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Objectives of the Guidelines

1. To explain how mineral fertilizers are used to supplement existing soil fertility levels in meeting plant nutrient requirements.
2. To explain how proper use of fertilizers improves and maintains soil fertility levels for long term soil productivity while protecting the environment.
3. To show how sound fertilizer management minimizes nutrient loses by erosion and leaching to ground water.
4. To promote economic crop production and environmental protection through site-specific use of best agricultural practices.
5. To create public awareness and to provide planners, policymakers and other leadership with an understanding of the critical role of fertilizers in sustaining agricultural production.

Introduction

North America is blessed with some of the best agricultural soils in the world but also contains many soils that have one or more severe growth limitations and require special management practices to be productive. The climate is equally variable relative to its suitability for crop production, The combination of variable soils and climate demands that best agricultural practices be defined on a site specific basis. Therefore, this publication will focus on science-based principles and concepts that must be adjusted and adapted to fit the circumstances of the specific grower, year, crop, field, and in some cases, area within the field.

Although the productivity of North American agriculture is obvious, it faces several challenges today. Nitrate in ground water of areas of intensive agriculture is sometimes elevated above desired levels. Algal blooms caused by elevated phosphorus (P) levels in surface waters sometimes impair desired use. Hypoxia (oxygen deficiency) in the Gulf of Mexico has been at least partially attributed to nitrate flowing into the Gulf via the Mississippi River that drains much of the Corn Belt. The perception by the public that agriculture is a major contributor to these problems, whether it is or not, is a major challenge to the industry in a region where the majority of the population understands very little about agriculture.

North American agriculture continues to be a dynamic industry as it implements technological innovations and responds to economic pressures and the environmental concerns mentioned above.

• Conservation tillage. Some form of conservation tillage, in which 30 percent or more of the soil surface is covered by crop residue, is now utilized on over 35 percent of US cropland (table 1). Conservation tillage reduces soil erosion and allows the sustainable use of more intensive cropping systems than soil or water resources would allow with conventional tillage.

Table 1

• Plant genetics. Advances in plant genetics continue to increase yield potential, maintain or improve pest resistance, allow greater tailoring of quality characteristics for specific end use, and offer growers a greater number of management options.
• Site-specific precision agriculture. Refinements in civilian use of the Global Positioning System (GPS), the advent of variable rate fertilizer application equipment, development of accurate yield monitors, development of computer software for information management and mapping, and improved understanding of soil variability have contributed to the adoption of site specific precision agriculture by a growing number of North American farmers. The promise of these precision systems is increased efficiency, profitability and environmental protection.
• Information technologies. The Internet and satellite-based systems are revolutionizing how information is obtained and sent by North American farmers. Information on markets, weather, crop conditions, the latest agronomic research, input and equipment suppliers, soil test data for their farm, remotely sensed imagery for their farm, and a multitude of other subjects are frequently obtained electronically. Farmer to farmer discussion groups are becoming popular and electronic financial transactions are becoming more common.
• Certification of crop advisers. The formal certification of individuals developing crop management plans with farmers has increased significantly the technical competency of these professionals. The Certified Crop Adviser program, administered by the American Society of Agronomy, currently (January 1997) has over 8,300 individuals certified and over 16,O00 that have taken the national exam and are working on their certification. This rigorous program includes technical written exams and continuing education requirements to maintain certification.

Commercial fertilizer is a critical component of North American agriculture. Total annual consumption of commercial fertilizer is now more than 45 million metric tons product in the U.S.A. and 4.2 million metric tons product in Canada. Corn (maize) is the leading user of fertilizer in North America. According to the U.S. Department of Agriculture, average use per hectare of corn planted in the U.S. is currently 140 to 145 kg N, 50 to 55 kg P2O5, and 60 to 65 kg K2O.

Nutrient use per acre of corn stopped increasing in the early 1980's. Nitrogen use per acre for corn has been relatively stable since then, while P and K use has declined. During this same period, corn yields have continued to increase at a rate exceeding 1.5 percent per year. Several factors are responsible for the increase in production per unit of fertilizer used, a major one being the implementation of best management practices (BMP's) discussed in this publication. These practices reduce nutrient losses and produce a cropping system that more efficiently utilizes all inputs, including commercial fertilizer. In the case of P and K, many corn farmers have succeeded in building their fields to the optimum soil test range where only maintenance amounts of P and K are now required for optimum production.

