Understanding soil phosphorus variability with depth for the improvement of current soil sampling methods

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Noise in soil test results can be reduced by measuring phosphorus below the top 3cm of soil from ground level. This is significant for improving current soil nutrient testing methods by allowing better geospatial predictions for whole paddock soil nutrient variability mapping for use in precision fertilizer application. In this study 200 cores were collected from predetermined grids at two trial sites at „Patitapu‟ hill country farm in the Wairarapa. The sites were selected according to accessibility and slope- Trial 1 was a 200m x 100m grid located in a gently undulating paddock. Trial 2 was a 220m x 80m grid located on a moderate to steeply sloped paddock. Each grid had cores taken at intervals of 5m, 10m and 20m. Core sites were mapped out on a Landsat 8 image (NASA) of the Trial sites using ArcGIS 10.2 (ESRI, Redlands Ca.) prior to going into the field; these were then marked out using a LEICA (real time kinematic GPS), pigtails and spray-paint on the ground. Cores were taken using a 30mm diameter soil core sampler. Trial 1 cores were cut into four sections according to depth: A – 0-30mm, B – 30mm-75mm, C- 75mm-150mm, and D- >150mm. Trial 2 cores were cut into three sections: A – 0-30mm, B – 30mm-75mm, C- 75mm-150mm. Olsen P lab results were collected for 120 of the 400 soil cores. These results were analyzed to compare the spatial variability of each depth. The results indicate that there is a significant decrease in variability from section A to section B for both trials. Section B and C for trial 1 have similar variability, whereas there is another significant drop in variability from section B to C in trial 2. Measuring samples below the top 3cm appears to effectively reduce noise, however measuring below 7.5cm for a steeply sloped paddock such as trial 2 may reduce variability too much as to no longer be representative of plant available P, and therefore misrepresenting the overall variability of soil P across a paddock or farm.