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a Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506
b Dep. of Biological & Agricultural Engineering, Kansas State Univ., Manhattan, KS 66506
c Dep. of Statistics, Kansas State Univ., Manhattan, KS 66506
* Corresponding author (schmidt{at}ksu.edu)
Because of the costs of additional soil sampling and analyses, high-density soil sampling is a major obstacle for producers when deciding whether to variably apply P fertilizer. Our objective was to develop an approach to evaluate the potential for site-specific P applications prior to high-density soil sampling. Soil samples (40–167) were collected from eight fields ranging in size from 40 to 170 ha. The soil P test frequency distribution (DT) was determined for each field using all the samples and assuming lognormally distributed soil P values. Sets (500) of random samples (n = 5, 10, 20, 30, and 50) were selected from all the samples for each of eight fields. With each sample set, the soil P test frequency distribution (Dx) was estimated. A deviation index (Udev) represented the amount that Dx deviated from DT. The generated populations of Udev were then used to evaluate the probability of exceeding any random Udev as a function of n, providing a measure of success in estimating DT. A P application based on the mean soil P test was compared with a P application based on DT, providing a measure of potential for a site-specific P application. The P application based on the mean soil P test deviated from the P application based on DT by 11.7, 9.4, and 5.1 kg P2O5 ha-1 for Fields A, B, and C, respectively, representing the average deviation for the entire field. Larger values represent greater potential return to a site-specific P application. The Udev that was obtained with 99% probability (n = 50) was used as the criterion from which to compare the success of using smaller sample sizes for estimating DT. To achieve the same success in estimating DT with 70, 80, and 90% probability, 18 to 26, 23 to 31, and 33 to 39 soil samples were required for Fields A, B, and C, respectively. This approach provides decision makers a practical tool for estimating DT prior to high-density soil sampling, with an estimate of the corresponding risk associated with different sample sizes.
Abbreviations: Dx, estimate of the soil P test frequency distribution based on simulations • DT, estimate of the soil P test population frequency distribution based on all soil samples • OM, organic matter • Udev, deviation index • Pdev, average summed deviation from the ideal P recommendation
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