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Fertilizer ratios, also called N-P-K numbers, are a series of 3 numbers that tell you how much of a fertilizer mixture, by weight, is made up of fertilizers rich in nitrogen, phosphorus, and potassium. If you have a sample of your soil studied, you will be provided with an ideal fertilizer ratio that will help make up for your soil's nutrient deficiencies. Most hobbyist gardeners can find premixed fertilizers that will suit their needs at a plant nursery or garden supply center. Urea provides plants with nitrogen, but other elements, like phosphorus and potassium, are important for plant health as well.  Fertilizers that you can safely mix and store with urea include:  Calcium cyanamide Sulphate of potash Sulphate of potash magnesium There are certain fertilizers that can be mixed with urea, but lose their effectiveness after 2-3 days because of the reactions that occur between the fertilizer's chemicals. These include:  Chilean nitrate Sulphate of ammonia Nitrogen magneseia Diamonnium phosphate Basic slag Rock phosphate Muriate of potash Some fertilizers will react with urea to either create a volatile chemical reaction, or to render the fertilizer mixture completely useless. Never combine urea with the following fertilizers:  Calcium nitrate Calium ammonium nitrate Limestone ammonium nitrate Ammonium sulphate nitrate Nitropotash Potash ammonium nitrate Superphosphate Triple superphosphate Referencing the list of fertilizers that are and are not effective to mix with urea, choose sources of phosphorus and potassium to add to your fertilizer mix.  Many of these are available at nurseries and garden supply stores. Add each of your chosen fertilizers together, according to the weights given by your fertilizer ratio.  Mix them together thoroughly. This can be done in a large bucket, in a wheelbarrow, or with mechanical mixers. Apply your fertilizer mixture as you would apply urea on its own, spreading it evenly across the soil.  Then water and till the soil in order to incorporate the fertilizer. Urea is less dense than other fertilizers.  If you are using spinning-type equipment to spread your urea-based fertilizer over large distances on your farm, keep your spread width below 50 feet (15.2 m) to spread the fertilizer mixture equally.
Determine your ideal fertilizer ratio. Combine urea with additional fertilizers to create a stable fertilizer mixture. Mix urea with certain fertilizers to fertilize plants immediately. Prevent unwanted chemical reactions from harming your crops. Blend urea with phosphorus and potassium-rich fertilizers for a well-balanced fertilizer. Spread your urea-based fertilizer evenly across your crops.