Nutrient management is among the most consequential decisions that a grower makes with respect to water quality and crop productivity. Because crops do not take up fertilizer with 100% efficiency, many growers apply organic and inorganic fertility in excess of crop demand to ensure that nutrients are not limiting to their crops. While this is often an economic decision, adding excess nutrients to the crop-soil system also creates an opportunity for nutrient losses from farms into the surrounding environment. One major loss pathway for excess nutrients is via nutrient-enriched water that drains from the surface of agricultural fields or percolates beyond the root zone of the crop and into groundwater storage. Nitrogen (N) and phosphorus (P) are generally the most limiting nutrients to crop growth, and are, therefore, added in the greatest quantities by growers and most frequently the nutrient constituents of concern in agriculturally connected waterways and aquifers. When present in excess, nitrates and phosphates can create environmental problems such as eutrophication of waterways, algal blooms, and contamination of drinking water. Recent research in the Tulare Basin and Salinas Valley has found that nitrate pollution of groundwater supplies is widespread and overwhelmingly the result of the agricultural activities in the area over the past six decades. As a result of this study, new regulations on N management have been introduced and are being phased-in throughout the state. The objectives of these regulations are to maintain crop productivity while also reducing environmental pollution due to the over-application of plant nutrients.
Fortunately, managing nutrients to optimize crop growth and water quality are not mutually exclusive. Applying the Right Amount of fertility, at the Right Time, to the Right Place, in the Right Form (4Rs) is likely to maximize the amount of fertilizer that is taken up by the crop and minimize the amount of fertilizer that is wasted or lost to the environment. Since the application of fertilizers is generally one of the highest input costs in agricultural systems, this approach saves farmers money while reducing their environmental footprint in surrounding bodies of water. However, such best management practices (BMP) tend to be highly specific to the crop and environment where they are applied. Further, they involve not only management of the nutrients themselves, but also the interaction of the nutrients with water that is added to the crop-soil system (whether via irrigation or rainfall). Therefore, BMP should be governed by a few fundamental principles, but adapted to the particular cropping context where they are applied. The objective of this practice page is to outline several of the key principles for managing nutrients to maintain water quality without sacrificing crop productivity. Also included are links to resources that will assist in better understanding and implementing BMP as well as links to case studies that exemplify context-specific applications of BMP.
SOURCE: The California Agricultural Water Stewardship Initiative