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FOR SITE SPECIFIC N-FERTILIZER RECOMMENDATION IN WHEAT M.Rüştü Karaman, Sabit Erşahin Gaziosmanpaşa University, Agricultural Faculty, Department of SoilScience, Tokat/Turkey ABSTRACT This study aimed to determine nitrogen fertilizer demands based on spatial variability of organic matter values on a field under wheat production. The study was conducted to determine spatial variability in organic matter of topsoils (0-30 cm) and subsoils (30-60 cm) in a 8.5 ha alluvial field near Tokat Airport, 25 km north of downtown Tokat, Turkey. Total amount of fertilizer recommended based on variable rate application program was compared with that recommended based on classical fertilizer recommendation program suggested by various fertilizer agencies in Turkey. For variable rate N fertilizer recommendation program, soil samples were taken based on a regular grid spacing of 25 x 25 m. For classical N fertilizer recommendation program, samples at each part of the field were averaged to represent that particular portion. A great variability occured in organic matter values for topsoils and subsoils, indicating that site-specific management practices of water and nitrogen are needed to decrease amount of fertilizer N used and amount of N leached. Application of site specific N fertilizer recommendation will be beneficial to use of suitable fertilizer N rate for optimal crop production. This approach will also help minimize amount of N fertilizer used and decrease adverse effect of N fertilizers on environment in large agricultural areas. The main data used for this paper were obtained from the project sponsored by The Scientific and Technical Research Council of Turkey, Project No: TARP-1871 INTRODUCTION All sources of plant-available N should be considered to maximize crop yield and quality, and to decrease the impact of these fertilizers on environment. The considerations on improved N fertilizer recommendation will increase fertilizer N use efficiency and lower overall N fertilizer consumption. The utilization of N fertilizers by crops generally is below 50%. Mechanisms, such as leaching, volatilization, and denitrification significantly affect N losses (Jackson et al., 1988; Webster et.al., 1992). Therefore, heavy nitrogen fertilization should be avoided and determination of the most effective N-fertilizer recommendation scheme depending on soil fertility analysis is essential to increase N-use efficiency for a better management practice. Different soil sampling methods may be used in soil fertility analysis for N fertilizer recommendations. In the simple random sampling, which is the classic sampling method, all points are randomly selected on the field, whereas in a sytematic sampling program, sampling points are located in a grid format (Berry, 1962). Compared with classic sampling method, systematic sampling is more expensive and greater expenditure of time and effort than random sampling. Thus, systematic sampling method has been principally restricted, as a result of these limitations (Wilding, 1985). Compared with the results from random sampling, results from systematic sampling are more convinent to determine spatial variabilities of soil properties. Determination of spatial variability of soil properties can make valuable contributions to the beneficial use of field soil (Arnold and Wilding, 1991). In site specific management of fertilizers, local needs of crops are considered. Knowledge of the spatial variability in soil organic matter content is essential in determining the local N needs of crops. The demands for more precise information about N fertilizer recommendations will increase with the developing of technology and other possibilities. In the soil fertility analysis, a knowledge of organic matter content of soils is essential in predicting the N demand of crops. The dynamic and complex chemistry of soil organic matter makes it a main source of plant nutrients. The soil organic matter fraction, with about 95 % of soil nitrogen, can supply most of the nitrogen needed for plant growth (Smith et al., 1991). A great variability in organic matter values occur in agricultural areas. Thus, application of variable method to determine N demand of crops will be beneficial to recommend suitable fertilizer N rate for large agricultural areas. In this study, a new approach was employed for N fertilizer recommendations in wheat production. In this approach, N fertilizer recommendation is based on the spatial variability of the soil organic matter content accross the field. This approach will help minimize amount of N fertilizer used and decrease adverse effect of N fertilizers on environment. MATERIALS AND METHOD This study was conducted to determine spatial variability in organic matter of