Commercial Horticultural Field Specialist
Iowa State University Extension and Outreach
Interpreting a soil report can seem like a daunting task to do yourself but it does not have to be. The article “Managing Soil pH in Horticulture Crops” addresses soil pH and can be found in the May 2017 Acreage Living Newsletter. Phosphorus and potassium are the two most common nutrients that need adjustment. A soil report will list phosphorus and potassium in pounds per acre (lbs/acre) or parts per million (ppm). The formula lbs/acre = ppm x 2 can be used to convert between the two numbers. Additionally, some labs will list phosphorus with two numbers, Bray P1 and Bray P2. Bray P1 phosphorus is readily available for plant uptake while Bray P2 will slowly be available through the season. (Note: I always calculate off the Bray P1.)
Optimum soil phosphorus and potassium levels have been established through greenhouse trials. In general, soils should be optimized at rates shown in Table 1 below; however, the Midwest Vegetable Production Guide has crop specific recommendations that vary slightly from the rates listed below. To calculate how much phosphorus or potassium to apply to the soil, subtract the value from the soil fertility report from the optimum rate listed in the table below. For example, if your fertility report has a potassium value of 280 lbs/acre and you plan to grow vegetables, then you would apply:
Table 1. Optimum soil fertility of phosphorus and potassium for fruits and vegetables.
The next step is to calculate how much area you are treating. For this example, we have a high tunnel of tomatoes with six, 90 ft long rows. We can assume a bed diameter (root zone) of 4 ft. The total root zone in this high tunnel then is 6 rows x 90 ft each x 4 ft wide beds = 2160 sq ft.
From our example above, we need to apply 60 lbs/acre potassium to the 2160 sq ft. One acre is equivalent to 43,560 sq ft. To calculate how much potassium to apply, we use the following formula:
Solving for x, we need to apply 3.0 lbs of potassium to the 6 rows of tomatoes in a 4ft wide band.
The next step is the complicated part. A bag of fertilizer states nutrient values as N-P-K but in actuality it is %N–%P2O5–%K2O. For example, a bag of 10-10-10 fertilizer is 10%N, 10% P2O5, and 10% K2O by weight. A conversion factor of 0.43 is required to convert P2O5 to P and a conversion factor of 0.83 to convert K2O to K. No conversion factor is necessary for nitrogen.
The following formulas will help you calculate lbs of P or K, which are color-coded to the following example.
So, we need to apply 3.0 lbs of potassium to the 6 rows of tomatoes in a 4ft wide band, how much 10-10-10 fertilizer must we apply? There are a couple ways to solve this, the following two formulas derive the same result.
Keep in mind when using a fertilizer like 10-10-10, you are also applying nitrogen and phosphorus. If these nutrients are not needed, then a 10-10-10 fertilizer is not the best choice to use. There are many fertilizer options available when individual nutrients are needed.
Interpreting Soil Reports
The next section will estimate fertility applications from sources such as compost.
From the previous section, we need to apply 3.0 lbs of potassium to 6 rows of tomatoes in a 4ft wide 90ft long band (2160 sq ft). The compost has an analysis of
- Actual analysis = 1.93% as K2O
- Lbs/1000 gals = % x 85.
- 39lbs K2O per ton
1. We start by determining how much potassium is in the compost by volume.
2. Like with dry fertilizer month, we have to convert K2O to K by multiplying K2O by the conversion factor of 0.83.
3. We now know there are 136lbs K per 1000 gals of compost. We only need 3 lbs of K. How many gallons do we need to apply?
Solving for X, we find that we need to apply 22 gallons of compost to supply 3 lbs of potassium to the 6 rows of tomatoes in the high tunnel.
4. However, compost is often sold by the cubic yard. There are 175 gal per cubic yard.
Solving for X, we find that we need to apply 0.126 cubic yards of compost.
5. Alternatively, we know there are 39lbs of K2O per ton. We can use this information to calculate how many pounds of compost we need to apply to supply 3 lbs of potassium to the 6 rows of tomatoes in the high tunnel. Remember, we have to convert 39lbs K2O to K. This time, we will convert K2O and calculate pounds of compost in one step (note: there are 2000 lbs per ton)
Solving for X, we find that we need to apply 187 pounds of compost.