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Understanding Soil Nutrients

by Michael Russell

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"Complete fertilizers" contain all three macronutrients - nitrogen (N), phosphorus (P), and potassium (K) - but don't let the name "complete" fool you. It doesn't mean that the fertilizer has all the nutrients that plants need, just that it contains all three of the major ones.

Bags of complete fertilizers contain three numbers, such as 5-3-3, for example. Each number represents a percentage of N-P-K in that bag, as measured by weight. In this case, a bag of 5-3-3 fertilizer contains 5 percent nitrogen, 3 percent phosphorous, and 3 percent potassium. To determine the amount in pounds of each nutrient in the bag, multiply the weight of the bag (say 50 pounds) by the percentage of each nutrient: 50 pounds x .05 = 2.5 pounds of nitrogen. You need to know the actual amount of nutrients in the bag because a soil test often recommends pounds of actual N-P-K to add per square foot of your garden.

Each of these three nutrients plays a critical role in plant growth and development. Here's what they do and their deficiency symptoms to watch for.

Nitrogen (N): This critical element is responsible for the healthy green foliage of the plants, as well as protein and chlorophyll development. Chlorophyll is the pigment that makes plants green and is a vital component in photosynthesis. Nitrogen moves easily in the soil and leaches out rapidly, especially from sandy soils and in high rainfall areas or irrigated gardens. Plants use lots of nitrogen during the growing season, so it's commonly the most deficient element. If you add too much nitrogen, however, plants will have dark green, leafy growth but less root development and delayed flowering and fruiting. Symptoms of nitrogen deficiency include slow growth and yellowing leaves, especially on older foliage. Animal manures, soybean meal, and cottonseed meal provide high levels of nitrogen.

Phosphorus (P): Plants need phosphorus for strong root growth; fruit, stem and seed development; disease resistance; and general plant vigor. Phosphorus doesn't move in the soil as easily as nitrogen does so you don't have to add it as frequently. Depending on where you live in the country, your soil may have plenty of phosphorus, but it may be unavailable to plants. Phosphorus availability depends on warm soil temperatures, pH range, and the levels of other nutrients, such as calcium and potassium, in the soil. Deficiency symptoms include stunted plants with dark green foliage, reddish-purple stems or leaves, and fruits that drop early. Rock phosphate and bone meal are good sources of phosphorous.

Potassium (K): This nutrient, sometimes called potash, is essential for vigorous growth, disease resistance, fruit and vegetable flavor and development, and general plant function. Potassium breaks down slowly so you won't have to add it often. Deficiency symptoms include yellow areas along the leaf veins and leaf edges, crinkled and rolled-up leaves, and dead twigs. Fruit trees may develop fruit with poor flavor or stunted fruits. Certain animal manures and mineral fertilizers, such as greensand, add potassium to the soil.

Michael Russell
Your Independent guide to Gardening
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©2006 Michael Russell. All rights reserved.

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