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Soil and Plant Testing |
| What's in the Bag?
Introduction
to Fertilizer Grades and Calculations
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| All fertilizers report the grade using three numbers, e.g., | 28-3-3 | |
| The first number is always the elemental nitrogen content, so | 28% N | |
| The second is phosphorus pentoxide, or
phosphate, P2O5, not
phosphorus. (Phosphorus, P2, comprises 44% of P2O5, so 3% x 44% = 1.3%) |
3% P2O5 | 1.3 % P |
| The third is dipotassium oxide, or potash, K2O,
not potassium (Potassium, K2, comprises 83% of K2O, so 3% x 83% = 2.4%) |
3% K2O | 2.4 % K |
| If other nutrients or chemicals are present,
they will be
reported in the detailed analysis Other nutrients are reported on an elemental basis, e.g., % sulfur or % iron |
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| Applying Fertilizer Determine the area of your field, plot, yard, garden, etc |
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| Divide the area into rectangles or squares | |
| Measure the length and width of each parcel | 60 feet long and 40 feet wide 30 feet long and 25 feet wide |
| Multiply the numbers together to find square feet | 60 ft x 40 ft = 2400 ft2 30 ft x 25 ft = 750 ft2 |
| Total area | 2400 ft2 + 750 ft2 = 3150 ft2 |
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| With the area and the fertilizer recommendation, you can determine how much fertilizer you need. |
| Most fertilizer bags come with setting
instructions for
common spreaders
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| Read the bag carefully: | Some products also have herbicides or insecticides in them. |
| Make sure the product is safe for your lawn or garden. | |
| Some herbicides kill certain grasses, flowers, shrubs, etc. |
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Following the suggested rates and settings on the spreaders should result in a green, healthy lawn. |
| Yellowing (chlorosis) is a sign of stress that could be caused by nutrient deficiencies, disease, or insect damage. |
| Chlorosis is a symptom in plants as a fever is a
symptom
in animals. By itself, neither is sufficient information to diagnose
the
problem.
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| Lawn
Fertilizer
Calculation Examples Example 1 |
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| 5000 ft2 lawn needs 1 lb N/1000 ft2 | Since the lawn requires 1 lb N for each 1000 ft2 , divide total by 1000 |
| 5000 ft2/1000 ft2=5 | multiplier (how many 1000 ft2 plots are in the lawn) |
| 5-1000 ft2 plots x 1 lb N/1000 ft2 = 5 lb N required | |
| Fertilizer:ammonium sulfate 21-0-0 | Ammonium sulfate is 21% N, 0% phosphate, and 0% potash |
| How much ammonium sulfate is required to provide 5 lb N? | Ammonium sulfate is 21% N, so |
| Divide N required by the nitrogen content of the fertilizer | 5 lb N required / 21% = 23.8 lbs ammonium sulfate |
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Example 2 |
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| 20 ft x 10 ft garden needs 2 lbs phosphorus per 1000 ft2 | Area of the garden is 20 ft x 10 ft = 200 ft2 |
| As before, 200 ft2 / 1000 ft2 = 0.2 | multiplier (how many 1000 ft2 plots are in the lawn) |
| 0.2-1000 ft2 plots x 2 lb P / 1000 ft2 = 0.4 lb P | |
| Fertilizer: 20-10-5 contains | 20% N, 10% phosphate, and 5% potash |
| phosphate P2O5 is 44% P | 10% phosphate x 0.44 = 4.4% P in fertilizer |
| Divide P required by the P content of the fertilizer | 0.4 lb P required / 4.4% P = 9 lbs of 20-10-5 |
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General recommendations for Panhandle lawns and gardens Nitrogen |
| If pH is high, or calcium and magnesium are high, you will need to apply iron - the most reasonable source is Iron Sulfate, FeSO4 |
| Phosphorus on your gardens, or if you remove grass clippings |
| Potassium: Seldom (if ever) needed here |
| Gypsum, sulfur, and caliche
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| Nutrient Sources Commercial fertilizers, standard mixes |
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| Compost - provides most nutrients, may need more nitrogen | If phosphorus soil test levels get too high, the lab will recommend no more phosphorus applications for some time. If so, stop manure and compost applications |
| Manure - provides most nutrients |
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