Nitrogen (N) and phosphorus (P) are the two primary nutrients most often associated with potential impacts on water quality. Both are also essential for optimum crop production. In recent years, The Fertilizer Institute (TFI) has promoted four general management practices for nutrient use that can be varied to impact crop yields and help reduce nutrient losses. These four management concepts have become known as the 4R’s. These are 1) the right rate 2) with the right source 3) with the right placement and 4) at the right time. The 4R program is founded on scientific principles and a combination of the basic research conducted by university faculty and data collected on actual farm fields.
The 4R nutrient stewardship principles are the same around the world, but vary depending on site specific characteristics. The scientific principles of the 4R framework, as listed by The Fertilizer Institute, include:
Right Rate: Assess and make decisions on soil nutrient supply and plant demand.
In the 4R educational programs there is increasing emphasis on analysis of soils and manures in an effort to achieve the most efficient nitrogen rate. These analyses are also important for water quality considerations because excessive nitrogen can leave the landscape via surface flow and tile drainage. Some undetermined amount originates from the soil organic matter via the process of mineralization. Nitrogen in tile lines is affected by total rainfall as well as the intensity and duration of rainfall events.
Soil samples collected to a depth of 24 inches in the fall after corn harvest and analyzed for nitrate-nitrogen can be used as an indicator of the general rate of nitrogen applied to fine textured soils. Relatively high amounts of carryover nitrate can indicate the use of N rates were higher than needed for optimum production. The amount of nitrate-nitrogen in the tile water combined with the amount of nitrate-nitrogen in the soil following harvest can indicate if excessive rates of nitrogen were applied.
Right Source: Ensure a balanced supply of essential nutrients, considering both naturally available sources and the characteristic of specific products, in plant available forms.
Both organic and inorganic sources of N have been used for optimum nitrogen management in Minnesota. Choice of N source is a major component of the 4R program from TFI. This is appropriate because the interaction between N source and production environment is very important. These interactions have been identified as a consequence of considerable research. From a water quality perspective, however, there should be small differences among the various N sources when properly used for grain production if other production practices are similar.
Right Place: Address root-soil dynamics and nutrient movement, and manage spatial variability within the field to meet site-specific crop needs and limit potential losses from the field.
Both organic and inorganic sources except for anhydrous ammonia can be either broadcast (with or without incorporation) or injected below the soil surface. As emphasized by the educational programs from TFI, placement of any nitrogen below the soil surface whenever possible is a good management practice. If broadcast on the soil surface, the nitrogen source should be incorporated by some type of tillage equipment to minimize loss.
Right Time: Assess and make decisions based on the dynamics of crop uptake, soil supply, nutrient loss risks, and field operation logistics.
For the diverse production environments in Minnesota, there are several options for the timing of both organic and inorganic sources of nitrogen. In general, maximum efficiency of nitrogen use occurs when the nitrogen source is applied as close as possible to maximum nitrogen uptake. When this occurs, more nitrogen will be used by the crop and less is susceptible to loss from the landscape. As a consequence of putting this concept into practice, sidedressing of the inorganic sources is being promoted and is becoming more popular.
The 4Rs help improve agricultural productivity while minimizing impacts to the environment. Efficiency of production helps meet global crop needs with less land than would otherwise be required. As edge-of-field data collected from Discovery Farms show, the water quality benefits of the 4R program are also important. Research based production practices associated with each of the 4R’s should minimize the amount of nitrogen and phosphorus leaving the Minnesota landscape.
Learn more about The Fertilizer Institute and the 4R program at http://www.nutrientstewardship.org/.