topsoils (0-30 cm) and subsoils (30-60 cm) on a 8.5 ha alluvial field near Tokat Airport, in 25 km north of downtown Tokat, Turkey. Total amount of N fertilizer recommended based on variable rate application program was compared with that recommended based on classical N fertilizer recommendation program suggested by various fertilizer agencies in Turkey. For variable rate N fertilizer recommendation program, soil samples were taken based on a regular grid spacing of 25 x 25 m. For classical N fertilizer recommendation program, soil samples at each part were averaged to represent that particular portion. Organic matter contents as well as other soil properties of the topsoil and subsoil samples were determined. Organic matter contents were determined by the Walkley-Black method from Jackson (1956). In addition, inorganic N (Bremner, 1965), available P (Olsen et al., 1954), C.E.C. (Richards, 1954), CaCO3 (Allison and Moodie, 1965) and pH (McLean, 1986) were also determined. The textural analysis of experimental soil was made with a Bouyoucos hydrometer (Gee and Bouder, 1986). Values of maximum, minimum, mean, standard deviation, coefficient of variance, kurtosis and skewness were calculated with the computer program StatMost (Kleinbaum et al., 1988; StatMost, 1995). RESULTS AND DISCUSSION Selected soil properties : The results of the maximum, minimum, coefficient of variance (C.V.), kurtosis and skewness values revealed that a great spatial variability occured in available P content, C.E.C. and organic matter content for topsoil and subsoil (Table 1 and 2). All other selected soil properties generally exhibited a medium varibility in both topsoils and subsoils. The variabilities of 31.79, 28.62 and 21.70% C.V. were observed in available phosphorus, C.E.C. and organic matter content for topsoil, respectively. A maximum variability of 62.93 % C.V. was observed for organic matter content in subsoil. 
 Spatial pattern of organic matter values in the field : Three-dimensional maps for organic matter contents for topsoils and subsiols indicated that considerable variability in organic matter content occured in the experimental area (Fig. 1). Results also indicated that organic matter contents of topsoils were greater than those of subsoils due to continuous addition of organic matter to topsoils. The representation of organic matter distribution for spatial location is also shown in contur plot (Figure 2). according to boundary values from General Directorate of Rural Services (Ülgen and Yurtsever, 1984); For topsoils, 1.4 % low, 16.4 % medium, 35.0 % sufficient, and 47.2 % was high; and for subsoils, 36.4 % very low, 40.7 % low, 17.1 % medium, 5.0 % sufficient and 0.8 % was high in organic matter (Table3 and Figure 2). Site-specific N fertilizer recommendation : A significant variability occured in N fertilizer rate recommendations depending on the spatial variability of organic matter content of the topsoils (Table 3). Based on the variable-rate N fertilizer recommendation program, of the 85000 m2 experimental area, 14 kg N ha-1 was recommended for 1190 m2, 153 kg N ha-1 was recommended for 13940 m2, 268 kg N ha-1 was recommended for 29750 m2, and 361 kg N ha-1 was recommended for 40120 m2. However, amount of N fertilizer recommended for the all experimental field based on the classical fertilization program, which consider field averaged organic matter content, is 108 kg N ha-1. Application of this classical recommendation program will lead overfertilization in 69870 m2 and underfertilization in 15130 m2 of 85000 m2 total experimental field.  This variability may reach to even higher levels for other agricultural areas. Site-specific N fertilizer recommendation considering spatial variability in organic matter values will supply suitable and more realistic N fertilizer rates for optimal crop production in large agricultural areas. Thus, the site-specific N fertilization will has more potential for both environmental and economic benefits for those agricultural areas. In conclusion; a great variability occured in O.M. values, indicating that site-specific management practices of nitrogen fertilizers are needed to decrease amount of N-used by the crops and amount of N leached. Thus, a knowledge of spatial variability of organic matter content of soils will be beneficial to recommend suitable fertilizer N-rate for large agricultural areas. If inorganic N content of soils will be essential in predicting the N demand of crops, which is known as Nmin method, application of variable method to determine N demand of crops will also be beneficial to recommend suitable fertilizer N-rate for these agricultural areas. By this approach, the site effect of N-fertilization on environment may be decreased. 
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