Fertilizer Use and the 4Rs

Fertilizer Use and the 4Rs

Fertilizers are a necessary component of sustainable crop production. When properly managed, fertilizers help address the challenge of increasing production in an economically viable way while retaining the ecological integrity of cropping systems. However, if nutrients are not adequately available within a crop production system, fertility is mined from the soil, and the crop will never attain optimal yields. Conversely, if nutrients are supplied in excess or without managing risks, the possibility of nutrient movement away from the cropping system increases, potentially negatively affecting the environment. In both situations, the profitability of the cropping system will be negatively impacted by lost yield or by lost inputs.
4R Nutrient Stewardship utilizes fertilizer best management practices (BMP) addressing the right fertilizer source, at the right rate, the right time, and in the right place. The 4Rs provide the foundation for a science-based framework to

achieve sustainable plant nutrition management. In short, 4R practices are good for the grower, good for the farming community, and good for the environment.
There is an existing need to improve the adoption of fertilizer best management practices to enhance the sustainability, efficiency and productivity of agricultural systems. Efficiency and productivity together are interwoven with sustainability. Striving to improve efficiency without also increasing productivity simply increases the pressure to produce more on lands less suited to agricultural production. Conversely, squandering resources to maximize productivity can result in increased environmental impacts and decreased profitability.
The essential plant nutrients play a vital role in providing adequate food supplies and protecting our environment.

Plant nutrients promote a more vigorous, healthy and productive crop. A vigorously growing crop has greater root systems, more above-ground residue, sustained green top growth, quicker ground cover, greater water use efficiency, greater nutrient efficiency and higher resistance to crop stresses caused by drought, pests, cold temperatures or delayed planting. Plant growth through the process of photosynthesis utilizes atmospheric carbon dioxide, a greenhouse gas, and generates life-sustaining oxygen. While many nutrients are essential to plant health, some nutrients pose a greater environmental risk than others when improperly managed. The two nutrients most often associated with mismanagement and non-point-source environmental concerns are nitrogen (N) and phosphorus (P).

Nitrogen and the Environment

When soil nitrogen supply becomes low, plant stresses are immediate and yield losses are assured. The large demand crops have for nitrogen (legumes are an exception) means that supplemental sources must be provided for efficient and sustainable crop production. All these sources, when added to soils enter the nitrogen transformation cycle and are eventually converted to plant-available ammonium and nitrate-nitrogen. To meet crop management objectives, fertilizer best management practices must ensure that adequate amounts of nitrogen are used for profitable production levels, while minimizing any potential negative effects on the environment. This is best achieved by utilizing practices that address the 4Rs.

Much of the concern about nitrogen in the environment is due to the potential movement of unused or excess nitrate-N through the soil profile into groundwater (leaching). Because of its negative charge, nitrate-nitrogen is not attracted to the various soil fractions. Rather, it is free to leach as water moves through the soil profile. Soil type has an influence on the amount of and speed with which nitrate-nitrogen moves through a soil profile, with movement greater in sandy as compared to clay soils. Nitrogen loss as ammonia volatilization from surface-applied sources and as dinitrogen gas (N₂) or nitrous oxide (N₂O) from soil microbial activity is also a concern.

Nitrate is more likely to move downward in sandy soil than in clay soil.
Source: IPNI

Phosphorus and the Environment

Phosphorus has been associated with environmental pollution through the eutrophication of lakes, bays and non-flowing water bodies. The symptoms are algal blooms, heavy growth of aquatic plants and deoxygenation. Since phosphorus is insoluble relative to other essential nutrients, environmental degradation is associated largely with phosphorus movement when soil erosion occurs. Except on some organic soils, very low concentrations of phosphorus are found in drainage waters as the result of leaching. The major form of phosphorus entering surface waters in most agricultural watersheds is particulate-phosphorus associated with either clay soil fractions or organic matter. These fractions are the most easily eroded, and have a relatively high surface area that contains enriched phosphorus levels compared to soil particles that have greater resistance to erosion.

Sediment-enriched phosphorus commonly contains two to six times that of soil phosphorus levels that are left behind. High-loading in surface runoff is usually associated with storm events. Storm flow concentrations of soluble phosphorus are often 10 times greater than base flow concentrations. Numerous research studies have shown that conservation tillage practices reduce soil erosion and the movement of phosphorus from agricultural lands. Conservation tillage is a BMP because it reduces erosion considerably by absorbing the impact of falling rain and slowing water runoff. If erosion is stopped, then phosphorus losses to the environment will be reduced to acceptable minimum levels.