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.
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
- Know the nutrient concentrations in your soil, listed on the report.
- 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.
- If you recently planted cover crops or used manure-based fertilizers, account for any nutrient credits.
- 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.
- 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.
The type of fertilizer to apply should depend on the nutrient concentrations of your soil and the crop you planted or intend to plant. For example, if the soil is deficient in P but is already very high in K, choose a fertilizer with a higher concentration of P and little or no K (for example 2-14-0 bone meal). A less appropriate ratio, in this case, is 8-2-4, which contains little P relative to K and N.
Each crop has unique nutrient needs. When growing several crops in the same field, it may be necessary to alter fertilizer applications within the field for each crop. Crop-specific recommendations are listed throughout the Nutrient Management for Commercial Fruit and Vegetable Crops In Minnesota.
Soil test reports list nutrient concentrations in parts per million (ppm). Some reports recommend how many pounds per acre (lbs/acre) of each nutrient to add, but not all do. The nutrient management guide tells you how many pounds per acre are needed based on the ppm values in your soil test.
If the soil test does not list recommendations, the next step is to calculate the rate (pounds per acre) of each nutrient, if any, to add. The rate is based on the difference between the existing concentration stated on the soil test, and the optimal nutrient level for your crop(s) listed in Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota. This guide takes some of the work out of calculating nutrient needs. It provides tables listing how much of each major nutrient to apple based on test results. Use this guide to work through the following example:
Kara's tomatoes: Kara wants to plant tomatoes. Her soil test shows a P concentration of 10 ppm according to the Olsen test. Go to Table 29 (page 22) of the guide. According to this table, if Kara's field contains 10 ppm of P (Olsen), and she is growing tomatoes, she should add 100 lbs/acre of P.
Next, Kara looks at the soil test again, and the K level is 190 ppm. Go to Table 30 (page 23). According to the table, if the soil contains 150 ppm of K, she should add 100 lbs/acre of K for tomatoes.
After determining P and K recommendations, the next step is to find out how much nitrogen to apply. This will help her decide which N-P-K fertilizer is best. Most soil tests do not provide a ppm concentration for N. However, they list the percentage of organic matter (%OM), which is correlated to soil N needs. Higher organic matter soil generally requires a lower annual N input than soil with low organic matter. Kara's report shows a %OM of 6.9%. This is considered high. See Table 28 (page 21). For high %OM soil, 90 lbs/acre of N should be applied for tomatoes.
The guide recommends applying half of this N before planting, and 1/2 a few weeks later when fruit begin to form. This is most relevant for farmers using synthetic nitrogen sources. However, for organic farmers using slow-release nutrient sources, a split application is not necessary. If Kara is using synthetic fertilizers, she should apply 45 lbs/acre N in the spring before planting, and 45 lbs/acre later. If she’s using organic or slow-release fertilizers, she should apply 90 lbs/acre N upfront.
In some cases, nutrient credits should be taken into account. Nutrients that are leftover from previous years are referred to as “credits”. If you conduct a soil test each year, it will take into account any P or K credits, but N credits should still be considered. If you are relying on a previous year’s soil test for your nutrient management decisions, all three nutrients should be taken into account. Examples of nutrient credits include:
Cover crops: Cover crops provide nitrogen to the crop that follows them. Leguminous cover crops (plants belonging to the legume family) fix nitrogen from the atmosphere. While non-legume cover crops do not fix nitrogen, they help to prevent nitrogen leaching, and so more nitrogen is available to the following crop.
This video provides a simple step-by-step tutorial for calculating nitrogen credits from a cover crop.
Organic fertilizers: Slow-release organic fertilizers are usually not 100% available in the first year of application. If you applied these products recently, make sure to account for the nutrients that continue to become available in the second and third years after application. Some examples include:
- For dairy manure, 40-60% of the nitrogen is available in year 1, and 30-40% will be available in years 2 and 3 (split between years). Some will be lost to the atmosphere or via leaching. If the dairy manure is fully composted at the time of application, only 5-20% is available in year 1.
- For poultry manure, 50-75% of the nitrogen is available in year 1, with 20-25% total being released in years 2 and 3. If the poultry manure is composted (e.g. 4-6-4 composted poultry manure), only about 40-50% is available in year 1.
- 70-90% of P and K should be available in year 1 in most manures.
These credits should be subtracted from your calculation of needed nutrients.
For example, if your soil test indicated that you need 70 pounds of nitrogen per acre, and your cover crop provided you with 50 pounds per acre, you can adjust your fertilizer needs: (70 pounds needed - 50 pounds provided by cover crop = 20 pounds needed from fertilizer).
The next step is to decide what type of fertilizer, compost, or manure to apply, and how much. There is a basic equation that is important during this process. It calculates how many lb/acre of a fertilizer to use, based on 1) the recommended lb/acre of actual nutrient to add, and 2) the concentration of that nutrient in the fertilizer:
Pound per acre of actual nutrient needed / fraction of nutrients in the fertilizer (6% = 0.06)
Taking the example from above, Kara needs 90 lbs/acre N, 100 lbs/acre P, and 100 lbs/acre K for her tomatoes, but based on the Nutrient Management Guide, she is doing a split application of N, so she only needs 45 lbs/acre N up-front.
