Variable Fertilizer Application Research Articles

Irrigated cotton lint yields as affected by phosphorus fertilizer and landscape position

Phosphorus (P) is the second most limiting nutrient in cotton (Gossypium hirsutum L.) production after nitrogen. Response to P fertilizer, however, is often hard to predict in the Southern High Plains, even with soil test-based applications. Landscape position has a strong influence on yields and perhaps on fertilizer response as well. The objective of this 5-year study (1994–1998) was to determine P fertilizer response in irrigated cotton in different landscape positions. We used an 825-m transect of end to end 15-m plots across a broad swale in an Amarillo fine sandy loam in Lamesa, TX that included three landscape positions, sideslope, bottomslope and drainageway. A randomized complete block design was used with 11 replicates and 5 P rates (0, 22.4, 33.6, 44.8, and 56 kg P ha−1). Analysis of variance showed a linear or quadratic response to P fertilizer in 3 of 5 years, and an effect of landscape position in 4 of 5 years. Four-year lint yield averages, excluding 1997, were 1355 kg ha−1 in the bottomslope position in the landscape, and 1210 and 1226 kg ha−1 on the sideslopes and in the narrow drainageway, respectively. Cross-correlation using the 55 plots as a transect revealed few effects of soil properties on lint yield, but negative correlation between yield and elevation. In 1997, the one year without a landscape effect on yield, more rain fell during the growing season than in the other 4 years. This suggests that yield potential is higher in lower landscape positions because of more favorable soil water relations. Cotton lint response to P fertilizer was evident only in the bottomslopes and in the drainageway, and was absent in the sideslopes, meaning that variable rate fertilizer applications could be linked to management zones based on landscape position.

Spring wheat (Triticum aestivum) yield and grain protein responses to N fertilizer in topographically defined landscape positions

A 3-yr field study was initiated in 1996 to examine the different grain yield and grain protein responses of wheat to varied N fertilizer rates in a typical glacial till landscape in Saskatchewan, Canada. Our objective was to assess the agronomic and economic feasibility of variable rate fertilizer (VRF) N application for wheat. Results suggest that spring soil water status largely determined the yield and the protein content of wheat both within different years of the study and between different landscape positions within a given year. Although grain yield was strongly related to spring soil water and was predictable on that basis, the grain yield response of wheat to fertilizer N additions was highly variable due, in part, to the dual role that N played in determining both grain yield and grain protein content. As a consequence of the unpredictable nature of the varied response of wheat to N fertilizer additions, there was little economic rationale for using VRF strategies in the 3 yr of this study. However, in the long-term, we believe that VRF N application strategies can be employed to manage N inputs from the perspective of managing and replacing harvested N. Key words: Variable rate fertilizer application, precision farming, nitrogen application, N fertilizer, Saskatchewan

COMPARISONS OF UNIFORM AND VARIABLE RATE NITROGEN AND PHOSPHORUS FERTILIZER APPLICATIONS FOR GRAIN SORGHUM

Variable rate fertilizer application has the potential to improve fertilizer use efficiency, increase economic returns,and reduce environmental impacts. This study was designed to examine differences in yield and economic returns betweenuniform and variable rate fertilizer applications. During the 1997 and 1998 growing seasons, a variable rate applicator,capable of varying two liquid fertilizers simultaneously, was used to evaluate three fertility strategies: conventional uniformN, uniform N and P, and variable rate N and P. The three treatments were assigned in six blocks within three 14ha grainsorghum fields (two blocks in each field) in a randomized complete block design. Thirtysix soil samples were taken in astaggered systematic grid from each field, and levels of soil nutrients were determined. Application rate maps for the variablerate N and P treatment were generated based on a fixed yield goal and sitespecific soil N and P levels across the experimentalplots, while application rates for the uniform N and P treatment were calculated from the same yield goal and average soilN and P levels for all three fields. Yield monitor data indicated that the variable rate treatment resulted in significantly higheryields than the uniform N and P treatment for both years (400 kg/ha higher in 1997 and 338 kg/ha higher in 1998). Moreover,coefficients of variation of yield monitor data for the variable rate treatment were smaller than those for the two uniform ratetreatments. A simple economic analysis showed that the variable rate treatment had positive relative economic returns overthe uniform N and P treatment ($27/ha in 1997 and $23/ha in 1998). However, if additional costs for soil sampling, equipment,and data analysis associated with variable rate application were considered, these returns would be much lower or evennegative. These results showed that variable rate fertilization can increase yield, reduce yield variability, and improveeconomic returns. More experiments are needed to evaluate the longterm agronomic, economic, and environmental viabilityof variable rate technology in the Rio Grande Valley of south Texas

