20/20/20 NPKS - What does it mean?

What do those numbers on the fertilizer bags mean?

Beside the major nutrients (N, P, K, S) there are also "micro-nutrients", or trace elements which are needed by plants in very, very small amounts. But if these are missing, the plants will not be able to complete their life cycle. Among these micro-nutrients are iron, zinc copper, calcium, manganese and magnesium. Most of these are present in adequate amounts in Saskatchewan soils. Complete house-plant fertilizers usually contain the major nutrients as well as the micro-nutrients. Read the label prior to purchase to ensure this.

Nitrogen - N

Nitrogen is needed for the green, leafy vegetative growth of plants. When an element is lacking, the plant will show deficiency symptoms. Deficiency symptoms for nitrogen include an overall pale yellow color of the leaves, and plants which are dwarfed or stunted. Nitrogen is mobile in the plant; that is, it moves from the older growth to the newer growth, where it is most needed. Therefore deficiency signs will appear first in older leaves.

Nitrogen moves easily through the soil in the soil water. For this reason it is said to be very "mobile." It is easily "leached" or washed downward by rain or irrigation water. If it is washed below the root zone of the plants, it will not be available for plant use. Therefore, it is the fertilizer element most often lacking and most often needing replacement.

Because of complex bacterial interaction, nitrogen is usually not "available" for plant use until the soil has warmed up in the spring and the soil temperature has reached 15.5C (60F). This is why plants may appear yellow and stunted in early spring when the soil is still cold, even if nitrogen fertilizer has been applied. As soon as the soil warms up, they will appear green and vigorous.

Too much nitrogen or a nitrogen imbalance can delay flowering, fruiting and seed set. The resultant growth is soft and succulent and may be more vulnerable to fungal and bacterial infection. As well, nitrogen can desiccate or "burn" the roots of plants if placed too close to seeds, seedlings or newly planted plants.

Phosphorus - P

Phosphorus is said to promote root growth, root branching, stem growth, flowering, fruiting, seed formation and maturation. When phosphorus is lacking, stems and foliage often have a red or purplish tinge. This is particularly noticeable on tomatoes and corn. Deficiency signs are seen in new growth first.

Phosphorus is very stable and non-mobile within the soil, so it is not easily leached by soil water. When used moderately, it may be placed fairly close to seeds and seedlings and will not "burn" or desiccate them.

Potash or Potassium - K

Potassium enables the plant to more readily withstand "stress" such as drought, cold, heat and disease. (In a lawn such "stress" may be in the form of human and pet traffic.) It also stimulates flower color and promotes tuber formation and a strong root system.

When potassium is lacking, leaves appear dry and scorched on the edges and have irregular yellowing. This is seen on older leaves first.

Potassium is usually readily available in prairie soils and therefore seldom needed as a fertilizer application. But sandy garden soils may show deficiencies. Corn deficient in potassium may be susceptible to fusarium infections.

Sulfur - S

Sulfur is essential to plant growth and metabolism. It contributes to the unique taste and flavor of cabbage, broccoli, Brussels sprouts, cauliflower and other members of the mustard family.

Plants that do not have enough sulfur are stunted, thin-stemmed and spindly. The younger leaves are light green or yellow. Fruit and seed maturity may be delayed when sulfur is lacking.

Various forms of sulfur may be added to basic soils to acidify them (or lower the pH).

The Numbers on the Fertilizer Bag

On a fertilizer bag, you will find three or four numbers with hyphens separating them. The numbers indicate, in order, the percentage of nitrogen (N), phosphorus (P), potassium or potash (K) and sulfur (S) - the letters in parentheses are the chemical symbols for the elements. Here are some common fertilizers and the proportion of the elements they contain.

Percentage of N P K S





















So, a 25 kg bag of 16-20-0-14 would provide (16/100 X 25 =) 4 kg of nitrogen, (20/100 X 25 =) 5 kg of phosphorus, no potash and (14/100 X 25 =) 3.5 kg of of sulfur. A 25 kg bag of 34-0-0 would supply 34/100 X 25 =) 8.5 kg of nitrogen but no other elements. A "complete" fertilizer contains nitrogen, phosphorus and potash. If one of these three elements is missing, the fertilizer is not "complete."


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