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Hüseyin Dikici, Recep Gündoğan Kahramanmaraş Sütçü İmam University, Department of Soil Science, Turkey The dramatic increase in worldwide fertilizer use has raised concerns regarding possible negative impact of fertilizers on the environment and renewed interest to seek new methods to manage fertilizers in a more efficient manner. Advances in technology, global positioning systems (GPS), and geographical information systems (GIS) have driven a recent movement toward precision agriculture or site specific management. Site specific management is defined as applying the right management at the right time, at the right place, and in the right way or simply managing by soil condition. Defining soil spatial variability is the first step for site specific fertilizer management. Then, the question how to manage this variability should be addressed. It is generally accepted that a more detailed soil sampling will result in more precise fertilizer recommendations. Grid soil sampling is intensively used for this purpose. In this study, the objective was to measure field variability of Olsen's phosphorus based on a 20.5 m grid in a cotton field in the Narli Plain of Kahramanmaras. Grid soil sampling is intensively used for this purpose. Based on the summary statistics of soil P analysis, the mean value for Olsen's P was 14.5 mg/kg. If we were to make our P recommendation for the whole field based on the mean value, no P fertilizer would be recommend. However, coefficient of variation for P was quite high (cv 31%) suggesting that there was a high within field variability. The Olsen's P values ranged from 5.5 to 24 mg/kg in the field, and the range was 18.6 mg/kg. These findings suggest that field was heterogeneous for P, and uniform P management would lower yield in some part of the field. If soils were uniform, there would be no need for site specific management. Currently, a field as big as 4-5 ha in size is accepted as a homogeneous area for making fertilizer recommendations. A composite soil sample is collected by taking 15-20 separate cores, at random, in a zigzag pattern across a field. This sub-sample is thoroughly mixed in a bag to represent the whole field. Then, according to the test results, a uniform fertilizer recommendation is made for the field. Relative levels of Olsen's P are classified as very low, low, medium, high, and very high. A 1.14 ha area (30.7%) in the field is considered low and medium in P and needed P fertilization. If P fertilizer recommendation was based on the mean P value, this area would suffer from P deficiency and lower yields. A 1.05 ha area (28.3%) was high in P, and there was no need for P fertilization for this class. And the remainder of the field (41%) was very high in P, and fertilization of this area with P would carry some serious environmental risks. Thus, uniform P fertilizer applications would underestimate P needs in some portions of the field. For agricultural research standpoint, we are in a stage of fine-tuning. Increased usage of agricultural chemicals within the last 50 years has caused environmental problems. We must seek new methods to increase effectiveness of agricultural inputs. Site specific management or precision agriculture has great potential to increase effectiveness of agricultural inputs. |