In Extension, we often get calls about why someone’s food plot is not growing as expected. While there can be a host of reasons why this is the case, the first thing one should examine is the soil pH and soil fertility. If the soil pH is not at the correct level, nutrients can be bound in the soil and are not available for the plants to absorb. For example, if you have a soil pH of 5.6, adding more fertilizer will not necessarily increase the health of the plants. We must first raise the pH before the plants can utilize the fertilizer we added. It can take three to six months to raise soil pH, so lime applications should be made well in advance of planting to raise the soil pH. Many of the plants we grow for food plots like a soil pH of 6.0 to 6.5 on the pH scale.
A common problem I see when discussing fertilization is the lack of understanding of what the numbers mean on a bag of fertilizer. Common analysis of bag fertilizer consists of 10-10-10, 17-17-17, 34-0-0, 0-45-0, and 0-0-60, to name a few. Those three numbers are the percentage of nitrogen (N), phosphorus (P), and potassium (K) that the bag contains. For instance, 10-10-10 has ten percent nitrogen, ten percent phosphorus, and ten percent potassium. So a 50-pound bag would have five pounds of nitrogen, five pounds of phosphorus, and five pounds of potassium. A bag of 0-45-0 would only contain phosphorus, and it would make up 45% of that bag. A 50-pound bag of 0-45-0 would contain 22.5 pounds of phosphorus.
I will not go into how to take a soil sample in this article. If you need help taking a soil sample, call your local Clemson Extension Office or read our soil testing factsheet on The Home and Garden Information Center webpage (https://hgic.clemson.edu/factsheet/soil-testing/).
Your soil sample report has been emailed or mailed to you; now what? First, you need to know the size of the area you plan to plant. This can be in square footage or acreage. Please do not guess at this number. There are many tools available (for free) to help you calculate the area of your planting. For those who have smartphones, I would recommend downloading an area calculation app. This app will allow you to walk or ride around the area to be planted with your smartphone and calculate the acreage or square footage. If you have a rectangular or square area to plant, you can always calculate the area using length multiplied by width. Google Earth can also be used to draw the area on an aerial photo and calculate the acreage. If you try to estimate this number, there is a good chance you will over or under estimate, which will result in you spending more money than needed or not providing enough soil amendments to help plant growth. With the cost of fertilizer, calculating your area correctly can save you a lot of money.
Figure 2 shows a new plot that I plan to establish as a fall food plot this year. We will use this as an example. The plot is rectangular, so I measured the length and width of the plot. I measured the plot with a measuring wheel to be 30 feet wide by 330 feet long (30 X 330 = 9,900 square ft.). I also used Google Earth, as seen in figure 2, to calculate the area. Google Earth calculated the area by my best guess of drawing the plot on the aerial photo as 9,894 square feet. We can also convert that to acreage. One acre is equal to 43,560 square feet. To convert square feet to acreage, take 9,900 square feet and divide by 43,560 (9,900/43,560 = .227). This plot would equal 9,900 square feet or .23 acres.
When you receive your soil sample report, it should look like Figure 3 and Figure 4. For this example let us assume that I will be planting oats. For oats, we will use the grasses for wildlife code, which is the second one under the crop section. Since our plot is not an acre, we will need to calculate exactly how much we need for our plot. Remember, the area of our plot is 9,900 square feet or .227 acres.
Lime: Need 2,700 pounds per acre
- Convert 2,700 pounds of lime per acre to per 1,000 square feet
- Divide 2,700 by 43.56 (2,700/43.56) == 61.98 pounds per 1,000 square feet
- 61.98 X 9.9 = 613.6 pounds of lime for the new food plot
OR
- Use the .227 acres for the new food plot
- .227 X 2,700 = 613 pounds of lime for the new food plot (Rounding creates a small difference in the actual pounds)
Nitrogen: Need 100 pounds per acre. Split the nitrogen application into 2-3 applications through the growing season.
- Convert 100 pounds of nitrogen per acre to per 1,000 square feet
- Divide 100 by 43.56 (100/43.56) == 2.30 pounds per 1,000 square feet
- 2.30 X 9.9 = 22.73 pounds of nitrogen for the plot
- Split into 2 applications == 11.36 pounds of nitrogen per application
- To calculate the amount of 34-0-0 we need
- Take the amount of nitrogen you need and divide by .34
- 11.36/.34 == 33.4 pounds of 34-0-0 for the new food plot
- Take the amount of nitrogen you need and divide by .34
Phosphorus: Need 100 pounds of per acre.
- Convert 100 pounds of phosphorus per acre to per 1,000 square feet
- Divide 100 by 43.56 (100/43.56) == 2.30 pounds per 1,000 square feet
- 2.30 X 9.9 = 22.73 pounds of phosphorus for the plot
- To calculate the amount of 0-45-0 we need
- Take the amount of phosphorus you need and divide by .45
- 22.73/.45 == 50.5 pounds of 0-45-0 for the new food plot
- Take the amount of phosphorus you need and divide by .45
Potassium: Need 60 pounds of potassium per acre.
- Convert 60 pounds of potassium per acre to per 1,000 square feet
- Divide 60 by 43.56 (60/43.56) == 1.37 pounds per 1,000 square feet
- 1.37 X 9.9 = 13.56 pounds of potassium for the plot
- To calculate the amount of 0-0-60 we need
- Take the amount of phosphorus you need and divide by .60
- 13.56/.60 == 22.6 pounds of 0-0-60 for the new food plot
- Take the amount of phosphorus you need and divide by .60
I know this is a lot of math can be confusing, but don’t worry. You can contact your local Clemson Extension Office and we can help you with these calculations. There is also a Clemson Fertilizer Blend Calculator located online that does many of the calculations for you. However, it does not take into account the area of your planting. This calculator can be found at: https://precisionag.sites.clemson.edu/Calculators/Fertility/fertBlend/
Author(s)
Jeff Fellers, Cooperative Extension, Forestry and Wildlife Agent
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