Estimating Fertilizer Amounts

Fertilizer instructions often are given as pounds of “actual nitrogen,” or some other nutrient. This can be confusing if you have a garden of unknown size and a bag of fertilizer. How do you know how much to put on?

For starters, you can assume that a pint of most chemical fertilizers weighs about a pound, and measure the fertilizer with two-cup (one pint) measuring cup from the kitchen.

For help in estimating the area of your garden, see How to Measure Area.

The percentage of each primary nutrient in the fertilizer is written on the label.

To find how much chemical fertilizer you need to provide a given weight of actual nutrient, divide the number of pounds of nutrient you need by the percentage of that element in the fertilizer.

For example, if you want to add 2 pounds of actual phosphate to your vegetable garden and your bag of fertilizer contains 20 percent phosphate. Divide the 2 pounds (the amount you want) by 20 percent (the percentage in the fertilizer): 2 divided by 0.20 equals 10. You will need to apply 10 pounds of the fertilizer to get the desired amount of phosphate.

How to Measure Area

Measure equal distances along the line, selecting a distance that will divide the line into 5 to 10 sections. If the longest dimension is 70 feet, you might select 10-foot segments. At each mark on the line, measure the width of the area perpendicular to the line. Write down all the measures. Add the widths together and divide by the number of widths to find the average width. Then multiply the average width by the length of the line (the height) to find the area.

Measuring Odd Shapes as an Averaged Radius

A second way to find the area of an odd shape is to treat it as a circle and find the average radius. To use this method, mark a paper plate into 8 equal segments as if you were cutting a pie. With a long nail or screwdriver, pin the plate to the ground near the center of the area being measured. Hook a tape rule over the nail or screwdriver and, laying it carefully along one of the lines on the plate, measure the distance from the center of the plate to the perimeter of the area being measured. Write down the distance, then move to each of the other 8 lines and measure the distances to the perimeter again.

When you are finished, add the distances together and divide by 8 to find the average radius.

Use the average radius in the formula for finding the area of a circle.

3.15 x radius x radius = area

If the average radius was 10.4 feet, multiply 3.15 x 10.4 x 10.4 = 341 square feet.

The accuracy of both of the above methods depends on the number of measurements taken. A larger number of measurements used to make the average width or radius gives more accurate results.