How Much Fertilizer Do I Need?

The first thing that needs to be done is to calculate how much area is in the place to be fertilized. The reader can do these simple calculations by going to our paper on how to calculate areas area_calculations.htm. This paper contains a variety of sizes and shapes that the average person will encounter when calculating how large of area to be fertilized .

Plant nutrient requirements can be supplied by a wide variety of fertilizers that are available in each area of the country. Spectrum Analytic serves a large number of states, and some foreign countries, and is not able to recommend specific fertilizers grades or brands that might be available in each area. For this reason, and to help explain proper fertilizer use, we have provided the following information.

What is in a Fertilizer Grade or Analysis

Fertilizer is identified by numbers indicating the percent of each nutrient that is contained in the product. Each fertilizer product might contain any combination of up to 13 nutrients. However, the majority of fertilizer products will have a 3 number identification, such as 5-10-5, 12-12-12, 26-3-3, etc. In all cases, these 3 numbers refer to the percentages of the major nutrients: Nitrogen (N), Phosphate (P2O5), and Potash (K2O) in that order. P2O5 is the fertilizer form of the plant nutrient Phosphorus (P), and K2O is the fertilizer form of the plant nutrient Potassium (K). These three numbers are called the fertilizer analysis or "Grade". Any additional nutrients that might be contained in a fertilizer are typically listed in smaller print below these three. They will often be in a format similar to 5S, 0.5 Zn, etc. Fertilizer products that contain some amount of all three major nutrients are often called "complete" fertilizers. Soil test nutrient recommendations are made in the same units (N, P2O5 and K2O). You have to determine how much of a particular fertilizer to apply to meet the recommendation.

Once you have a nutrient recommendation from a soil test report, you must convert it to an amount of fertilizer product that will supply those nutrients. This requires some basic arithmetic. For example, if the recommendation is for 3 lb. of N/1000 ft2 and your fertilizer grade is 20-5-10, first divide the 3 lb. N by 20 (the percent of N in this fertilizer grade, which is the first number in the analysis). Next, multiply that result by 100. The result is the amount of 20-5-10 to apply per 1000 ft2 to get 3 lb. N per 1000 ft2.

Formula

Amount of Fertilizer to apply/1000 ft2 = (lbs of Nutrient recommended % of Nutrient looking to calculate) 100

Example

3 lbs N/1000 ft2 recommended, fertilizer grade is 20-5-10

Amount of Fertilizer to apply/1000 ft2 = (lbs of Nutrient recommended % of Nutrient looking to calculate) 100

• = (3 20) 100

• = .15 100

• = 15

When the 15 lb./1000 ft2 of 20-5-10 is applied, you will also be applying some P2O5 and K2O. To find out how much, multiply the amount of fertilizer applied (15 lb.) times the percent of P2O5 or K2O in the fertilizer (5 and 10), then divide that result by 100.

Formula

(Lbs of fertilizer applied nutrient %) 100 = lbs of plant food/1000 ft2

Example

15 lbs 20-5-10 was applied/1000 ft2

  • 5% P2O5, 10% K2O

(Lbs of fertilizer applied nutrient %) 100 = lbs of plant food/1000 ft2

  • = (15 5) 100
  • = 75 100
  • = 0.75 lbs P2O5/1000 ft2

Next calculate K2O from 15 lbs 20-5-10/1000 ft2

(Lbs of fertilizer applied nutrient %) 100 = lbs of plant food/1000 ft2

  • = (15 10) 100
  • = 150 100
  • = 1.5 lbs K2O/1000 ft2

Therefore, 15 lb. of 20-5-10 per 1000 sq. ft. supplies 3 lb. N, 0.75 lb. P2O5, and 1.5 lb. K2O

These amounts of P2O5 and K2O may be lower or higher than the recommendations on the soil test report. If either or both are close to the recommended amount, don't worry too much about it. In nearly all situations, N will be the most critical of the three nutrients and you will want to be most accurate with it. If your application results in a significant shortage of P2O5 or K2O, you can make additional applications of a fertilizer grade that more closely meets the remaining need at some other time. If there is a large excess of these nutrients, you should consider using a different grade of fertilizer that more closely matches the recommendation. Many of the discount chain stores will only have 1 or 2 analysis in inventory, if you shop around at some of the larger nurseries or go to a farm supply store that sells to farmers you will find a large selection of fertilizer analysis.


MEASURING FERTILIZER

It is much easier for most people to apply fertilizer by volume than by weight. This can be a problem when small amounts are needed. The following list includes typical weight-to-volume conversions for a variety of fertilizers.

