De. Clay et al., Precision farming protocols. Part 2. Comparison of sampling approaches forprecision phosphorus management, COMM SOIL S, 31(19-20), 2000, pp. 2969-2985
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
.