Fertilizer and Lime Applications

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.

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 5 S, 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.

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.

Example

* 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 ft² 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.

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.

^* “Typical” complete dry fertilizer. 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.

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

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

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.

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) × 10
• = .115 × 100
• = 11.5 lbs/1000 ft² of 26-3-3 required

Putting this together we find the following

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.

• 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).
• Split applications of lime into no more than 90 lb/1000 ft² (9 lb./100 ft²) spring & fall.
• Split applications of sulfur into no more than 10 lb/1000 ft² (1 lb./100 ft²) spring & fall.

• Do not apply much more than is recommended. Excess N makes plants more succulent and susceptible to disease.
• Too little N reduces plant vigor and growth, plus reduces the uptake of most other nutrients.
• 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.
• 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.

• 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.
• 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.

• 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.
• 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.
• 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 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.