as K throughout this article.

Steps for calculating fertilizer rates based on a soil test

  1. Know the nutrient concentrations in your soil, listed on the report.
  2. Using tables in the nutrient management guide, and the numbers on the soil test (step #1) determine the pounds per acre of actual nutrient to add.
  3. If you recently planted cover crops or used manure-based fertilizers, account for any nutrient credits.
  4. Decide which nutrient source to apply based on the relative amounts of nutrients needed. For instance, if more N is needed but little P or K, choose a source with relatively higher N and lower P and K.
  5. For each nutrient, divide the pounds per acre of actual nutrient needed by the fraction of nutrients in the fertilizer to get the amount of fertilizer to apply.

Example: If your fertilizer has 30% N and your crop requires 100 lb/acre of N, divide 100 lb/acre by 0.2. This equals 333.33 pounds per acre of fertilizer needed to achieve your nitrogen requirement.

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For organic flowers

When your fertilizer needs don't match the fertilizer ratios of available products

Since organic growers have fewer fertilizer options, there is rarely a perfect organic product to meet the needs of a particular crop in a particular soil. Instead, most growers will have to use one or two primary products and then supplement, or get a custom mix from a fertilizer supplier.

While it may be tempting to over-fertilize certain nutrients, it is important to avoid doing so. Too much fertilizer is not just expensive, it is damaging to the environment. Many organic farms in Minnesota have too much phosphorus due to their reliance on manure, which has been shown to have a negative impact on water quality.

There are two different approaches growers can use to select a nutrient source or sources that fit their nutrient needs.

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Authors: Annie Klodd, Extension educator, fruit and vegetable production and Natalie Hoidal, Extension educator, local foods and vegetable production 

Reviewed by Paulo Pagliari

Reviewed in 2021

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