GRAZING MANAGEMENT AFFECTS SOIL PHOSPHORUS
AND POTASSIUM LEVELS
J.R. Gerrish, P.R. Peterson, and J.R. Brown1
Abstract
Soil fertility can be affected by pasture utilization and manure distribution patterns of
grazing cattle. Management to keep manure evenly distributed in a pasture to maintain soil
fertility should be a producer goal. This research examined factors affecting soil fertility
changes in grazed pastures. Grazing systems with 3, 12, or 24 paddocks and either having
water available in each paddock or using a lane to provide water access were compared.
Use of a lane to provide access to water for livestock resulted in declining P1 soil test
values over a two year period. A trend (p=.07) existed for decline in exchangeable
potassium soil tests in pastures using lane access to water. Phosphorus and potassium soil
tests generally increased nearer to water in larger pastures grazed at relatively low stock
density. Fertility gradients toward water source were the least in smaller paddocks grazed
at a higher stock density. Providing water in every paddock and placing water to keep
cattle travel distance to water under 800 ft should result in less soil nutrient transport off
the grazable pasture area.
Introduction: Distribution of soil nutrients in a pasture varies across the landscape and is
affected by many factors. Parent material, pre-settlement plant community, slope and
aspect all contribute to the fertility status of a pasture. Human factors include both past and
current management practices. Tillage, hay removal, and fertility application are examples
of human effects. The grazing animals themselves will also affect soil nutrient availability
and distribution. Differences in grazing distribution patterns, preferred loafing sites, and
watering sites can all create nutrient gradients in pasture (Borrow, 1967; West et al.,
1989). Mathews et al. (1994) reported greater concentration of exchangeable potassium
in the front one-third of both continuously and rotationally grazed pastures. The research
reported by both West et al. and Mathews et al. were both conducted on very small
pastures with limited number of animals. Such research, while well designed and
interesting, frequently yields information that may be of little consequence in commercial
settings.
The challenge to a producer is to maintain soil nutrients at or near optimum levels for
as great a part of the pasture as is economically feasible. Choice of forage-livestock
management system can have a profound impact on the efficiency of nutrient return to
grazing lands by the grazing animal. In this research project we have sought to determine
some of the critical factors affecting soil nutrient status as it relates to grazing
management.
Materials and Methods: A 3 year grazing study was conducted at the University of
Missouri-Forage Systems Research Center located in the deep loess and drift region of
north Missouri. Six grazing systems were established on pastures consisting of well
established, diverse cool season grass-legume mixtures. Two cells each were subdivided
into 3, 12, or 24 paddocks. One paddock system within each level of subdivision had stock
water available in each paddock while the other system used lanes to provide access to
water. Pastures ranged in size from 31 to 42 acres with individual paddocks ranging from
1.3 acres for the 24 paddock cell to 14 acres for the 3 paddock cell. Each system was
grazed by both cow-calf pairs and yearling steers. Stocking rate was 2.0, 1.6, and 1.25
acres/cow-calf equivalent from mid-April through mid-July and 3.3, 2.7, and 2.1
acres/cow-calf equivalent from mid-July through mid-November for the 3-, 12-, and 24-
paddock systems, respectively.
Detailed soil sampling was conducted on a total of 14 paddocks, three each from the
12- and 24-paddock systems and one from each of the 3-paddock systems. Each of the 14
monitor paddocks was sub-sampled on a grid basis with each paddock being divided into
60 to 80 sampling blocks. Soil samples were taken in the fall of 1992, 1993, and 1994
from 12 blocks in each paddock. Block sizes were approximately 30 X 30 ft, 40 X 40 ft,
and 80 X 80 ft for the 24, 12, and 3 paddock systems, respectively. Sampled blocks were
selected to provide a cross section of landscape positions in each paddock and provide a
transect that could be spatially related to the water source.
Changes in soil P and K levels due to level of paddock subdivision or water access
were analyzed using analysis of variance procedure. Spatially dependent changes in P and
K levels were determined through linear regression procedures.
Results and Discussion: Lane effects: Both level of subdivision and water access had a
significant effect on soil P levels (Table 1). The interaction term was not significant.