Crop Nutrition and Nutrient Losses from Soils

Crops require an adequate supply of nutrients to maintain satisfactory yields and quality. Commercial fertilizer and organic nutrient sources are used to make up the difference between crop needs and soil supplying ability. Soil testing is the major tool used in determining supplemental nutrient needs.

Nutrient removal in the harvested portion of crops represents the largest single loss of nutrients from soils. Table 2 shows nutrients removed in the harvested portion of several North American crops.

Table 2

Nutrient removal is a critical factor when evaluating the sustainability of a farming system. If the nutrients removed from a field are not replaced the system will not be sustainable, The two nutrients most susceptible to depletion through crop removal are P and K. Unlike N, which can be partially replaced by rotation with legume crops, there is no biological method of replacing P and K. Once soil supplies are depleted through crop removal, the only method of replacement is through the importation of outside sources. The source can be organic residues or commercial fertilizer.

Other types of nutrient losses from soil include soil erosion, denitrification, leaching, and volatilization. Through soil erosion nutrients move off the field with the enriched soil particles that are eroded. Nitrogen may be lost through the processes of denitrification and leaching. Denitrification occurs in warm waterlogged soils that have ample organic residues. Micro-organisms convert or denitrify nitrate-N (NO3-N) to unusable N gases. Leaching occurs when water moves NO3-N through the soil and out of the crop rooting zone. The amount lost depends on the NO3-N level, the volume of water available, and the soil texture.

Sound N management involves practices that minimize these two N losses. Volatilization is another N loss mechanism that occurs under some circumstances where ammoniacal-N is present. Volatilization an occur either from the soil surface or from the surface of growing plants. In this process N is lost from the soil in the form of ammonia gas. Various N additives are utilized by producers under certain circumstances to help reduce N losses.

Integrated Site-Specific Crop Management

Soil or applied nutrients can be efficiently utilized only when they are part of a well managed integrated cropping system. All controllable factors of production must be set at optimum levels such that none limit the effectiveness of other factors and prevent the target yield from being attained. Improper plant population, planting too late, poor pest control, inadequate liming, or using a poorly adapted variety are examples of practices that lead to inefficient nutrient utilization. Likewise, a deficiency of any one nutrient can markedly reduce the response to other nutrients and the associated nutrient use efficiency. Balanced nutrition in which optimum levels of all nutrients are used is a critical part of successful crop management.

However, some production factors such as the amount of rainfall or the occurrence of killing frosts, are not controllable. Execution of a plan that includes the best our current level of understanding has to offer may still result in poor nutrient use efficiency, There are no guarantees in crop production. Wise producers implement management practices that have the highest probability of succeeding over the long term, recognizing that in any one year they may fail.

No universal optimum fertilization recipe exists for any crop. Effective nutrient management plans can be developed only with extensive knowledge of the grower, the field, the crop to be grown and the year. They are based on use of appropriate diagnostic tools such as soil testing and plant analysis as well as past observations and consideration of current soil physical and biological conditions.

In other words, the plans must be site-specific. If within-field variability is substantial, profitability and environmental soundness may be improved by considering the variability existing within the field boundaries.

Site-specific precision management involves recording yield, soil test, soil properties and other observations along with a precise description of the location within the field where the data were collected. Input applications are varied based on maps that are created from geo-referenced records of soil test values, soil yield potential, previous yield histories, and nutrient applications that can be coded in the computerized record keeping system. New computer software allows the geo-referenced records to be analyzed and displayed as management maps.

