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Fertilizer

Fertilizer composite

 

Fertilizer containers always have three numbers. These three numbers are the percents of nitrogen, phosphorus and potassium (N-P-K), listed in that order. A bag with the numbers 16-4-8, has 16% nitrogen, 4% phosphorus and 8% potassium, the rest of the fertilizer is inert filler.

 

Nitrogen, the first number, is the nutrient needed in the greatest quantities and most suggested rates of application on fertilizer containers are based upon delivering the proper amount of nitrogen. A rule of thumb for many plants is that they will need 2-4 lbs of nitrogen per 1000 sq. feet per year, either from existing soil nitrogen or from fertilizer.

 

Often it is difficult to calculate the amount of fertilizer to use due to the various  concentrations of nutrients in different brands. The tables below will help with these calculations.

  

Pounds of Fertilizer Needed per 1000 Square Feet to Supply Various Amounts of Nitrogen

 

Amount of Nitrogen Delivered

1 Pound Actual Nitrogen per 1000 sq. ft.

1 ½ Pounds Actual Nitrogen

3 Pounds Actual Nitrogen

 

N-P-K Percent in Product

lbs of Product

to apply

lbs of Product to apply

lbs of Product to apply

45-0-0(urea)

2.2

3.3

6.5

33-0- 0  (ammonium nitrate)

3.0

4.5

9.1

27-7-7

3.7

5.6

11.1

21-0-0

4.8

7.1

14.3

20-20-20

5.0

7.5

15.0

16-4-8

6.2

9.4

18.8

12-4-8

8.3

 12.5

25.0

10-10-10

10

15.0

30.0

5-10-10

20.0

30.0

60.0

 

 

 

 Application Rate for Granular Fertilizer to Apply 1 Pound of Nitrogen for 1000, 100 and 10 Square Feet

 

 

1000 Square Feet

100 Square Feet

10 Square feet

 

 

 

 

N-P-K Source

Pounds

Cups

Pounds

Cups

Tablespoons

16-4-8

6

12

.5

1

2

10-10-10

10

20

1

2

4

12-4-8

8

16

.75

1.5

5

5-10-10

20

40

2

4

8

 

Click here for recommendations about what type of nitrogen fertilizer to use and how much for top or side dressing of vegetables and ornamentals.

 

Organic Fertilizers

 

There are many sources of organic fertilizers with a wide variation in the concentration of the nutrients N-P-K. The advantages of organic fertilizers are that the nutrients are released more slowly, with less loss into ground water. Organics also contain many of the lesser "micro-nutrients" not found in commercial preparations. They also add organic material to the soil, improving structure and overall tilth.

 

Disadvantages are that larger volumes of material must be used which is generally more expensive. Because of slow release, rapidly growing crops, such as vegetables, may also need a quick release commercial preparation to supplement the demand.  Many people are surprised by the actual concentrations of nutrients in some organic materials. The table below lists the concentrations in organics.

 

Nutrient Content of Organic Materials

 

Material

% Nitrogen

% Phosphorus

% Potassium

Availability

 

Blood Meal

15

1.3

0.7

Slow-med.

Bone Meal

4

21

0

Slow

Coffee Grounds

2

0

0

Slow-Med.

Compost(unfortified)

2.5

0

1.5

Slow

Cottonseed Meal

7

1.3

1.2

Slow

Dried Blood

12-15

3

0

Med.-Fast

Fish Meal

8

7

0

Slow

Granite Dust

0

0

5

Slow

Grass Clippings

1

0

2

Slow

Greensand

0

1.5

5

Slow

Guano

12

0

0

Med.

Hay (mix)

1.1

0.4

1

Slow

Hay (Alfalfa)

2.5

0.5

2.1

Medium

Leaves

0.9

0.2

0.3

Slow

Manure (Cattle)

1-2

1

2

Medium

Manure (Horse)

0.4

0.2

0.3

Medium

Manure (Pig)

0.5

0.3

0.5

Medium

Manure (Poultry)

3-5

2-3

1-2

Medium

Milorganite (Dry)

5

3

2

Medium

Mushroom Compost

0.5

60

1

Slow

Peat

2

0.5

0.8

Very Slow

Red Clover

2.1

0.5

2

Slow

Sawdust

4

2

4

Very Slow

Soybean Meal

6.7

1.6

2.3

Slow-Med.

Wood Ashes

0

1-2

3-8

Fast

 

 

Click here to view OSU's Organic Gardening fact sheet HLA-6436, "Healthy Garden Soils".

 

 

It is not necessary to fertilize every year, apply only when needed. If a plant is not performing well, it may have a nutrient deficiency. This often produces yellow leaves which are smaller than normal. The plant may have overall stunted growth with fewer blossoms or fruits than expected.  Obtaining a  soil test will offer guidance whether fertilizers are needed and if so, which nutrients are deficient.

 

Many people think that fertilizer is plant "food" and the more you apply the better the plants performance. Fertilizer is not plant food. These mineral nutrients allow plants to be able to manufacture food from sunlight. Too much is much worse than too little—it can kill plants and pollute the environment. Always follow the labeled directions.

 

The best time to fertilize is either spring or fall. In fall wait until a month past the first frost. This insures the plant is dormant and will not put on new growth sensitive to the cold. The roots are still very active at this time and will add significant growth before the soil cools. In spring fertilize before new growth begins. Depending on the product it generally takes 3-4 weeks for the fertilizer to be available in root zones of plants.

 

Do not use nutrients which are not need. Do not use fertilizer with phosphorus and potassium (the second and third numbers) in previously fertilized areas unless a soil test indicates a need. A review of soil tests from homeowners in the Tulsa area showed that about 75% of them had adequate or, more often, excessive amounts of phosphorus. Adding more is not only harmful to plants but runoff pollutes our waterways.