How Much Fertilizer?

If you want to know how much fertilizer to use for gardening, you will find information about cations, organic and inorganic forms of nutrients, and much other information that is of little use to the typical gardener except in interpreting soil testing results. Here is a summary of information from various sources that may be helpful:

Summary

Soil fertility is determined by how much nutrition plants can obtain from the soil, and also by how easy it is for roots to penetrate the soil. In addition to nutrients from the soil, plants have other requirements such as light, water, and temperature. As for soil fertility, the soil should be loose enough for roots to penetrate without difficulty. The nutrition available to plants from the soil is determined by the acidity of the soil, the cation binding sites in the soil, and the anions in the soil. The soil should typically be slightly acid for best nutrition, with a pH of 6.0 to 6.8. As for cations, a cation is basically the positively charged portion of a molecule. An anion is a negatively charged portion of a molecule. Plant nutrients, including nitrogen and potassium, have to be present in the soil in the form of cations for plants to absorb them. However, phosphorus and some other nutrients can be absorbed as anions. In organic matter, plant nutrients are bound up in large molecules that plant roots cannot absorb. These nutrients must be broken down by bacteria before plants can use them. This takes time, so organic fertilizers do not supply plant nutrients right away, but it is necessary to wait until the nutrients are broken down, little by little, over time. Chemical fertilizers, in contrast, are available immediately.

Now, different soils have different quantities of cation binding sites. These sites are on the surface of the soil particles. It is the percentage of the sites that are occupied by a particular nutrient that determines how much of that nutrient can be absorbed by a plant. If a soil has few cation binding sites, then it can be nourished by a smaller amount of fertilizer, but the fertilizer will be exhausted sooner. If a soil has many cation binding sites, then it takes more fertilizer to nourish the plant, but the fertilizer will last longer. Soil tests may tell the number of cation binding sites in the soil and the percentages occupied by various nutrients. In a soil test, “The cation nutrients (Ca, Mg, K) are expressed in parts per million (ppm). If you multiply ppm by 2, you will get the approximate number of pounds/acre of the nutrient (in the top 6 inches of soil). Soil labs usually report the level of each of these cations from very high to very low, relative to the CEC of each particular soil.” [http://www.seedsofchange.com/digging/soil_tests.aspx, August, 2012] The soil test will also indicate the amount of phosphorus (phosphates) and possibly other anions in the soil.

Most of the cation binding sites should be occupied by calcium, it turns out. The rest should be occupied by various plant nutrients in certain ratios. “Calcium dominates the exchange sites, and for best crop performance, should occupy 65 to 85 percent of the sites . Magnesium is best between 10 and 20 percent, while potassium should be about 3 to 5 percent. The remaining sites are occupied by hydrogen, sodium (high alkaline soils), ammonium, and trace elements. Even these ranges are not necessarily the best in all cases, but they can be used as guides for balancing your nutrients.” [from http://www.seedsofchange.com/digging/soil_tests.aspx, August 2012.]

Soil tests may recommend how much of various nutrients to add to the soil. The soil test will need to be repeated each year because nutritional content of the soil will change. If a soil needs a certain number of pounds of nitrogen per 1000 square feet, then the amount of fertilizer to apply depends on the percentage of nitrogen in it. For example, if it is 10-10-10 fertilizer then it is 10 percent nitrogen, phosphorus, and potassium, so if one needs one pound of nitrogen per 1000 square feet, then one has to apply 10 pounds of this fertilizer per 1000 square feet to obtain this much nitrogen. The same calculation applies to the other nutrients. Frequently a fertilizer will specify a fourth number, which indicates the amount of sulfur it contains.

