Friday, May 21, 2010

Fertilizer 101

What is NPK?
NPK is the abbreviation that stands for the elements nitrogen, phosphorus and potassium. These nutrients are the three most important chemicals that plants need to thrive.

What do the numbers mean?
Usually represented by a number like 10-10-10, they are the percentage by weight of the three nutrients. The numbers are always given in the same sequence, NPK, with the first number percentage nitrogen, the second phosphorus, and the third potassium. Fertilizers with different numbers, but the same ratios are equivalent, but have different strengths. Two pounds of 5-10-5 fertilizer has the nutrients as one pound of 10-20-10. Fertilizers that contain all three of these elements are considered ‘complete’. A zero in any of these spots means that a fertilizer does not contain that nutrient. A 4-12-0 fertilizer contains nitrogen and phosphorus, but no potassium.

Nitrogen- Foliage Growth
Nitrogen is needed by all plants and is easily washed out of the soil. It stimulates shoot and leaf growth, and regular application is particularly important for green, leafy plants like grasses, lettuce, and foliage plants. Too much nitrogen, however, can cause other types of growth (such as flowers, fruit and roots) to slow down, and even stop. It can also cause fast growth, but weaken a plant, making it more prone to disease or pests. A low nitrogen fertilizer can be used to encourage flowering and fruiting over foliage growth. Nitrogen is absorbed or washed from the soil within a few weeks of application, and should usually be applied every month during the growing season.
Signs of nitrogen deficiency: old leaves turn yellow, slow or no growth, small new leaves.

Phosphorus – Roots and Flowers Growth
Phosphorus binds with other elements in the soil to form stable compounds. This means it does not leech out of the soil as quickly as nitrogen does, and therefore does not need to be applied in quantity as often as nitrogen does, especially for established plantings and tree. Phosphorus can usually be applied once a year, or when putting in new plantings and trees, and will remain in the soil.
Signs of phosphorus deficiency: leaves are dull green or grayish, purple foliage on otherwise green plants, short stems, few flowers, small fruit.

Potassium (Potash) – Overall Plant Health
Potassium is more soluble in water than phosphorus, but does not leech out of the soil as quickly as nitrogen does. A single application can last several months. It helps with root growth, disease resistance, and temperature tolerance.
Signs of potassium deficiency: Older leaves crinkle and curl upward, leaves turn yellow and scorch begins on leaf edges and progresses in toward veins, shoots die back late season.

Organic vs. Inorganic
The primary difference between organic and inorganic fertilizers is where the raw materials are sources from. Organic fertilizers use naturally occurring ingredients (either animal or plant based, ‘organic’ refers to from materials that were once living), like bone meal, as a nutrient source. Inorganic fertilizers use synthetic chemicals or mineral, like ammonium nitrate, to supply the nutrients. Typically (but not always) organic fertilizers are slow release, while inorganic are released more quickly. Nutrient content varies from fertilizer to fertilizer, but an organic and an inorganic fertilizer with the same NPK numbers will have equivalent nutrient amounts.

Fertilizer Applications:
Fertilizers come in several different forms. Most commonly they come in granules that are meant to be scattered on the soil. Watering will dissolve them in to the soil. Others are liquids or soluble crystals meant to be dissolved in water. These can be applied while watering, either with a watering can, a hose attachment, or through an automatic watering system. Some come in solid chunks or bricks. These are always slow release forms, and are placed individually above the root zone of the plants.
Follow the directions on the fertilizer package for specific application instructions.

Secondary Nutrients and pH:
Calcium, magnesium and sulfur are considered secondary nutrients. Secondary nutrients are no less important than NPK, but seldom need to be added in quantity to soils. Calcium (in the form of lime) is often added to make soil less acidic, while sulfur is used to make soil more acidic. If there is a concern about soil pH, have the soil tested before adding these amendments. There are fertilizers that are mildly acidic, meant for acid loving plants like azaleas, and can be chosen with those specific plants in mind.

Always follow package instructions for application amounts compared to the size of the area you want to fertilize. More than the recommended amount is NOT necessarily better!



After putting this together for work, I thought I'd share it here as well.


Paul said...

Thanks Heather for the explanations! I always wondered what those numbers meant when I was buying fertilizer for my indoor plants. Now I'll probably make better choices next time I'll "re-pot" one of my Bonsai.

What do you think would be the best combination for a Japanese Black Pine?

Thanks in advance!
All the best!

Just discovered your blog... I'll sure keep on reading more of your articles! Please, Keep'em coming! :)