What do the three numbers on a fertilizer package mean?
All fertilizer packages have three numbers printed on them; for example, 10-15-5. These numbers represent the percentage of the three primary plant nutrients, nitrogen, phosphorus and potassium. In the above example, the fertilizer would contain 10% nitrogen, 15 % phosphorus and 5% potassium. This is important to know because different plants and different situations require different types of fertilizer.
· Nitrogen is important in the photosynthesis process and for many growth and developmental processes. It moves readily in the soil and roots have easy access to it. But during periods of heavy rainfall, the nitrogen is leach beyond the root zone and can contribute to groundwater pollution. Deficiency symptoms include stunted growth and chlorotic (yellow) and pale green leaves. These symptoms usually begin in older tissue because nitrogen is mobile within plants and moves from older to younger tissue when in short supply. Plants with excess nitrogen will exhibit dark green leaves, reduced yields and delayed maturity.
· Phosphorus stimulates early growth and root development and promotes the formation of seeds. It is also important for the energy systems of the plant. Soil phosphorus is often tied up chemically in relatively insoluble compounds and therefore does not leach as much as nitrogen will. Phosphorus fertilizer uptake by the plant is more efficient when in the presence of nitrogen. Although some Hawaiian soils have adequate levels of phosphorus, many are low in this element. Deficiency symptoms include slow growth, dark green leaves and a purplish color on older leaves. Excess phosphorus can interfere with the uptake of micronutrients.
· Potassium is involved with cell division, starch and protein synthesis and sugar movement within the plant. It increases size and quality of fruits and vegetables and increases disease resistance. Potassium is usually abundant in soils, but much of it is tied up in soil minerals and is unavailable to plants. Soils in high rainfall areas are usually low in potassium. Deficiency symptoms include slow growth, small fruits and chlorosis on the leaf tip and margins.
You can buy fertilizers containing a single nutrient, like ammonium nitrate and urea, containing 33% and 45% nitrogen, respectively. Or, one containing a double nutrient like ammonium phosphate, which is 11% nitrogen and 48% phosphorus. You can also purchase what is called a complete fertilizer which contains all three primary nutrients. The term complete should not be interpreted to mean that the fertilizer supplies all the nutritional needs of the plant.
Many plants have similar nutritional requirement, but many have differing requirements. A lawn, which is grown for its foliage and not fruit or flowers, will basically need nitrogen applications, with P and K added only occasionally, depending on their soil content. Bananas require ample potassium; a fertilizer similar to 10-5-20 would be appropriate. Papayas do well with a more balanced fertilizer. Vegetables in general like a high phosphorus content, like 10-30-10. In determining which fertilizer to use, it is always best to have the soil analyzed; at least the primary nutrients, N-P-K.