Potassium levels were not significantly different due to either factor although a strong
trend (p>F=.07) existed due to water access but not level of subdivision. The data in
Table 1 does not include the sample block where water was located. In most cases, soil
nutrients tend to accumulate in the watering area (Gerrish et al, 1993) but does not
contribute to the productivity of the pasture. The greatest apparent P loss occurred in the
24 paddock unit with lane access to water. Cattle travelled the greatest distance to water
in this system and were observed to spend a good deal of time in the lane. Manure
distribution measurements (Peterson and Gerrish, 1995) indicate that approximately 13%
of the dung was deposited in the lane.
The higher apparent loss of P in the 24 paddock system is related to the sampling
procedure used and the more dense manure concentration in the 24 paddock system
(Peterson and Gerrish, 1995). The student collecting soil samples had been instructed to
avoid sampling in dung pats but in the 24 paddock system such a high proportion of the
total area was visibly affected by dung that microsite areas of P concentration would have
been avoided in the sampling process. On an annual basis, mean manure concentration per
500 ft2 in the 24 paddock system was 2 to 3 times greater than in the 3 paddock system.
The mean manure concentration per 500 ft2 and apparent P loss for the 12 paddock system
was intermediate between the 3 and 24 paddock systems. Saunders (1984) described
differences in soil nutrients from sites either affected or unaffected by dung and urine. The
impact of dung pats on pasture growth is more long term than is the impact of urine. This
effect is due to two factors. Grazing animals avoid dung sites longer than urine sites and
the subsequent growth tends to become more mature around a dung pat. The second factor
is the relative nutrient content of dung versus urine. Most P is excreted in dung and the
stability of P in both organic compounds and the soil results in a more long term effect on
soil and plant growth. Urine is the primary excretory path for K and soluble N, thus the
effect of urine is more short term.
The lane effect was not significant in the 3-paddock system. This is probably due to
a confounding factor of natural water being available through much of the 3 paddock lane
system during most of the grazing season. All stock tanks were equipped with water meters
to monitor daily water intake. Very little water was actually drunk from the stock tank in
the lane. Through most of the season, water would have been available within a few
hundred feet of any point in the paddock.
The changes measured over the course of this study would not warrant P and K
maintenance fertilization on an annual basis. Pastures using lanes could effectively be
fertilized on a three year frequency based on our results. Grazing systems supplying water
in every pasture and having optimal placement of that water, would apparently require
very infrequent fertilization. We would recommend soil testing on a 3 to 4 year frequency
to monitor nutrient status and plan fertilization strategies.
Even though the absolute change in soil tests for a particular pasture may not appear
to be significant, the movement of nutrients within the system may be a point of concern.
While average fertility may not decline at a biologically significant rate, some areas of the
pasture may be declining in fertility at a greater rate as other areas are increasing in
fertility level.
Spatial effects: Soil P and K distribution patterns in grazing cells of different
configuration were examined. A 3-paddock cell with individual paddock size of
approximately 10 acres and a 24-paddock cell with individual paddocks of approximately
1.3 acres were compared. Changes in both Bray P1 soil test and exchangeable K levels
from 1992 to 1994 were spatially dependent in the 3-paddock grazing cell but not in the
24-paddock cell (Figures 1 and 2). Distance traveled to water was approximately 500 ft
maximum for the 24-paddock cell while maximum travel distance was about 1250 ft in the
3 paddock cell. Grazing distribution would likely be different in these two situations with
a higher degree of utilization gradient developing toward water in the larger paddocks
(Gerrish et al, 1995). Greater grazing pressure in the front of the pasture resulted in a
significant manure deposition gradient toward water in the 3 paddock cell while the
manure deposition gradient was less extensive in the 24-paddock cell (Peterson and
Gerrish, 1995).
A factor which also explains why greater nutrient gradients are likely to develop in
systems having fewer paddock subdivisions is a time function. In the 3-paddock cell, the
livestock have the opportunity to deposit manure around a particular watering site for 33%
of the grazing season. In the 24-paddock cell, only about 4% of the grazing season is spent
around a particular watering site, assuming water is made available in each paddock.
Observation of the herd behavior in these two cells also provides some answers to the
difference in fertility patterns. The cattle in the 3-paddock cell function very much as a
herd. Rarely would there be animals in all parts of the 10 acre paddock simultaneously,
rather they would remain congregated. In the 24-paddock system, the animals could
frequently be seen scattered over the entire area.