A common commercial fertilizer that closely matches this need would be a 10-26-26 product. Since she needs 100 lbs/acre of both P and K, we can calculate:
100 pounds per acre needed / 0.26 (fraction of P and K in the fertilizer) = 384.62 pounds of fertilizer per acre.
Next, we’ll calculate the amount of nitrogen provided by this amount of fertilizer:
385 pounds of fertilizer used to meet P & K needs * 0.1 (amount of N in the fertilizer) = 38.5 pounds of N per acre
Since Kara still needs 10 pounds of nitrogen to meet her 45 lbs/acre goal, she can use a nitrogen source such as urea (46-0-0) to fulfill the final 10 pounds. She can use the urea fertilizer for her second nitrogen application once when fruit begins to form.
For organic growers
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.
Try to find a fertilizer that has an N-P-K ratio relatively similar to the ratio you need based on your soil test.
However, let’s say Kara (see example above) lives near a turkey manure composting facility and has ready access to 4-6-4 fertilizer made up primarily of composted turkey manure.
Kara needs 90 lbs/acre N, 100 lbs/acre P, and 100 lbs/acre K. Since she is using organic fertilizer, she will apply all 90 pounds of N up-front rather than doing a split application.
Divide the recommended nutrient amount that you calculated earlier by the percent nutrient (fractional basis) in the fertilizer. Divide the P recommendation (100 lbs/acre) by the fraction of P in the bag (6% would be 0.06):
Nutrient recommendation / % nutrient in the product expressed as a fraction = 100 / 0.06 = 1,666 lbs per acre
However, keep in mind that the phosphorus in composted manure is only about 90% available to plants. So in reality, the equation should be:
100 / 0.06(0.9) = 1,852 lbs per acre
This means Kara would need to apply 1,852 lbs/acre of a 4-6-4 fertilizer in order to satisfy the phosphorus demands of her crop and soil. She could also do this same calculation for N and K.
But since P is the most limiting nutrient in her soil at this point, it is useful to start the calculation based on P demands. From here, she can calculate how much N and K are being provided.
Only about 40-50% of the nitrogen in composted poultry manure is available in year 1, and around 90% of the potassium is available. So, by multiplying the % available nutrient by the % nutrient by weight in the product, then multiplying by the total amount of product being applied, we get:
Nitrogen: 1852 * 0.04(.5) = 37.04 pounds N (assuming 50% availability in year 1)
Potassium: 1852 * 0.04(.9) = 66.67 pounds K
In this case, by using 4-6-4 composted poultry fertilizer, Kara has under-applied nitrogen and potassium. She would need to supplement her nitrogen needs with another product such as blood meal (13-0-0), and her potassium needs with a product like potassium sulfate (0-0-50).
- 4-6-4 was clearly not the best choice in this scenario. It was used as an example because it is a common fertilizer used by organic growers in Minnesota.
- 1,852 lbs/acre is a substantial amount of fertilizer to apply. If Kara can use a fertilizer with higher nutrient concentrations, like a 10-20-10, then she would only have to apply 500 lbs/acre of the fertilizer in order to meet her soil's P demands.
- Remember to include nitrogen credits. If Kara had used a legume cover crop, she could have supplemented her nitrogen needs substantially.
By using two products with different ratios of nutrients, you can mix them at different rates to more closely match your soil’s nutrient needs.
Sticking with the poultry manure example, various companies sell fertilizer composed primarily of composted poultry manure with nutrient ratios such as 4-6-4 and 8-2-4.
Kara’s nutrient needs are 90 lbs N, 100 lbs P, 50 lbs K (see above example for more context on the example of Kara’s tomatoes). At first glance, the 8-2-4 option doesn’t look ideal for Kara. However, the actual N content is much lower than the P and K content due to availability. Therefore, a higher N product may be appropriate.
If you cannot find a suitable single fertilizer source to meet your needs, try the same calculations as above, but with different combinations of two or more fertilizers.
Remember that for composted poultry manure, only around 40-50% of the N is available in year 1, and 90% of P and K will be available.
- Using approach 2, Kara would still need to supplement some nutrients with more targeted sources like blood meal (13-0-0), rock or colloidal phosphate (0-25ish-0), and potassium sulfate (0-0-50). Exact ratios will vary based on product, so make sure to check the label. However, this approach may allow you to use less supplemental product and keep costs lower.
- These calculations assume that there is no nitrogen credit from previous years; if Kara used the same products last year, she could assume that around 15% of the nitrogen applied last year will be available this year.
- Since she is using a slow-release fertilizer, Kara should take this year’s application into account when calculating her fertilizer needs next year. About 15% of the nitrogen in the composted turkey she applies this year will be available next year.
Reviewed in 2021