Urea is a naturally occurring substance that has a high nitrogen count. In the usual fertilizer ratio of nitrogen, phosphorus and potassium, urea is 46-0-0. All plants need nitrogen, which directs the formation of green leafy growth, protein formation and drives the production of chlorophyll. Nitrogen is ever present in the air but plants are not able to harvest the nitrogen from the atmosphere. Urea acts very quickly on plants and contains large amounts of salts. Salts can build up in potted soils and can eventually poison the plant.
Know Your Plant
Not every houseplant is a heavy nitrogen user, which means it may not need the addition of nitrogen. Potted soils often have urea with ammonium sulfate incorporated into them, which slows the release of the urea. This provides enough nitrogen for most potted plants as long as they are re-potted every few years. Bamboo, potted roses and orchids are commonly fertilized with urea. Other plants may be sensitive to such a high formulation, so a nursery professional or extension horticulturist should be consulted regarding the nitrogen needs of your particular plant.
Urea Effects in Soil
Urea turns into ammonia with air exposure within a short period of time. For this reason it is mixed into soil or applied as a drench so the solution seeps in before it become toxic. Urea contains high amounts of sodium which act upon a plant hydration system much like it does in humans. Sodium buildup can be prevented by soaking the plant until water pours out of the drainage holes. This will flush out any excess salts and clear the soil. Urea becomes a nitrate when it is mixed in soil and contacts water. Within a few hours of application, foliage is noticeably greener.
Application on Potted Indoor Plants
Indoor plants are almost always potted. These plants can only get their required nutrients from the limited area of the soil. They do not have the advantage that outdoor plants have, where extra organic matter is constantly layered into the soil to provide more nutrients. Potted plants should usually be fertilized annually. However, urea is a component of a strong acid, uric acid, and is still slightly caustic even after processing. Urea should be diluted to at least half distillation and the soils should be well watered after application. A good use of urea is in foliar sprays, where the plant can immediately use what it needs and the rest evaporates.
Types of Urea Fertilizers
There are a few different formulations of urea, each of which has a different release rate. Straight urea is not commonly used in potted plants. It provides the highest level of nitrogen and is quickly broken down, often within two days. Ureaformaldehydes have only about 38 percent nitrogen. The formula decomposes with the assistance of micro-organisms and is therefore slower to release in winter. Isobutylidene diurea releases urea slowly as it dissolves. It is only slightly soluble in water so the process takes longer than the other formulations. It also only provides 31 percent nitrogen. The best formulation for potted indoor plants is crotonylidene diurea. It is a long-term, slow release formulation.