Herd behavior is highly dependent upon the ability of individuals within the herd to
maintain visual contact with their herdmates. In the smaller paddock, visual contact could
be maintained at all times within almost the entire paddock area. Water placement was at
midpoint on a slope and facilitated a good field of vision even in what is generally a rolling
landscape. In the 3-paddock cell, water was at a lower point than most of the pasture and
two swales cut the 10 acre pasture area making it impossible for visual contact to be
maintained among the herd on any more than about one-quarter of the paddock. The herd
in the 3-paddock system consisted of 9 cow-calf pairs and 13 yearling steers.
In larger herds and pastures, the animals can spread over several folds in the landscape
and still maintain visual contact among the herd. Another mechanism that operates in
larger herds is the breakup of the total herd into smaller social units or grazing herds The
cow group in this research was not large enough to divide into grazing sub-herds which
may have resulted in more uniform grazing distribution across the landscape.
Several practical soil fertility management recommendations which can be made based
on these results. One of the most obvious is not to apply fertilizer within 100 to 200 feet
of watering sites in a pasture. Shade areas where the animals are observed to spend a good
deal of time should also be avoided. While the nutrient gradient does not seem to extend
as far into the pasture area around shade compared to the gradient toward water, the
nutrient level near to the shade is frequently even higher than the concentration around
water. When designing rotational grazing systems, placement of water in each individual
paddocks will result in much more uniform manure distribution and maintenance of
fertility. If the net loss of phosphorus and potassium to lanes is converted to economic
value, it is clear than just savings in fertilizer cost will pay for the water reticulation
system in 3 to 7 years.
Literature cited:
Borrow, N.J., 1967. Some aspects on the effects of grazing on the nutrition of
pastures. J.
Aust.
Inst.
Agric.
Sci.
33:254-262.
Gerrish, J.R., J.R. Brown, and P.R. Peterson. 1993. Impact of grazing cattle
on
distribution
of soil
minerals.
p.66-70. In
American
Forage
and
Grassland
Council Proc.
Des
Moines, IA,
29-31
March,
1993.
Gerrish, J.R., P.R. Peterson, and R.E. Morrow. 1995. Distance cattle travel
to
water
affects
pasture
utilization
rate.
American
Forage
and
Grassland
Council Proc.
Lexington
KY,
12-16
March,
1995.
Mathews, B.W., L.E. Sollenberger, P Nkedi-Kizza, L.A. Gaston, and H.D. Hornsby.
1994. Soil sampling procedures for monitoring potassium distribution in grazed pastures.
Agron. J. 86:121-126.
Peterson, P.R. and J.R. Gerrish. 1995. Grazing management affects manure distribution
by beef cattle. In American Forage and Grassland Council Proc. Lexington, KY, 12-16
March, 1995.
Saunders, W.M.H., 1984. Mineral composition of soil and pasture from areas of grazed paddocks, affected and unaffected by dung and urine. N.Z. J. Agr. Res. 27:405-412.
West, C.P., A.P. Mallarino, W.F. Wedin, and D.B. Marx. 1989. Spatial variability of
soil chemical properties in grazed pastures. Soil Sci. Soc. Am. J. 53:784-789.
Table 1. Change in soil phosphorus and potassium levels in grazing systems differing in
level of subdivision and water access.
P1 soil test K soil test
Water -------------------- ---------------------
System Access 1992 1994 Change 1992 1994 Change
--------------------------------------------------------------------------
---- (lb/A) ---- ---- (lb/A) ----
3-paddock Paddock 20 21 1 226 223 -3
Lane 27 26 -1 229 199 -30
12-paddock Paddock 19 20 1 224 212 -12
Lane 24 20 -4 252 228 -24
24-paddock Paddock 23 19 -4 246 224 -22
Lane 27 19 -8 227 200 -27
LSD = 2.5 n.s.
--------------------------------------------------------------------------
Figure 1. Change in Bray P1 soil test in two grazing systems each having water
available in each paddock.
Figure 2. Change in K soil test in two grazing systems each having water available
in each paddock.
1Research Assistant Professor, University of Missouri-Forage Systems Research Center,
RR1 Box 80, Linneus MO 64653; Assistant Prefessor, Plant Science Department, McGill University,
Montreal, Quebec, Canada; Professor Soil Science, School of Natural Resources, University of Missouri.
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