Method for Precision Nitrogen Management in Spring Wheat: II. Implementation

By accounting for spatial variation in soil N levels, variable-rate fertilizer application may improve crop yield and quality, and N use efficiency within fields. The main purpose of this study was to demonstrate how site-specific wheat yield and protein data, and a geographic information system may be used in developing precision N-recommendations for spring wheat. The three steps in the procedure include: (1) estimate the amount of N-removed in wheat in the year in which the crop is harvested, (2) estimate the N-deficit, defined as the amount of additional N needed for raising protein concentration in a future crop to a specified target level, and (3) estimate the total N-recommendation by summing the mapped values of the N-removed and the N-deficit. A map for variable-rate application of fertilizer is derived by specifying cutoff values to divide the range in the total N-recommendation into classes representing N management zones. A field experiment was conducted within an annually cropped wheat field (101 ha) in northern Montana to determine whether the proposed method could improve grain yields and protein levels. The N-removal and N-deficit were estimated from site-specific wheat yield and protein data that were acquired during harvest of 1996. In 1997, which was a dry year, an experiment was conducted in the same field that consisted of a randomized complete block design arranged as pairs of strip plots. Variable- or uniform-rate N treatments were randomly assigned to each pair of strips. Both treatments received nearly the same amount of fertilizer, however, N in the variable treatment was varied to match patterns in grain yield and protein levels that previously existed in 1996. Yields were not significantly different between management systems, but proteins were significantly enhanced by spatially variable N application. In addition, variability in protein levels was reduced within the whole field. Field areas deficient in N fertility could be identified without having to sample for soil profile N.

Spatial and temporal variability of corn grain yield: site-specific relationships of biotic and abiotic factors.

Inadequate information on factors affecting crop yield variability has contributed to the slow adoption of site-specific farming (SSF). This study was conducted to determine the effects of biotic and abiotic factors on the spatial and temporal variability of irrigated corn grain yields and to derive information useful for SSF. The effects of water (80% evapotranspiration (ET) and 50% ET), hybrid (drought-tolerant and -susceptible), elevation, soil index (SI)(texture), soil NO3–N, arthropods, and diseases on corn grain yield were investigated at Halfway, TX on geo-referenced locations. Grain yields were influenced by interrelationships among biotic and abiotic factors. Grain yields were consistently high under high water treatment, at higher elevations, and on soils with high SI (high clay and silt). Soil NO3–N increased grain yields when water was adequate. Management zones for variable rate fertilizer and water application should, therefore, be based on information on elevation, SI, and soil NO3–N. The effects of arthropods, diseases, and crop stress (due to drought and N) on corn grain yield were unpredictable. Spider mite (Oligonychus pratensis) and common smut (Ustilago zeae) damage occurred under hot and dry conditions in 1998. Spider mite infestations were high in areas with high soil NO3–N. Moderate air temperatures and high relative humidity in 1999 favored southwestern corn borer (Diatraea grandiosella) and common rust (Puccinia maydis) incidences. Knowledge of conditions that favor arthropods and diseases outbreak and crop stress can improve the efficiency of scouting and in-season management of SSF. Management of SSF can be improved when effects of biotic and abiotic factors on grain yield are integrated and evaluated as a system.

Evaluating Farmer Defined Management Zone Maps for Variable Rate Fertilizer Application

In the U.S.A. intensive grid soil sampling has conventionally been used to develop prescription maps for variable rate (VRT) fertilizer application. Grid sampling can provide an accurate basis for variable rate application; however the cost and labor requirements suggest other approaches may be more economical. This research was initiated to determine if farmer-developed management zone maps based on soil color from aerial photographs, topography, and the farmer's past management experience can be effective in developing variable rate application maps. The accuracy of farmer-developed management zone maps was investigated on two center pivot irrigated fields near Wiggins, Colorado. Using aerial photographs as a template, the farmer defined high, medium, and low productivity management zones. Cluster analysis confirmed that management zones represent different suites of soil characteristics. In field one, soil organic matter (SOM), clay, nitrate, potassium, zinc, conductivity and corn yield data followed the trends indicated by the management zones. In field two, however, the medium productivity zone had the highest values for these parameters. Farmer developed management zones appear to be effective in identifying different management zones; however, ground truthing is needed to develop accurate VRT maps from the zones.