Too much fertilizer in soil can be harmful. Nitrogen is typically needed in the amount of 1 pound per 1000 square feet. For soil with little organic matter, twice this much may be needed. “Where phosphate levels are believed to be low, the standard application rate without a soil test is ¼ to 1 pound triple super phosphate (0-46-0) or ammonium phosphate (18-46-0) per 100 square feet. “ [http://www.ext.colostate.edu/mg/gardennotes/232.html, August 2012] ”Where potash levels are believed to be low, the standard application rate without a soil test is ¼ to ½ pound potassium chloride (0-0-60) or potassium sulfate (0-0-50) per 100 square feet.” [http://www.ext.colostate.edu/mg/gardennotes/232.html, August 2012] “A common recommendation for vegetables is to apply 1 pound of a 10-10-10 fertilizer or 2 pounds of a 5-10-5 (or 5-10-10) fertilizer per 100 feet of row. “ [http://www.ext.colostate.edu/pubs/garden/07611.html, August, 2012]

There are also other nutrients that plants need in small quantities, in addition to nitrogen, phosphorus, potassium, and sulfur.

Various plants differ in their nutritional requirements. They also differ in the method in which the fertilizer should be applied and the time at which it should be applied. Some kinds of fertilizer should not be applied directly to the roots because they can damage the roots. Organic fertilizers are less likely to burn the roots; indeed, plants can be grown in pure compost without trouble.

Soil testing is needed to determine how much and what kind of fertilizer to apply, and how much if any lime to apply. Lime makes a soil less acid, and sulfur in various forms increases acidity.

Information from Web Sites

Here are some rules of thumb and information from various web sites:

From http://www.ext.colostate.edu/mg/gardennotes/232.html :

By law, all products sold as fertilizer require uniform labeling guaranteeing the minimum percentage of nutrients. The three-number combination (fertilizer grade or analysis) on the product identifies percentages of nitrogen (N), phosphate (P2O5), and potash (K2O), respectively. For example, a 20-10-5 fertilizer contains 20% nitrogen, 10% phosphate, and 5% potash.

Nitrogen is the nutrient needed in largest quantities as a fertilizer. Nitrogen is annually applied by manufactured fertilizer, organic fertilizers, and/or organic soil amendments. Application rates are critical, as too much or too little directly affect crop growth.

Application rate is based on the soil organic content. As the organic content increases, nitrogen will be slowly mineralized (released) by the activity of soil microorganisms. Standard application rates for gardens are given in Table 3.

Nitrogen fertilizer can be broadcast and watered in, or broadcast and tilled into the top few inches of soil. It can be banded 3-4 inches to the side of the seed row. Do not place the fertilizer in the seed row or root injury may occur.

For additional information on fertilizers refer to the CMG GardenNotes #234, Organic Fertilizers, and #711, Vegetable Garden: Soil Management and Fertilization.

From http://pubs.ext.vt.edu/426/426-323/426-323.html

Side-Dressing - Dry fertilizer is applied as a side dressing after plants are up and growing. Scatter fertilizer on both sides of the row 6 to 8 inches from the plants. Rake it into the soil and water thoroughly.

Recommended application rate: Apply one pound of 10-10-10 or two pounds of 5-10-5 (or 5-10-10) per 100 feet of row.

From http://aces.nmsu.edu/pubs/_h/h-119.pdf

Table 1. Conversion from Pounds Per Acre to Pounds

Per 100 or 1,000 Square Feet

Rate per 1,000

Rate per acre Rate per 100 sq. feet sq. feet

lb lb ounces lb + ounces

100 0.23 4 2 , 5

200 0.46 7 4 , 9

400 0.92 15 9 , 3

500 1.15 18 11 , 8

600 1.38 22 13 , 12

700 1.61 26 16 , 1

800 1.84 29 18 , 6

1,000 2.30 37 22 , 15

2,000 4.59 73 45 , 15

General Formula: Desired pounds = [(pounds per acre)÷ 43,560] x (square

feet to be fertilized)

1 acre = 43,560 square feet, 1 pound = 16 ounces, 454 g = 1 pound.

From http://extension.oregonstate.edu/catalog/pdf/ec/ec1503.pdf

Plants can use chemical fertilizers as soon as they are

applied. Soil bacteria and fungi must act on most organic

nutrient sources to change them into a usable form. Thus,

if you use mostly organic fertilizers, you may need to add

a small amount of a source of more readily available

nitrogen early in the season to ensure adequate plant

nutrition until the organic sources become available to

plants. Options include

liquid fish, blood meal,

and chemical fertilizer.