Fertilizer Measurement Data

Fertilizer Product

Lbs/ft3

Lbs/gal (dry vol)

Lbs/cup

Cups/lb

Tbs./lb

Tsp./lb

"Typical" Complete

Dry Fertilizer*

26-3-3

62

8.29

0.52

1.9

31

93

16-8-8

68

9.09

0.57

1.8

28

84

12-12-12

70

9.36

0.58

1.7

27

82

18-24-12

62

8.29

0.52

1.9

31

93

5-10-5

78

10.43

0.65

1.5

25

74

5-10-10

78

10.43

0.65

1.5

25

74

6-24-24

70

9.36

0.58

1.7

27

82

Urea (46-0-0)

47

6.28

0.39

2.5

41

122

Ammonium Nitrate (33.5-0-0)

62

8.29

0.52

1.9

31

93

Calcium Nitrate (15.5-0-0)

70

9.36

0.58

1.7

27

82

Potassium Nitrate (13-0-44)

66

8.82

0.55

1.8

29

87

Ammonium Sulfate (21-0-0-24S)

66

8.82

0.55

1.8

29

87

Triple Superphosphate (0-46-0)

66

8.82

0.55

1.8

29

87

Diammonium phosphate (18-46-0)

63

8.42

0.53

1.9

30

91

Monoammonium phosphate (11-52-0)

61

8.15

0.51

2.0

31

94

Sulfate of Potash (0-0-50-18S)

93

12.43

0.78

1.3

21

62

Muriate of Potash (0-0-61)

67

8.96

0.56

1.8

29

86

K-Mag (0-0-22-22S-11Mg)

75

10.03

0.63

1.6

26

77

Ground Limestone

87

11.63

0.73

1.4

22

66

Elemental Sulfur (90%S)

70

9.36

0.58

1.7

27

82

Aluminum Sulfate (14.4%S)

67

8.96

0.56

1.8

29

86

*Complete fertilizers with similar nutrient ratios will have similar weights/volume. Many fertilizer grades with a low total nutrient analysis contain a significant amount of lime. This could be an important factor in your fertilization choices, depending on your soil pH and needs of your plants.

Fertilizer Product

Lbs/gal (Liquid vol.)

Lbs/cup

Cups/lb

Tbs./lb

Tsp./lb

Most manufactured liquid fertilizers

11.0

0.69

1.4

22

67


Determining How Much Fertilizer to Apply to Meet a Recommendation

This process may seem complicated at first, but it is really a sequence of simple steps. Take it one step at a time and you shouldn't have a problem.

Let's say you have a recommendation of 4 lb. N, 2 lb. P2O5, and 3 lb. K2O per 1,000 ft2. For simplicity, we'll assume that your area to fertilize is 1,000 ft2. Most people have a limited selection of grades or analyses of fertilizers to choose from. Sometimes you won't be able to apply the exact amount of nutrients recommended in a single application, or maybe not at all. Don't worry too much about this. The main goal is to apply the correct amount of N and try to get close to the recommended amounts of P2O5 and K2O. If your application of P2O5 or K2O is a little off, you can adjust future applications accordingly. Let's assume that you can buy the fertilizer grades: 12-12-12, 6-24-24, 26-3-3, and 5-10-15.

In developing a fertilizer application that meets a soil test nutrient recommendation, it is often most difficult to apply the correct amount of K2O. This is because most available fertilizer grades tend to be high in either N and/or P2O5, but there are fewer grades available that are high in K2O. If this is the case in your area, you should talk to some suppliers and ask them to stock something with higher K2O, like the 5-10-15 in our example.

The recommendation is for 3 lb. of K2O/1000 ft2). We will get this from our 5-10-15 because it has the highest percentage of K2O in it. Based on the previously described method, we calculate the amount of fertilizer needed as follows

Formula

Amount of Fertilizer to apply/1000 ft2 = (lbs of Nutrient recommended % of Nutrient to calculate) 100

Example

3 lbs K2O/1000 ft2 recommended on the soil test results, grade fertilizer available is 5-10-15.

Amount of Fertilizer to apply/1000 ft2 = (lbs of Nutrient recommended % of Nutrient to calculate) 100

• = (3 15) 100

• = .2 100

• = 20 lbs of 5-10-15/1000 ft2 to get the recommended 3 lbs K2O/1000 ft2

This amount of 5-10-15 also supplies some N and P2O5. Again, based on previous instructions, we find out how much N and P2O5 as follows

Formula

Amount of additional nutrients supplied/1000 ft2 = (amount of fertilizer applied/1000 ft2 % of nutrient to calculate) 100

Example

20 lbs of 5-10-15/1000 ft2 was applied, how much N and P2O5 were also supplied with this application

Amount of nitrogen supplied/1000 ft2 = (amount of fertilizer applied/1000 ft2 % of nutrient to calculate) 100

• = (20 5) 100

• = 100 100

• = 1 lb N/1000 ft2 also applied

Amount of P2O5 supplied/1000 ft2 = (amount of fertilizer applied/1000 ft2 % of nutrient to calculate) 100

• = (20 10) 100

• = 200 100

• = 2 lbs P2O5/1000 ft2 also applied

Therefore, we find that by applying 20 lb. of 5-10-15 /1000 ft2. we can get 1 lb. N, 2 lb. P2O5, and 3 lb. K2O/ 1000 ft2. This leaves us 3 lb. short of the recommended N. To get this, we must apply some 26-3-3. This will force us to over-apply P2O5 and K2O a little bit, but this is not a problem. To determine how much 26-3-3 to apply to get the needed 3 lb/ 1000 ft2 of N, we use the same formulas.