Using Site-specific Approaches to Advance Potato Management in Irrigated Systems

Potatoes (Solanum tuberosum L.) are grown extensively throughout the Pacific northwestern United States as a high value crop in irrigated rotations with other row crops such as wheat (Triticum aestivum L.) and both field and sweet corn (Zea mays L.). Center pivots are the predominant irrigation systems. Soil texture ranges from coarse sands to finer textured silt loams and silts and can vary within one field, particularly in fields with hilly topography. Site specific management is being evaluated as an approach to help to optimize inputs (water, seed, agricultural chemicals) to maintain or enhance yield and reduce potential negative environmental impacts from these farming systems. Currently, variable rate fertilizer application technology and harvest yield monitoring equipment are commercially available for potato. Variable rate seeding and variable rate irrigation water application technologies are developed but not fully commercialized and variable rate pesticide application equipment is in development. At the Irrigated Agricultural Research and Extension Center in Prosser, Wash., we have a team of research scientists, interested individuals from local industry, and other key organizations (e.g. local conservation districts) who are working together to evaluate different site specific technologies, improve the ability to use available tools, and to improve decision-making ability by conducting research both on farm and in research plots.

645 Using Site-specific Approaches to Advance Potato Management in Irrigated Systems

Potato (Solanum tuberosum L.) is grown extensively throughout the Pacific Northwest as a high-value crop in irrigated rotations with other row crops such as wheat (Triticum aestivum L.) and corn (Zea mays L.)—both field and sweet. Center pivots are predominant irrigation systems. Soil texture ranges from coarse sands to finer textured silt loams and silts and can vary within one field, often with very hilly topography. Site-specific management is being evaluated as an approach to help to optimize inputs (water, seed, agricultural chemicals) to maintain or enhance yield and reduce the potential of negative environmental impacts in these farming systems. Currently variable rate fertilizer application technology and harvest yield monitoring equipment are commercially available for these systems. Variable rate seeding and variable rate irrigation water application technologies are developed but not fully commercialized and variable rate pesticide application equipment is in development. At the Irrigated Agr. Res. and Ext. Ctr. in Prosser, Wash., we have a team of research scientists (both university and USDA/ARS), interested individuals from local industry, and other key organizations (e.g., local conservation districts) who are working together to evaluate different site specific technologies, improve the ability to use available tools, and to improve decision-making ability by conducting research both on farm and in research plots.

Soil Nutrient Relationships with Topography as Influenced by Crop

Variable-rate fertilizer application is often based on grid soil sample data from a single year of data in an annual crop rotation. The objectives of this study were to determine if crop history influences spatial dependence (the degree of spatial variability) of nutrients in a rotation including both annual crops and alfalfa, and to compare grid-based and topography-based sampling strategies for representing within-field nutrient levels. A site in the Red River Valley of North Dakota was observed over three years from 1994–1996. The site was divided into one field of continuous alfalfa (Medica sativa L.) and an adjacent field seeded to spring wheat (Triticum aestivum L.) in 1994, barley (Hordeum vulgare L.) in 1995 and alfalfa in 1996. Samples were taken from a 16.2-ha site each fall in a 33-m grid and analyzed for NO3-N, P, SO4-S and Cl. Topography was determined by measuring elevation in a 33-m grid with a laser-surveying device. Spatial dependence was determined by calculating the semivariogram and using regression analysis to assess the relationship between the semivariogram and the semivariogram model. Spatial dependence of NO3-N and P was strongest following spring wheat and barley, while spatial dependence for SO4-S and C1 was strongest for vigorous stands of alfalfa. When the continuous alfalfa stand declined following winter kill, NO3-N and P spatial dependence intensified. Topography based sampling was correlated with the 33-m grid by giving each 33-m sampling location its value as directed by a topography sampling, then correlating that topography based value with the original 33-m sampling value. Topography-based sampling was correlated with the 33-m sampling grid for all nutrients following spring wheat and barley, but not in continuous alfalfa until the stand began to decline in vigor. Following alfalfa seeding in the annual crop field, topography relationships with NO3-N and P decreased, while topography relationships with SO4-S and Cl increased. Topography samplings of sulfate-S and chloride were most highly correlated to 33-m grid values in vigorous alfalfa. Lack of NO3-N spatial dependence in the vigorous alfalfa stands suggests that a composite or field average soil test might be sufficient to provide soil NO3-N information under similar conditions.

