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Ratio of fertilizer (N-P-K)

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Bags of fertilizer
Photo: warrenski

How plants respond when fertilized depends largely on the ratio between the three elements (N-P-K). To obtain the ratio, divide all three numbers on the label by the smallest number.

Examples:

  • 20-20-20 fertilizer has a 1-1-1 ratio
  • 15-30-15 fertilizer has a 1-2-1 ratio
  • 10-5-5 fertilizer has a 2-1-1 ratio

When choosing fertilizer, it is very important to check its ratio. For instance, to encourage plants to root, you need to choose a fertilizer that is higher in phosphorous than in nitrogen or potassium, i.e. one with a 1-2-1 ratio.

The following are the main fertilizer ratios used by gardeners:

  • Rooting: 1-2-1
  • Flowering and fruiting: 1-1-2, 1-2-2, 2-1-2
  • All-purpose: 1-1-1
  • Leafy growth: 2-1-1, 3-1-1

N.B.: Although natural fertilizers often have different ratios from these ones, they are no less effective.

Caution! A fertilizer containing 30% of a given element is not necessarily better than one with only 8%. You also have to consider how long the nutrients will last in the soil. Let’s look at an example:

Fertilizer containing 30% quick-release nitrogen VS Fertilizer containing 8% slow-release nitrogen
The nitrogen contained in this fertilizer is released quickly. The plant absorbs the amount of nitrogen it needs, but the rest is not used. There is a high risk of leaching and burning the roots. VS The plant absorbs small amounts of nitrogen as the fertilizer gradually breaks down. Since little is lost through leaching, most of the fertilizer can be used. There is little risk of burning the roots.

As a rule, it is better to use slow-release fertilizer. Quick-release fertilizers are used mainly to address specific problems (to pep up a weak plant, correct a deficiency, etc.) or to quickly increase yields (to encourage annuals to bloom profusely, for instance). Be careful not to overuse these fertilizers.

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