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manure was likely applied to agricultural land, the loading potential was combined
with agricultural land in the Geographic Information System to derive the animal
loading potential in metric tons per hectare of agricultural land within each water-
shed. The results indicate that total amounts of nitrogen (N) and phosphate (P
2
O
5
)
produced from animal manure ranges from 23,000 to 27,000, and from 10,000 to
11,400 metric tons, respectively, for the periods of 1987, 1992, 1997, and 2002.
These nutrients, if applied uniformly to all cropland (around 1.31 million ha) in the
region, would average around 17-21 kg/ha for nitrogen, and 8-9 kg/ha for phos-
phate (Table
6.4
). These amounts seem quite small on a per unit area basis.
However, animal production facilities are concentrated in certain locations in the
region and the manure produced from those facilities are often either applied to the
adjacent cropland or disposed of locally to reduce transportation and labor cost.
As shown in Fig.
6.3
, the amount of phosphate produced from manure ranges
from 3 to 114 kg/ha in the northeast and northwest portion of the Saginaw Bay
Basin, and in certain locations, it amounts up to 114 kg/ha. Consequently, these
locations can be targeted for implementation of manure management programs for
minimizing the pollution potential to the surface and subsurface waters. This also
indicates that agricultural statistics data at a finer scale (below county level) would
reveal more useful information than would the county level data in animal manure
management. Large livestock operations, difficulty to identify at the county level,
could be more easily identified at the 5-digit zip code level for manure management
(He and Shi
1998
; He and Croley
2007b
; He et al.
2008
; He and DeMarchi
2010
).
Large quantities of fertilizers are applied to both farmland and non-farmland
(residential lawns, parks, and golf courses, etc.) each year. If improperly applied,
these chemicals also represent a potential threat to both surface and groundwater.
Estimating loading potential of fertilizers , however, is challenging because no
fertilizer information is collected at county level on an annual basis (U.S. Geologi-
cal Survey
2000
; U. S. EPA
2004
; He and DeMarchi
2010
). U.S. Geological Survey
estimated the county level manure and fertilizer application rates for the period
of 1982-2001 based on the state level fertilizer sales data and agricultural statistics
data (Alexander and Smith
1990
; Ruddy et al.
2006
). The results show that approxi-
mately 92,000-110,000 metric tons of nitrogen (N) fertilizer and 32,200-11,400
metric tons of phosphate were applied to cropland in the study area each year,
averaging about 70-83 kg/ha/year for N fertilizer, and 25-62 kg/ha/year for P
2
O
5
fertilizer (Table
6.4
). These estimates only show amounts of fertilizers applied to
the study area each year and do not consider uptake of the fertilizer by crops. Lack
of soil testing, plant uptake of nutrients, and mineralization and volatilization
information makes it very difficult and speculative to estimate nutrient budget
and excessive nutrients remaining in the soil each year. Thus no attempt was
made to estimate excessive nutrients in the soil each year. Instead, only fertilizer
loading potential was estimated in the study area (He and DeMarchi
2010
).
Table
6.4
shows that total nitrogen and phosphate applications had declined for
the period of 1987-2002 in the study area. This was attributable to the implemen-
tation of best management practices for reduction of nutrient loadings to the rivers
and bay (U. S. EPA
2002
; He and DeMarchi
2010
). However, nonfarm applications
