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Application Of Pesticide & Fertilizer

It doesn't take an Einstein to have a successful garden, but some basic arithmetic can assist any gardener. Any time fertilizer, soil amendments, or pesticides are applied to the garden, it is important that the correct amounts be calculated so the applied material will help, not hurt, the plants. The following example will illustrate proper garden calculations.

Mr. MacGregor has a garden that is 30 feet long and 20 feet wide. His soil test recommendations are to apply 2 pounds of actual nitrogen per 1,000 square feet.

Using his new watering system, he wishes to apply one inch of water per week.

His cabbage plants occupy an area 5 feet wide and 12 feet long, and while they are fenced so that rabbits can't get them, cabbage loopers can and he would like to spray them with pesticide.

First Mr. MacGregor must calculate his garden area by multiplying length by width: 30 ft. X 20 ft. = 600 sq. ft. If his garden had been irregular in shape he could have divided it into rectangular sections, calculated the area of each section, and added them together.

To calculate the amount of fertilizer to spread over the garden, Mr. MacGregor checks the analysis of his fertilizer of choice. He is using 5-10-10. The first number in this analysis gives the percentage nitrogen by weight, or 5%. He divides the amount of actual nitrogen desired by the percentage, expressed as a decimal (1% = .01, 5% = .05, 15% = .15, etc.): 2 lbs. divided by 0.05 = 40 lbs. for a 1,000 square foot garden.

Since Mr. MacGregor's garden in 600 square feet, or six tenths of 1000 sq. ft., he multiplies the amount of fertilizer required for 1,000 sq. ft. (40 lbs.) by six tenths, resulting in the proper amount for his garden, or 24 lbs.

His irrigation system has an easy-to-read in-line meter which indicates that the system uses 10 gallons of water per minute (gpm). An inch of water is equivalent to 28,000 gallons of water per acre. An acre is equal to 43,560 square feet. In gallons per square foot, that is 28,000 gal / 43,560 = 0.642 gal/sq ft. This can safely be rounded to 0.65 gal/sq ft, and for Mr. MacGregor's garden, the result: 0.65 gal/sq ft X 600 sq ft = 390 gallons. At 10 gpm, that will require: 390 gal / 10 gpm = 39 minutes.

Mr. MacGregor's neighbor uses the low-tech but equally accurate, and simpler, method of measuring the water falling on her garden from a sprinkler with a rain gauge.

Standard spray applications require about 1 quart of spray for 125 sq ft of area. Mr. MacGregor's cabbage occupy 5 ft X 12 ft = 60 sq. ft., so he will require 60 sq ft / 125 (sq ft/qt) = 0.48 qt. This can be rounded to 0.5, or 1/2 quart. The label states that 4 tablespoons of concentrated powder are used to prepare one gallon. One gallon equals 4 quarts, so: 1/2 qt. spray X 4 T/gal divided by 4 qt. = 1/2 tablespoon concentrated powder to mix 1/2 quart spray.

Too little water, fertilizer, or pesticide can result in garden problems. Too much can cause even worse problems, and costs extra money as well. Careful garden calculations will ensure that garden inputs have their desired effects.