Kratsch, Heidi 2010, Fertilizing You Vegetable Garden, Extension, University of Nevada, Reno, Fact Sheet 10-68

Vegetable plants need water and sunlight to grow, but they also need nutrients to support growth. A good soil rich in organic matter may provide many of those nutrients, but our soils in northern Nevada are more mineral than organic – so they need a little help. You can provide the help they need by fertilizing your plants. But what kind, how much and when? Nutrients. The three major nutrients plants need are nitrogen, phosphorus and potassium. Nitrogen is the main nutrient that supports plant growth. It makes plants green and leafy. Provided at the right time and in the proper amounts, nitrogen can give plants the kick-start they need to produce a bountiful harvest. Be careful, though – too much nitrogen can cause plants to grow too fast and become leggy, and it can interfere with flower and fruit development in vegetables such as tomatoes and cucumbers. Phosphorus is the nutrient that supports root, flower and fruit development but it also supports plant growth. Too little phosphorus may cause stunted growth and reduced yield. Potassium is required for overall plant development. Timing of application and application amounts of these nutrients are important because nitrogen, phosphorus or potassium applied in excess can be leached into groundwater and pollute our waterways.

What the numbers mean. Commercial fertilizers, by law, have a code on the front of the label that tells you the relative proportion by weight of nitrogen (N), phosphorus as P2O5 (P), and potassium as K2O (K), in that order. For example, a 5-10-15 fertilizer contains 5 percent nitrogen, 10 percent phosphorus, and 15 percent potassium. This is an example of a complete fertilizer because it contains all three major nutrients. An incomplete fertilizer contains only one or two of the major nutrients. Examples of incomplete fertilizers include 46-0-0, which is 46 percent nitrogen by weight and no phosphorus or potassium; and 3-15-0, which is 3 percent nitrogen and 15 percent phosphorus by weight, and no potassium. Many different fertilizer formulations exist. A soil test can tell you if your soil is deficient in a particular nutrient. Nevada soils are generally low in organic matter and overall fertility, so incorporating a balanced fertilizer containing equal proportions of the three major nutrients prior to planting can be helpful. As an alternative, you can work into your soil well-aged manure or compost as a source of slowly released nutrients. Never use fresh manure or pet wastes as a pre-plant fertilizer. They may contaminate the soil with harmful bacteria that can transmit food-borne illness. Also, fresh manures are high in nutrient salts which can burn tender young plants.

Other nutrients. Plants need many other nutrients such as magnesium, calcium and sulfur, and micronutrients such as iron, manganese, zinc and others in smaller amounts. Fortunately, our soils are not usually deficient in these nutrients, and unless your soil pH is outside the optimal range for most plants (5.5 to 7.5) or extremely sandy, they should be available for uptake by plant roots. Soils that are too acid (low pH) or alkaline (high pH) will bind to certain nutrients and make them less available to plants. You can have your soil pH tested by a soil testing lab. Contact your local Cooperative Extension office for a list of soil testing labs in your area.

Natural or synthetic? Natural organic fertilizers come from living sources that decompose over time into an inorganic form. Commercial synthetic fertilizers are manufactured from inorganic sources. Plants take up all nutrients in their inorganic form, so the source of the nutrients does not matter to the plant. For example, the N that plants take up from decomposed compost is the same as the N taken up from a commercial granular fertilizer. The difference is that nutrients, especially nitrogen, from natural sources are released slowly over time and are more likely to be taken up by plant roots rather than leached into groundwater. Some synthetic fertilizers are also manufactured to be released slowly and may last all season. Natural organic fertilizers tend to have lower concentrations of nutrients than synthetic fertilizers, however, and some are high in only one or two of the major nutrients, but many have the added benefit of improving your soil’s structure and water-holding capacity. For more information on garden soils, refer to Nevada’s Soils – Worth the Toil, University of Nevada Cooperative Extension Fact Sheet-09-14.

Whether natural or synthetic, fertilizers come in a variety of forms. Soluble powders and crystals are mixed with water before applying. Liquid forms are usually sold as concentrates, and they need to be diluted before applying. Granular forms are cultivated into the soil, and need to be watered in. Liquid forms, including dry powders applied as liquids, are fast-acting but are easily leached. Granular forms need to be watered in after application to release the nutrients into the soil, and to avoid burning tender young plants.

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