Nutrient Management in Agriculture

Nutrient management is the science-based planning and day‑to‑day practice of supplying crops with the right nutrients, in the right amounts, at the right time and place—using soil tests, crop needs, and site conditions to minimize losses to air and water while maximizing yield and return on investment.

The drivers for nutrient management in agriculture are cost savings on inputs, long-term soil/crop health, efficient irrigation and reduced environmental impact. For row crops, maximized yield has traditionally been the primary goal through the last 50 years. During that time, excessive use of inputs to guarantee highest yield has not done so, due largely to soil depletion. In addition, excess nutrients, notably water-soluble nitrate, have leeched into groundwater in concentrations where it is now present in the drinking water for a substantial portion of the population. It has become a priority for growers

The 4Rs / SMART approach

• Right Source – Nitrate fertilizer is available in many forms, the three most common being urea, anhydrous ammonia and a Urea/Ammonium nitrate mixture called UAN. All of these are delivered to the soil as ammonia (NH₃), which converts to ammonium (NH₄⁺) in the soil and binds to soil particle so that it is essentially immobile. Microbes in the soil can convert ammonium to soluble nitrate (NO₃), a reaction that requires oxygen present in the soil. Plants can take up ammonium or nitrate, but as nitrate is mobile in the soil, it is usually what is available to plant roots.
•  Right Rate – Growers and orchardists try to avoid excessive truck rolls to the field in order to lower equipment and labor costs. As a result they may fall into a habit of applying one or two large applications of fertilizer per year in quantities of sufficient size to “guarantee” yield performance. They may include inhibitors to prevent nitrification, keeping nitrate more or less immobile as ammonium. As an alternative, applying lesser quantities of fertilizer at greater frequency provides nitrate in development phases when plants need it most. Nitrate is absorbed, not leached into lower soil layers where it may be hidden from soil tests, and can eventually leach into groundwater. This can lead to less nitrate applied per season, less residual nitrate, and no need for inhibitors.
• Right Time – Growers should actively pre-empt scheduled rate applications for unexpected usually weather-related events. Light rains following a broadcast urea application can water in the fertilizer, heavy rains can wash it away. A warming trend followed by rainstorms can nitrify ammonium in the soil, then leach it down past the root zone where it is unavailable. Very warm weather following an application can trigger volatization of greenhouse gases. Because growers are adding less fertilizer more often they have mor latitude to delay an application for a few days to allow the situation to stabilize following a weather incident.
• Right Place – When plants are just post-emergent through early stages of growth they have very small roots. Fertilizer side dressed down the middle of rows by granular broadcasting or coulter-injecting anhydrous may be too far away for the young plant to access. One method for improving side dress placement uses proprietary Y-Drop applicators, which can be deployed with high-clearance systems for side-dress well into the growing season. Growers can better realize their right rate objectives by spoon-feeding fertilizer to the plants, avoiding excessive applications of fertilizer
• Assessment – SMART planning tailors the 4Rs to site‑specific soils, weather, crops, and management. The synergies growers can realize with consistent application of 4Rs to their respective cropping systems promotes healthier plant with fewer inputs and efficient application of irrigation.

Typical Plan Components

A SMART plan that effectively assists a grower in reducing inputs is a dynamic document. It does not have to be perfect or all inclusive, it can expand as time, experience and resources will allow to be more effective. SMART plans are very site specific, they take into account geographic variations, soil types, micro-climates, any and all factors that may affect the amount of N in the soil. A grower might do well to split the plan into multiple plans, one for each field or field type. Local Natural Resources Conservation Service (NRCS) field offices have baseline information and guidelines that will help the grower get started as well as more general information on nutrient management. Typical plan components are as follows:

