Fertilizer recommendations call for urea when plants need high nitrogen levels quickly. Fast-growing turf grasses and vegetable crops need plentiful nitrogen to stay healthy and productive. When soil tests show that phosphorus and potassium -- the other two primary plant nutrients -- are present in good supply, nitrogen may be all that's needed. Adding unnecessary nutrients can be toxic to plants and to soil. Urea fertilizer is 46 percent nitrogen and contains no phosphorus or potassium. Unlike slow-release organic fertilizers, which provide nitrogen through decomposition, urea's nitrogen is immediately available. However, unless applied properly, much of urea's benefit can be lost.