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Soil Improvement Information Series

Fertilizer Tips

Giving plants the proper type and amount of fertilizer is one of the most important steps to success in the home garden. This is true for growing vegetables, flowers, fruits, shrubbery, or trees. This page will deal with the fertilizer that generally comes from a bag. We will explain those mystical numbers on the label of a fertilizer bag. These numbers show percentages by weight of the most important plant nutrients contained in the fertilizer: Nitrogen (N), Phosphorus (P), and Potassium (K).

The first number indicates the percentage of nitrogen. Nitrogen is the most important of the three. Plants use it to build proteins, enzymes, chlorophyll, and hormones. A plant with adequate amounts has healthy green leaves and strong growth. A plant with too little nitrogen is a pale, slow growing plant. However, too much nitrogen results in a plant that is succulent, over stimulated, and very attractive to bugs and some diseases.
Nitrogen is constantly moving through the soil, atmosphere, living things, and can be rapidly leached away with too much watering. And, it is usually the element in short supply. Close attention to the levels of nitrogen in your garden soil is important. Just the right amount must be maintained. Use a slow release fertilizer for best results. For a quick jolt, use blood meal (12-1-1) carefully. For a slower application use fish emulsion (5-2-2) or cottonseed meal (7-2-2).

The second number indicates the percentage of phosphate (or phosphorus). Phosphate is essential in the plant's metabolic processes, in seed production, and in proper root developement. A plant with a phosphate deficiency has stunted growth, and sometimes with a purplish color to the leaves. Phosphorus does not move easily through the soil, so the plant roots must extend themselves searching for adequate supplies. Phosphorus becomes available to the plants when there is lots of water available, plenty of organic material, and the soil pH is close to neutral. Because phosphorus tends to remain in the soil rather than easily leach away, it is possible that too much phosphate gathers in the soil. So be carefull. Bone meal (1-11-0) is an excellent organic source of phosphorus.

The third number indicates the percentage of potassium (or potash). Potassium is needed in fairly large quantities to regulate metabolic reactions within the plant. Photosynthesis being one of these important activities. Potassium behaves much like nitrogen, in that it dissolves quickly. When applied to the surface of the soil, potassium will move quickly unto the root areas. Some plants require lots of potassium, more than any other nutrient. A plant deficient in potassium usually is weak and the leaf edges appear scorched. There isn't much potassium in most organic material. If your garden soil is deficient, you may add granite dust or other rock powders. Some rock powders have a potassium content as high as seven per cent.

A soil test should be performed to determine your gardens soil requirements and what nutrients need to be added. This test involves collecting soil samples from various parts of your garden. Then mail the samples to a testing lab. Soil testing labs will make an analysis of your garden soil and most will recommend what nutrient and how much should be added. Your county extension agent can recommend to you the best local testing lab.

From a nutrient standpoint, plants do not care if the nutrients come from organic sources or from chemical sources. However, organic nutrient sources are generally more forgiving if you apply too much at one time.

Nutrient percentages for common fertilizers (By weight)
Fertilizer Nitrogen Phosphate Potassium
Nitrate of soda 16 0.0 0.2
Ammonium nitrate 34 0.0 0.0
Blood meal 12 1.0 1.0
Triple super phosphate 0.0 45 0.0
Cottonseed meal 7.0 2.0 2.0
Muriate of potash 0.0 0.0 60
Bone meal 3.0 23 0.0
Chichen litter (estimate) 36 80 34
Fish emulsion 5.0 2.0 2.0

Remember to apply fertilizer according to the label on the bag. What is a good element mix for a good all-purpose fertilizer? Generally a 5-10-10 (5% nirogen) or a 10-20-20 (10% nitrogen) is a good choice for most applications.

How much fertilizer to use varies according to what plants are being grown and when. To broadcast fertilizer over the vegetable garden, 5 pounds of 5-10-10 is applied well before planting the seeds.

For the Pacific Northwest Region, it is advised to stop fertilizing landscape plants in mid to late July. This is necessary for garden shrubby, perennial beds, and landscape trees and is connected to the natural rhythm of these plants. In early spring, when these plants are growing and expanding, leafing out, starting flowers and seed, and adding a greater root network, they need nutrients. However, most shrubby plants and landscape trees in our region begin to slow down their rapid growth in late July, and for some plants August. This is because the amount of rainfall drops usually quite a bit and the daylight hours begin to grow shorter. Adding fertilizer during this part of the natural rhythm keeps the plants from progressing toward a natural dormancy that is necessary to help it withstand frosts and to survive the winter months.

However, plants grown in containers, or grown for flower color need to be fertilized very two weeks throughout the summer. Vegetable gardens will also need a side dressing of fertilizer during July and August.

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This web site was first published February 24, 2002.

This page was last updated September 23, 2015.