• Soil and tissue testing schedule and targets—Soil and tissue testing provide snapshot glimpses of a plant’s overall health. Taken at regular intervals they can provide rough trend data. For Phosphorous and Potassium, which are fairly stable in the soil and many micronutrients which are also stable, scheduled samples can be adequate. Nitrate, which is soluble and highly mobile, s difficult to track using static tests alone. Growers should always log all results and perform simple analysis’ on each nutrient concentration’s acceleration or decrease in the soil to monitor trends and make adjustments based on the soil’s needs and not at regular intervals.
• Nutrient budgets—In addition to the three well-known macronutrients nitrogen, phosphorous, potassium, there are three sub-nutrients sulfur, calcium, magnesium and eight micronutrients. All fourteen are required, the micronutrients essentially in trace amounts. Micronutrients overall are relatively stable in soil, but they can be sensitive to reaction with macronutrients and environmental factors. Zinc in particular is a component of many plant enzymes that facilitates important plant metabolic reactions. Zinc deficiency is an issue worldwide, as the soluble zinc divalent cation (Zn²⁺) binds readily with phosphorous in high pH soils, which limits availability of both nutrients. Manganese is another micronutrient that can be affected by high pH, and the availability of oxygen, as it can be oxidized into less soluble forms. High manganese concentration can limit availability of zinc. High iron levels can limit manganese availability, while high manganese levels can limit iron uptake. These relationships between nutrients are common, and the grower should be familiar with common nutrient issues for the region and the grower’s fields when compiling a nutrient budget.

Many growers begin a nutrient budget by compiling all of the potential input and output sources for nitrogen, the most prevalent nutrient by volume and the most volatile. Legumes can contribute substantially to nitrogen levels, as can mineralization of soil organic matter. Nitrogen can leave the soil profile through leaching, volatization or nitrification. The two grower activities that most influence nitrogen are direct addition of chemical nitrate fertilizer and removal through harvesting. With a basic nitrogen budget in hand, the grower can work through phosphorous, potassium, and the sub-nutrients. Balancing nutrients and their relationships in the soil is very regional and complex. A qualified agronomist can be quite valuable in assisting the grower with a successful budget.

• Field risk screening—Risk screening assesses the potential for nutrient loss due to factors such as field contour, natural/constructed drainage, proximity to ground/surface water, permeability of the soil through the entire soil profile, anticipated weather during the crop cycle, and types of crops. As part of the assessment residual or natural levels of certain nutrients, notably nitrogen and phosphorous, are evaluated to inform the applications rates of nutrients during the crop cycle. As every field and cropping system is different and some influencing factors are not obvious, it can be appropriate for the screening to be performed by an objective and well-experienced third party.
• Application methods— the plan may advocate for a departure from application methods used in previous seasons for reasons of cost, effectiveness or safety. Distribution methods such as Y-drop deliver fertilizer in close proximity to plant roots, increasing the opportunity for uptake. Anhydrous ammonia, while inexpensive and widely available, can be difficult to handle safely, and if improperly installed can be outgassed into the atmosphere.
• Timing windows Application timing is crucial to good nutrient management. For best plant development, the objective is to add appropriate quantities of nutrients when plants are at maximum vegetative growth phase, ensuring optimal uptake and minimizing excess residual fertilizer. Timing is also important to avoid problems presented by climate and weather. Early season rains can wash pre-emergent broadcast fertilizer into the drainage system. Late Summer derecho winds can blow dry fertilizer away. Rising soil temperatures can cause excessive nitrification of ammonium to nitrate, leading to leaching or volatization. Today’s grower should be attentive to the weather during the entire growing season. The plan should include specific strategies for taking best advantage of weather conditions.
• Irrigation—The plan may include migration to a different system of irrigation. While furrow irrigation is a less costly investment, it is also difficult to accurately and uniformly apply small amounts of water. Strategies for controlling leaching can involve smaller amount of nitrate fertilizer and smaller amounts of water. Drip and pivot sprinklers can deliver this level of control, but they have nozzles that may clog easily, especially with hard water.
• Recordkeeping and performance tracking—A grower’s county or state government may have specific requirements for reporting/record keeping. For each field and by crop/growing cycle, growers should keep records of manure introduced over the season. For each fertilizer application, date, type of fertilizer, amount, application method and growing stage at application. Growers should also record irrigation events by date/duration and rainfall. End of season, total yield can be recorded. Results of any soil tests and the latest risk assessment should be incorporated by reference or attached.

Why is nutrient management important?

Effective nutrient management does more than save money on nutrients. Aligning nutrient applications with plant demand and water events improves plant health and nutrition. Better crop quality can lead to better market prices. Healthier plants have been shown to ward off diseases and pests more effectively, requiring fewer pesticide/herbicide inputs. A reduction in nutrient, pesticide and herbicide inputs can greatly improve percentage of soil organic material, enhancing infiltration and the holding capacity of the soil for moisture. On a broader level, when growers practice good nutrient management there will at some point result in less nitrate and salinity in the groundwater that supplies public wells. Fewer greenhouse gases released will promote better air quality, and ultimately help the climate improve. It is good for everyone.