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What Is NPK in Soil Testing?

by Victoria Lee Blackstone

Like taking your little ones to the pediatrician for their checkups, testing the soil in your garden lets you know if there are problems to address. Specifically, soil testing alerts you to nutritional deficiencies or excesses so you can correct them and grow healthy plants. Be on the lookout for the abbreviation NPK -- it represents nitrogen, phosphorus and potassium, the three primary elements essential to healthy plant growth.

1. Plant Nutrients

The three numbers displayed on a fertilizer bag, such as 10-10-10, represent the nitrogen, phosphate and potash (N-P-K) percentages the product provides. Although phosphorus is only an element of phosphate, and potassium is an element of potash, NPK is the catchall term. From the results of a soil test, the NPK analysis helps you determine which fertilizer your garden soil needs to correct deficiencies. If your garden soil has excessive amounts of an element such as phosphorus, the fertilizer you need may show something like 8-0-24, which indicates by the middle number that it doesn’t add any phosphate.

2. Nitrogen

Plants need nitrogen primarily for the healthy development and growth of their shoots and leaves. Nitrogen is more soluble in water than other elements, and soil is depleted of nitrogen more quickly. Slow-release fertilizers may supply nitrogen more efficiently to your plants. When you use compost in your garden, you supply your plants with an organic source of nitrogen. Microbes and earthworms break down plant waste into usable nitrogen, and supply this element through their nitrogen-rich excrement.

3. Phosphorus

Phosphorus is important for root development and growth, establishing new plants and productive flowers, fruits and vegetables. Phosphorus derived from phosphate compounds is not highly soluble in water, so it tends to build up in soil. During heavy rainfall, undissolved phosphate is carried in storm runoff, which pollutes waterways. Phosphorus is also dependent on soil pH for its availability to plants. If your soil pH is below 5.5 or above 7, plant roots may be unable to uptake phosphorus, even if you’ve supplied this element through fertilizing. For this reason, it’s important to test your garden’s soil pH along with its NPK values.

4. Potassium

Potassium fortifies plants against diseases and other stresses, and is an essential element in facilitating photosynthesis. Symptoms of potassium deficiency include scorched leaves, weak stems and undeveloped fruit. Because potassium may help increase a plant’s winter hardiness, commercial nurseries apply fertilizers containing this element in autumn to strengthen their plants against winter damage. Do not discourage ants in your compost piles, because they increase the levels of potassium and phosphorus by moving minerals around as they feed.

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