Computers use the maps to automatically change input rates and blends during application. The system of earth-orbiting global positioning satellites (GPS) established by the US government allows field operations and measurements to be precisely located within a field during the operation. Site-specific nutrient management attempts to capitalize on the inherent variability of soils by avoiding over fertilization of less productive areas and under fertilization of the more productive areas. Ultimately that should lead to the most agronomically sound, economically efficient, and environmentally responsible nutrient management plan for each field. This technology is still being developed and much is yet to be learned about how to implement it most efficiently,

Quantity of Nutrients to be Applied

The quantity of a nutrient to apply is obviously a critical part of any management plan and impacts greatly the profitability and environmental impact of the practice. If too little of a nutrient is used, crop yield and/or quality suffers and profitability can be severely reduced. Other nutrients may be utilized less efficiently and if crop growth is decreased significantly, soil erosion increases due to reduced soil cover. If too much of a nutrient is applied, profitability is again reduced and the excess may contribute to water quality problems.

The quantity of N applied in a given year is especially important because of the potential for loss of N that is not utilized by the crop during the year of application and because of the severe yield reduction than can occur if inadequate N is applied. The following factors should normally be considered in selecting a N application rate:

• Local research. Land Grant universities conduct research that helps define the specific factors that need to be Considered in a local area when determining N needs.
• Yield goal. The yield goal for the crop to be grown is usually an important factor and needs to be carefully selected. It should be an optimistic but realistic estimate.
• Soil tests. Profile soil nitrate levels or organic matter in the surface horizon are important in some regions.
• Previous crop credits. Leguminous previous crops reduce the supplemental N need of the crop that follows.
• Manure credits. Animal manures or other N containing wastes applied in the recent past need to be considered in assessing N needs.

More flexibility exists in annual rates of nutrients such as P and K that are immobile in most soils. The supplying ability of soils for these nutrients doesn't change abruptly. Therefore, the management focus is on building soil test levels to an optimum range followed by replacement of nutrients removed by crops to maintain soil fertility at optimum levels. Many factors influence the optimum soil test level range including crop rotation, yield potential, land tenure, and the opportunity costs of the producer.

Timing and Method of Application

Timing of fertilizer application is most critical for mobile nutrients such as N. Nitrogen is very soluble in water and can be lost in runoff during intense rainfall or leached from the soil profile. Immobile nutrients such as P or K can be safely applied at any time if incorporated into soils where erosion is not a problem.

Large amounts of K should not be applied to sandy soils at one time. In humid regions for optimum crop use efficiency and minimum potential for environmental damage, fertilizer nutrients should be applied as near to the time the crop needs them as is practical.

In high residue systems injecting N below the soil surface often reduces immobilization, volatilization and denitrification and results in more efficient utilization of the N applied. Therefore, in conservation tillage, injected N is often superior to top dressed applications. At low soil test levels and conservative application rates, band application of P and K often performs better than broadcast application. At high soil test levels there generally is little difference among application methods. The exception is in no-till or ridge-till systems, where banded K may give additional response even when soil test levels are high or very high. In environments were early season conditions are poor for nutrient uptake, a starter band applied near the seed row often gives good response regardless of soil test level.

For all fertilizer application methods, equipment should be adjusted to ensure uniform spread at the correct rate. Equipment should be well maintained and frequently calibrated.

Foliar application is the most efficient way to apply micronutrients that are needed only in small quantities by crops and may become unavailable if applied to the soil. Foliar application may also be an effective means of rescuing crops from inseason nutrient deficiencies.

Types of Fertilizer to be Applied

Commonly used commercial fertilizers have similar agronomic effectiveness when compared at equal nutrient application rates. Source selection is primarily a matter of price, convenience and quality of dealer service. There are some exceptions in the case of N sources. For example, ammoniacal N sources, like anhydrous ammonia or urea, are preferred for fall application because they are less subject to leaching and denitrification losses. Nitrate forms of N may be preferred for application to pastures in environments with high volatilization potential where urea forms could result in significant N loss.

Sources of further information

Potash & Phosphate Institute - PPI
655 Engineering Drive, Suite 110 - Norcross, GA 30092-2837 - USA
Tel: +1 770 4470335 - Fax: +1 770 4480439

The Fertilizer Institute - TFI
501 Second Street, N.E. - Washington, D.C. 20002 - USA
Tel: +1 202 6758250 - Fax: +1 202 5448123