Estimation of the Importance of Spatially Variable Nitrogen Application and Soil Moisture Holding Capacity to Wheat Production

Crop growth modelling techniques were used to investigate the performance of a wheat crop over a range of weather conditions, nitrogen application rates and soil types. The data were used to predict long term benefits of using spatially variable fertilizer application strategies where fertilizer application rate was matched to the soil type, against a strategy of uniform fertilizer application. The model was also run with modified soil properties to determine the importance of soil moisture holding capacity in the variability of crop yield. It was found that the benefits of spatially variable nitrogen management when fertilizer was applied at the beginning of the season were modest on average. The range of results for different weather conditions was much greater than the average benefit. A large proportion of the variability of crop performance between soil types could be explained by differing soil moisture holding capacity. Devising techniques for managing this variability was concluded to be important for precision farming of cereals.

Use of spectral radiance for correcting nitrogen deficiencies and estimating soil test variability in an established bermudagrass pasture

The use of variable rate technology has become increasingly popular for applying plant nutrient elements. The most widely used method for determining variable fertilizer rates is presently based on soil testing and yield mapping. Three field studies (Bumeyville 1995, Burneyville 1996, and Ardmore 1996) were initiated in established Midland bermudagrass [Cynodon dacrylon (L) Pers.] pastures to determine the relationship between spectral radiance at specific wavelengths with forage nitrogen (N) removal and biomass, and to determine field variability of soil test parameters. Variable N (applied to 1.5 × 2.4 m subplots within 2.4 × 45.7 m main plots), fixed N and check treatments were evaluated at each location. Spectral radiance readings were taken in the red (671±6 nm), green (570±6 nm), and near infrared (NIR) (780±6 nm) wavelengths. The normalized difference vegetation index (NDVI) was calculated as NIR‐red/NIR+red. Variable N rates were applied based on NDVI. The highest fixed variable N rate was set at 224, 336, and 672 kg N ha‐1 for Burneyville, 1995, 1996, and Ardmore, 1996, respectively. At Bumeyville, soil samples were collected in all variable rate plots (1.5 × 2.4 m) and analyzed for various soil test characteristics. NDVI, red, green, and NIR spectral radiance readings were correlated with bermudagrass forage N removal and yield. Correlation of forage yield and N removal with red, NIR, and NDVI were best with maximum forage production, however, when forage production levels were low correlation decreased dramatically for the red wavelength compared with NIR and NDVI. Forage yield and forage N removal in variable rate treatments increased when compared to the check while being equal to the half‐fixed and fixed rates where higher N rates were applied. Also, variability about the mean in variable rate plots was significantly lower than half‐fixed and fixed rates which supports adjusting N rates based on indirect NDVI measurements. Variable N rate plots reduced fertilizer inputs by 60% and produced the same yield as fixed rate plots, while fixed and half‐fixed rates did not increase N content in the forage over that of the variable rate treatment. Soil sample data collected from small consecutive plots (<4 m2) was extremely variable indicating that intense sampling would be needed if variable fertilizer application were to be based on soil test results.

Evaluating the Profitability of Site-Specific Farming

Site-specific farming (SSF) practices are being adopted at an accelerating rate, but evidence of their profitability has been mixed or missing. This contribution evaluates the profitability of SSF practices by synthesizing quantitative and qualitative research results within the context of the economics of information technology. The profitability results from nine published field research studies on variable rate (VR) fertilizer application are reviewed using partial budgets adjusted to include minimum costs and grid cell areas for each study. Profitability results correlated closely with the gross revenue earning potential of the crop, so VR fertilizer application was unprofitable on wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.), sometimes profitable on corn (Zea mays L.), and profitable on sugarbeet (Beta vulgaris subsp. L. vulgaris). Although the formal published literature has ignored the profitability of yield mapping, production economics and farmer interviews suggest that yield mapping is profitable when it reveals yield patterns that can be managed at acceptable cost and when the information has compensating off-field value. Manageable yield variability includes not only VR input management, but also whole-field improvements such as field drainage, land leveling, windbreaks, and fencing. Off-field value can come from cheaper on-farm experimentation and greater negotiating power with landlords. Farmers and agribusinesses committed to field crop production for the long term should develop SSF capabilities. But because SSF practices are site-specific, their profitability potential too is site-specific. This site specificity extends beyond the farm field to the crop rotation, and the capabilities and opportunities available to the farm or agribusiness manager.

