Nitrogen is relatively mobile in moist soil and therefore need not be applied near the plant to be effectively used. However, surface application or shallow incorporation can result in reduced uptake if the seedbed remains dry during the early growing season. Excessive levels of nitrogen applied initially or during the growing season will delay the maturity of potatoes, as well as increasing the risk of leaching as a result of heavy rainfall events. Splitting the total nitrogen application is often used as a means of limiting nitrate leaching losses. Part of the nitrogen is supplied pre-plant or sideband during the planting operation.
Several methods are available for making split nitrogen applications after crop emergence.
The most common method is top-dressing, which is the broadcasting of fertilizer onto a field when a crop is growing. Granular ammonium nitrate is the preferred nitrogen source for top-dressing if it is not possible to incorporate the fertilizer after application. Urea can be top-dressed, but it must be incorporated by hilling or irrigation shortly after application. Liquid nitrogen can be dribbled onto the hill at the base of the plants, but it must also be incorporated by hilling or irrigation shortly after application. Care must be taken to avoid contacting the foliage with liquid nitrogen or the leaves will burn.