Formula

Amount of Fertilizer to apply/1000 ft2 = (lbs of Nutrient recommended % of Nutrient to calculate) 100

Example

3 lbs/1000 ft2 of additional N recommended, grade fertilizer available is 26-3-3

Amount of Fertilizer to apply/1000 ft2 = (lbs of Nutrient recommended % of Nutrient to calculate) 100

• = (3 26) 100

• = .115 100

• = 11.5 lbs/1000 ft2 of 26-3-3 required

Putting this together we find the following

Nutrients Applied (lbs/1000 ft2)

N

P2O5

K2O

20 lbs/1000 ft2 of 5-10-15

1

2

3

11.5 lbs/1000 ft2 of 26-3-3

3

0.4

0.4

TOTAL

31.5 lbs/1000 ft 2 of fertilizer

4

2.4

3.4

You can see that with the recommendation in our example, using 12-12-12 or 6-24-24 would not have been as appropriate because they have the same amount of P2O5 and K2O. Using either of them over an extended time would supply either too much P2O5 or too little K2O. Over-applying P2O5 for one or two seasons is not normally a problem, but over a longer period, it could cause soil imbalances that might reduce the uptake of other nutrients. Excess P2O5 is also a pollution concern in some areas.

Your recommendation may suggest that you split-apply the fertilizer, or specific nutrients at different times. If so, you can apply all or part of either fertilizer in this example to satisfy the recommendation.


General Suggestions for Applying

Lime and Fertilizer

Soil pH

1. There is a correct soil pH range for all plants. When the soil pH is either below or above this range, nutrient uptake is reduced and plant performance is hurt. Therefore apply only the recommended amounts of lime (to increase the soil pH) or sulfur (to lower the soil pH).

2. Split applications into no more than 90 lb/1000 ft2 (9 lb./100 ft2) spring & fall.

3. Split applications into no more than 10 lb/1000 ft2 (1 lb./100 ft2) spring & fall.

Nitrogen

1. Do not apply much more than is recommended. Excess N makes plants more succulent and susceptible to disease.

2. Too little N reduces plant vigor and growth, plus reduces the uptake of most other nutrients.

3. Grasses (don't forget that corn is also a grass) tend to need more N than other plants. However, where possible, it is usually best to split the total N recommendation into multiple, smaller applications spaced throughout the growing season.

4. Do not apply N to most woody perennial plants after about mid-September. Excess N in the fall can increase the plants susceptibility to winter damage.

Phosphorous

1. If your soil test is Poor or Medium, you can apply more phosphorous than is recommended. However, a higher rate of application will primarily increase the soil test and is not likely to improve plant growth in the year it is applied.

2. If your soil phosphorous is already High or Very High, it could be interfering with the uptake of some micronutrients like zinc (Zn), copper (Cu), or others, and more will only make the problem worse.

Potassium, Magnesium, and Calcium

1. These three elements tend to compete with each other for uptake by the plant. An excess of one can suppress the uptake of the others.

2. Calcium (Ca) and magnesium (Mg) are contained in lime, so most soils with a pH between 6.0 and 7.0 will have adequate amounts for plant growth. However, acid-loving plants such as rhododendrons, azaleas, some conifers, blueberries, and others may need Ca or Mg from fertilizer sources, since lime may not be an option.

3. While most fertilizers are "salts", potassium (K) is one of the saltier fertilizers. Therefore, application rates that are significantly higher than recommended have the potential for causing salt damage to the plants.

Micronutrients

Micronutrients include the elements boron (B), copper (Cu), manganese (Mn), zinc (Zn), iron (Fe), and molybdenum (Mo). Plants need very small amounts of any of the micronutrients, and an excess of most of them can be very toxic to plants. For example, when a farmers corn crop needs additional boron, a typical recommendation is 0.5 to 1.0 lb./acre (43,560 sq. ft.). Since the need is so small, and the risks from excess application are high, homeowners are advised to apply these nutrients as part of a manufactured or pre-mixed fertilizer that contains the very small amounts needed.


Conversion Factors

Dry Volume

3 teaspoons (level)

equals

1 Tablespoon

16 Tablespoons (level)

equals

1 cup

2 cups

equals

1 pint

2 pints

equals

1 quart

4 quarts

equals

1 gallon

Liquid

80 drops

equals

1 teaspoon

3 teaspoons

equals

1 Tablespoons

1 fluid oz.

equals

2 Tablespoons

8 fluid oz.

equals

1 cup

16 fluid oz.

equals

1 pint

2 pints

equals

1 quart

4 quarts

equals

1 gallon

1 gallon of water

equals

8.34 pounds

Weight

28.35 grams

equals

1 ounce

16 ounces

equals

1 pound

454 grams

equals

1 pound

Area

1 acre

equals

43,560 ft2

1 yard2

equals

9 feet2

1 foot2

equals

144 inches2