Topography-Based Sampling Compared with Grid Sampling in the Northern Great Plains

A practical variable-rate fertilizer application should be based on information gathered at low cost that represents field fertility levels. The number of soil samples gathered and analyzed may limit the effectiveness of some variable-rate fertilizer applications. Topography-based soil sampling is attractive because it suggests a lower number of samples needed to characterize fertility levels and patterns in a field than some current grid sampling recommendations. A 40-acre North Dakota field consisting of Barnes loam (fine loamy, mixed, Udic Haplo-borolls), Swenoda loam (mixed, Pachic Udic Haploborolls), and Wyard clay loam (fine loamy, mixed, frigid Typic Haplaquolls) soil types was sampled in a 110 ft grid each fall from 1994 to 1996. Nitrate N, P, sulfate S, and chloride level patterns were similar in all 3 yr. Correlation of both area- (sample cores obtained from a wide area within a topography zone) and point-based (a topography zone represented by a composite taken from a point location) topography sampling with the 110 ft grid values was compared with correlation from a 220 ft, 330 ft, and 5 acre grid. Area-based topography sampling for nitrate N was superior in correlation to the 220 ft grid values in 2 of 3 yr. Area-based topography sampling with P was superior to the 220 ft grid in only 1 of 3 yr, but was superior to the 5 acre grid in all years. The 220 ft grid was superior to topography sampling for sulfate S in all years, but area-based topography sampling was better than the 330 ft grid in 2 of 3 yr and superior to the 5 acre grid in all years. Area-based topography sampling for chloride was superior to the 220 ft grid in 1 of 3 yr, but was more highly correlated with the 110 ft grid than the 330 ft and 5 acre grid in all years. Area-based topography sampling was more highly correlated than point-based topography sampling in seven of 12 comparisons. Topography-based sampling for nitrate N better represented fertility patterns than did grid sampling.

Variable rate fertilizer application for corn and soybean

Some have argued that variable rate application (VRT) of fertilizer is appropriate and profitable for the highly productive and fertile fields of the central cornbelt, but the proposition has not been tested. In this field experiment treatments of variably and uniformly applied phosphorus (P) and potassium (K) fertilizer and a control treatment with no P or K fertilizer were applied in a randomized complete block design at six locations in central Illinois in 1994. Individual experimental units were from 1,000 to 1,500 m in length and about 20 m in width. Fertilizer application rate was based upon a 100 m grid soil sampling. The variable rate treatment rate for each plot was based upon interpolated values of the soil fertility of each portion of the experimental unit. Uniform rate treatments were based upon the median soil fertility value for the entire field. Yield estimates were obtained with grain yield monitors. Neither corn nor soybean yield was significantly affected by fertilizer application treatment. We propose this lack of an effect is due to insurance built into the current fertility recommendations. A response to variable treatments will not be seen if there is no response to fertilization. Variable rate application will probably be most useful in fields with areas of very low soil tests.

A futuristic view of soil and plant analysis and nutrient recommendations

The wide scale adoption of site specific management (SSM) and variable rate fertilizer application (VRA) in the next 5 to 10 years portends changes in soil testing and plant analysis. To develop site specific nutrient recommendations, more detailed information about the soil's fertility status will be required which could mean greatly increased samples numbers for soil or plant analysis. Discussions of initial research on the large numbers of samples required to produce quality maps for SSM and VRA among scientists, agricultural industry and producers, however, indicates that this may only be a short phase as the expectation of using remote sensing (RS) to replace or compliment limited soil and plant analysis is already occurring. Soil and plant analysis numbers may increase during the next 5 to 10 years as there is a slow transition to SSM and VRA. During this time, however, there will be research to related RS imagery to soil and crop management problems. After 10 years of this transition, there may be more adoption of RS for primary diagnosis and management with limited soil and plant analysis done from ‘reference’ areas to assure the accuracy of remotely sensed image interpretation.

Self‐surveying — a strategy for efficient mapping of the spatial variability of time constant soil parameters

Precision Fanning addresses the small‐scale variability of soil fertility parameters in the field and consequently transforms them into variable fertilizer applications. Geocoded information about spatially variable soil characteristics is required in a close enough distance of 30 to 50 meters to meet the spatial correlation which is a prerequisite for the interpolation of data in order to obtain Digital Agro Resource Maps (DARMs). The self surveying approach efficiently provides geocoded information of time constant soil parameters, such as geomorphology, and organic matter and clay contents in optional density. It combines Gobal Positioning System (GPS) navigation and positioning with human sensory capabilities.

