What Is the 4R Concept for Plant Nutrient Management
Common knowledge is that there are 16 nutrients that are considered essential for optimum plant growth and development, and they are grouped according to the relative amount of each nutrient that plants need. These groups are: 1) primary or macronutrients–comprised of carbon [C], hydrogen [H], oxygen [O], nitrogen [N], phosphorus [P], and potassium [K]; 2) secondary nutrients–comprised of calcium [Ca], magnesium [Mg], and sulfur [S]; and 3) micro or trace nutrients–comprised of boron [B], chlorine [Cl], copper [Cu], iron [Fe], manganese [Mn], molybdenum [Mo], and zinc [Zn]. Click here for an article from Texas A&M AgriLife Extension Service that provides details related to the 16 essential plant nutrients.
In the past, a plant nutrient was defined based on the concept of essentiality–i.e., without an essential plant nutrient a plant cannot develop normally, and no other nutrient element will substitute for it. Now, 4R nutrient stewardship has been defined as nutrient management that produces a sustainable outcome, which suggests that it is the outcome realized from applying a nutrient-containing material rather than the essentiality of the applied nutrient that should guide nutrient applications to plants.
Elements other than those considered essential may provide benefits to some plants/in some environments even if they are not essential. This then leads to a proposed new definition for a plant nutrient that is under consideration, which is “a mineral plant nutrient is an element which is essential or beneficial for plant growth and development or the quality attributes of the harvested product of a given plant species grown in its natural or cultivated environment”. Notice the highlighted phrase “essential or beneficial” in this proposed new definition. See the chart in Table 1 of the article titled “What Is a 4R Plant Nutrient?” for elements that are listed as beneficial in addition to those mineral elements that are listed as essential. Note that the chart shows 5 to 7 elements that are considered beneficial vs. essential.
This leads to the new concept that should be considered when applying plant nutrients–i.e., the 4R approach of plant nutrient application that is based on fertilizer stewardship, or the responsible use of applied plant nutrients. 4R nutrient stewardship provides a framework to achieve goals such as increased crop production, increased producer profits, enhanced environmental protection, and improved sustainability. A 4R plant nutrient is any nutrient that enhances both plant growth and development as well as the quality of the harvested product.
The 4R concept incorporates the 4R’s that include 1) using the Right fertilizer source applied at 2) the Right rate at 3) the Right time and 4) in the Right place. Using this concept should avoid poorly managed nutrient applications that 1) decrease profitability, 2) increase nutrient losses to the surrounding environment, and 3) contaminate soil, water, and air both at/near to and far away from the site of fertilizer application. A brief narrative about each of the 4R’s follows.
Right fertilizer source. Are the applied fertilizer nutrients available for immediate or delayed crop uptake? What nutrients and how much of each are available in the soil?
Right rate. Does the amount of fertilizer applied match how much the crop can use? Is the amount of fertilizer applied based on an accurate soil analysis? Is the fertilizer application equipment properly calibrated? Does the amount applied match historical crop yields or current crop yield goals?
Right time. Does the time of fertilizer application(s) match peak crop demand for the applied nutrients? Is the soil at the site of fertilizer application prone to saturation following application? Which fertilizer nutrients can be applied in the fall preceding a following summer’s crop? Which fertilizer elements are subject to runoff if applied a considerable time before crop need?
Right place. Are applied fertilizers placed where a crop can successfully access the nutrients–i.e., was the fertilizer applied in the root zone of the crop being grown? Were site-specific factors such as field slope, soil texture, distance to surface waters, and field drainage patterns considered when fertilizer was applied? Should variable rate fertilizer application be considered for a particular field with differing soil properties?
4R nutrient stewardship requires that best management practices [BMP’s] be used to ensure that added fertilizers are used with optimum efficiency. The goal of using fertilizer BMP’s is to match nutrient supply with a particular crop’s requirements so that nutrient losses from crop fields are negligible or minimized. It is important to note that fertilizer BMP’s will be different for cropping sites based on differences in soil properties, climatic conditions, and crop management conditions such as tillage system. 4R stewardship will be most effective when used in consort with other conservation practices such as no-till and cover cropping.
The 4R nutrient stewardship approach considers the economic, social, and environmental aspects of nutrient management. In actuality, 4R nutrient stewardship is merely using fertilizer BMP’s to ensure that both essential and beneficial nutrients 1) are applied only in an amount needed for the crop being grown, 2) stay on the site of application, 3) are used efficiently by the fertilized crop, 4) are applied based on the needs of the crop being grown, 5) are applied based on specific properties of the application site, and 6) do not negatively impact the surrounding environment. It comes down to applying the right source of a nutrient at the right time at the right rate and in the right place, and is intended to be an essential tool in the development of sustainable agriculture systems.
The following links are to sources that provide more information about the 4R Nutrient Management Concept.
What Is a 4R Plant Nutrient? [Nov.-Dec. 2023 issue of Crops & Soils Magazine]
What are the 4Rs. [Nutrient Stewardship]
The 4R’s of Nutrient Management. [Michigan State Univ. Ext.]
4R Pocket Guide. [Nutrient Stewardship]
Composed by Larry G. Heatherly, Jan. 2024, larryh91746@gmail.com