Methods of applying fertilizer

Three ways to apply fertilizer are described

below. Regardless of the method you use, keep the

following facts in mind:

• The nitrogen in chemical fertilizers is highly

water-soluble and is carried to the roots by

irrigation and rain. Thus, you don’t need to mix

these materials into the soil, but you do need to

water your garden if rain doesn’t fall within a

day after you apply them.

• Organic sources of nitrogen are most accessible

to plants if mixed into the top 2–3 inches of soil.

• Phosphate moves slowly in the soil. You’ll

obtain best results by banding phosphatecontaining fertilizer 2 inches below the seed

when you plant or by tilling it into the soil

during spring preparation.

• Work potassium fertilizers into the soil using the

banding or broadcast methods. Do not allow

potassium fertilizers to contact plant roots.

From http://www.ext.colostate.edu/pubs/garden/07611.html

Fertilizer nutrients required by vegetables in the highest quantity are nitrogen (N), phosphorus (P) and potassium (K). Other nutrients, including iron, copper, manganese and zinc are needed in much smaller amounts. With the exception of nitrogen and phosphorus, most of these nutrients are most likely available in the soil at adequate or even excessive amounts. Adding nutrients that are not needed can cause deficiencies of other nutrients and can cause an imbalance of nutrients. Nobody can tell you what your soil really needs without referring to a soil test conducted by an analytical laboratory.

A common recommendation for vegetables is to apply 1 pound of a 10-10-10 fertilizer or 2 pounds of a 5-10-5 (or 5-10-10) fertilizer per 100 feet of row. The first number is the percentage by weight of nitrogen, the second the percentage by weight of phosphorus and the third number is the percentage by weight of potassium in the fertilizer product. Thus, 100 pounds of a 5-10-10 fertilizer contains 5 pounds of nitrogen, 10 pounds of phosphorus and 10 pounds of potassium. As already mentioned, applying nutrients not needed can cause problems with the vegetables you are growing.

When an excess amount of nitrogen is applied to fruit crops such as tomatoes and squash, it is common to have all vines and no fruit. With corn, missing a nitrogen fertilizer application as the corn starts to tassel can result in poor ear production. When excessive amounts of nitrogen are applied to root crops such as turnip, carrot and parsnips, you may end up with many leaves and small roots.

When phosphorus is applied, but not needed, it can kill off the symbiotic mycorrhizal-forming fungi required by the plant and reduce the vegetables’ ability to absorb iron and other micronutrients. Excess soil phosphorus also shuts down the plant’s ability to produce phytochelates, organic molecules produced by roots to increase its iron uptake.

From http://extension.usu.edu/smac/htm/soil/

Example: Suppose the soil test recommended that you apply 100 lbs of nitrogen per acre. Using the 32-10-10 fertilizer, you would need to apply 312 pounds per acre (100 lbs of nitrogen divided by 32% (.32) = 312 lbs of fertilizer).

From http://ohioline.osu.edu/hyg-fact/1000/1601.html

Make small furrows about three inches to each side of the row and two to three inches deep. Use caution, however, so that seeds or roots do not come into direct contact with the fertilizer. In lieu of a soil test recommendation, apply two to three pounds of fertilizer per 100 feet of row. Select the fertilizer analysis according to the type of vegetable grown. For leafy vegetables, high nitrogen fertilizers, such as 12-12-12 or 15-15-15 analyses are appropriate. For vegetables grown for their fruits, seeds, roots, or bulbs, such fertilizers as 6-24-24, 6-12-18, and 8-16-16 or equivalent are satisfactory. Growing healthy, productive plants in the vegetable garden involves attention to many cultural details. Since fertilization is just one of these details, fertilizers should not be considered a cure-all for all gardening problems. When properly used, fertilizers are extremely valuable in obtaining good gardening results. However, fertilizers cannot correct or improve poor soil structure that requires additions of organic matter. Nor can it compensate for an incorrect pH or unfavorable weather conditions. Choosing inappropriate vegetable cultivars, and not practicing weed or pest control cannot be corrected by applying fertilizers. The use of fertilizers is just one of a number of integrated gardening practices that lead to gardening success.