SPATIAL VARIABILITY AND SAMPLING STRATEGIES FOR NO3-N, P, AND K DETERMINATIONS FOR SITE-SPECIFIC FARMING

NO3-N, P, and K nutrients data were analyzed to examine the extent of spatial variations in their concentration levels and to develop sampling strategies to obtain their representative mean concentrations and correlated characteristics in spatial domain. The study was conducted at two farms located within the Coastal Plains of Virginia. The nutrients’ concentration values varied with coefficients of variation ranging from 13% to 40% at the two sites. Spatial variations along and across the rows at study site were the same at Brandon farm, but were different at the Renwood farm. The sample size required to determine the representative mean values of NO3-N and K nutrients could be decreased by about 40% to 60% if the associated error in the mean values was increased from 5% to 10% level. The concentration values of three nutrients exhibited spatially correlated dependence on their adjacent measurements. The correlation structures of nutrients could best be described by the exponential forms of the semi-variogram model for each site, except for K at Brandon site for which a spherical model was found to be more appropriate. Based on the spatial variability and correlation length of nutrients, sampling grids of 32 × 32 m size for NO3-N, P, and K determinations at the Brandon site, and of 18 × 32 m for NO3-N, 29 × 59 m for P, and 23 × 44 m for K determinations at the Renwood site are recommended. However, due to cost and practical considerations, sampling grid of 18 × 32 m size for the three nutrients is suggested for the Renwood site. A reduction of 12% in P fertilizer at Brandon and of 25% in K fertilizer at Renwood site could be achieved if fertilizer is applied at a variable rate based on the spatial variability of the nutrient levels of the soil. However, the requirement for K fertilizer for Brandon and P fertilizer for Renwood site would be approximately 6% higher compared to those for the conventional strategy. The information presented here could be used as a guideline for refining the existing sampling practices for implementing the variable-rate fertilizer application technology. However, its economic feasibility is yet to be studied.

Field Soil Sampling Density for Variable Rate Fertilization

Variable rate fertilizer application being commercially performed today is most often based on a soil test map. The sampling density used to develop a map is often selected without background information regarding field soil test variability. The objective of this study was to determine how many samples should be taken from a field in order to locate and describe major areas of fertility affecting variable rate fertilizer applications. Two 40 acre fields were sampled in an 82.5 ft grid each fall from 1989 to 1992. Soil pH, Bray P1, and available K levels were determined on each sample and maps were made using inverse distance squared estimates. Data were taken from the samplings to represent a 165 ft and 330 ft grid pattern. Maps were developed from these less dense grids and compared with the 82.5 ft grid values. In 1992, a separate 220 ft grid sampling was taken. The 220 ft grid estimates were more highly correlated with the 82.5 ft grid values than were the 330 ft grid estimates, however, membership of 220 ft and 330 ft grid estimates within soil test categories were similar. Fertilizer P and K applications were made in one field following the 1992 sampling. Spring 1993 sampling showed the success of the 220 ft grid in directing a variable rate application of P and K. Comparisons to theoretical P and K applications directed by a 330 ft grid map showed the superiority of the 220 ft grid compared with the 330 ft grid.

Mapping Soil Test Phosphorus and Potassium for Variable-Rate Fertilizer Application

Variable-rate fertilizer (VRF) application requires knowledge of the spatial variability of soil test P and K within a field. The objectives of the study were to: (i) evaluate cell (area) vs. point soil rumpling, both on a grid basis, and (ii) compare methods for mapping location specific soil test data. Soils in two central Wisconsin fields were rumpled with two methods; study site soils included two alfisols and one entisol. The grid-cell method involved dividing fields into 318-ft square cells and compositing soil cores to give one sample per cell. The grid-point method involved soil sampling at grid intersection points spaced on a 106-ft square grid

Assessment of Spatially Variable Nitrogen Fertilizer Management in Winter Wheat

Spatially variable N application may improve N use efficiency, grain yield, and net returns of winter wheat (Triticum aestivum L.) in fields exhibiting wide ranges of soil characteristics. The objectives of this study were to (i) determine the variability in optimal economic grain yield and in the amount of N required to produce a unit of grain at optimum yield, the unit N requirement (UNR), among landscape positions; (ii) evaluate landscape position as a criterion for dividing fields into units of equal productivity; and (iii) assess the economic benefits of spatially variable N fertilizer application [...]