Precision farming protocols. Part 2. Comparison of sampling approaches forprecision phosphorus management

Research is needed to compare the different techniques for developing site- specific phosphorus (P) recommendations on a field-wide basis. The objectiv e of this study was to determine the impact different techniques for develo ping site-specific P recommendation maps on yield and profitability. Enterp rise analysis combined with a crop simulation model and detailed field char acterization was used to estimate the value of spatial P information in a s ystem where N was not limiting. The systems evaluated were continuous corn (zea mays) and corn and soybean (Glycine max) rotations where sampling and fertilizer applications were applied annually and semi-annually, respective ly. The sampling techniques tested were: (i) an unfertilized P control; (ii ) whole field; (iii) whole field plus historic information (feedlot); (iv) landscape positions; (v) soil type; (vi) soil type plus historic informatio n (feedlot); and (vii) 90-m grid sampling. The finding of this study were b ased on soil samples collected from a 30 by 30-m grid. The value of the spa tial information was dependent on the crops response to P, the accuracy of the different sampling techniques, crop rotation, and the length of time be tween sampling dates. All of the sampling techniques produced different app lication maps. The recommendation map based on a single composite sample un der fertilized 56.5% of the field. Increasing the sampling density reduced the percentage of under-fertilized land. If corn had a low P response, then simulation/enterprise analysis indicated that applying P did not increased profits. For all scenarios tested: (i) the soil type + historic sampling a pproach had higher potential profits than the 90-m grid sampling approach; and (ii) there was no economic benefit associated with the 90-m grid sampli ng. However, if research shows that amortization of sampling and analysis c osts over 3 or 4 years is appropriate, then it may be possible to derive ec onomic benefit from a 90-m grid sampling. For a corn/soybean rotation, wher e fertilizer was applied when corn was planted and N and P was not applied to soybeans, enterprise/simulation analysis (2.8 Mg ha(-1) soybean yield go al and a moderate P model) showed that soil + historic sampling approach in creased profitability $3.74 ha(-1) when compared to the uniform P treatment .