Soil Pitting to Improve Arid Land Revegetation
By David A. Bainbridge
Associate Professor
United States International College of Business
Alliant International University
San Diego, CA 92131
Soil pitting is a practical and proven surface modification treatment to
improve water infiltration and retention and reduces evaporation and
increases surface storage and the time available for infiltration to
occur. Pitting can double absorption rates.
Pits also capture blowing litter, seeds, and fine dust and protect
seedlings from wind and sand blast. The increased availability of water in
and around the pits provides suitable niches for plant establishment and
stimulates plant growth.
The size of the pit will depend on the soil type, the rainfall variation,
species type and seeding method, and equipment availability. Large shallow
sloped pits provide a range of conditions for seeds to germinate. Where
seeds germinate in a given year will depend on rainfall amounts, timing,
and temperature.
Seed
germination location varies from year to year.
Implements may use disks, tines, or scrapers to make the pits. The
typical Western Australia pitter uses four disks or tines producing four
rows of pits with each pass. These break the hard surface crust which
often forms on flat, overgrazed and degraded rangeland. These pitters
makes pits that are 3 feet long (1 m long), 6-8" wide (15 to 20 cm
wide) and 4-5 inches (10 cm) deep. Scrapers and blades can also be used to
create pits in any size.
Several types of disk plows have been used as pitters, but most are
modified one-way disk or brushland plows. The standard disks are cut and
half of the disks removed. The brushland spring-mounted disks are
preferable on more difficult sites.
Cut Out Disk Pitter
Disk pitters can also be made by offsetting the axle location in the disk,
creating an eccentric rolling motion, leaving alternate pits.
Pitting without seeding has led to mixed success in the Southwest. In
degraded areas with limited seedbanks seeding should be done along with
the pitting. A broadcast seeder may be mounted on the pitting implement or
the site can be broadcast seeded by hand. These do not allow precise seed
placement and many seeds may be misplaced and fail to germinate.
Few pitting studies have been done in the California desert's with
predominant winter rain and very dry summers. However, from limited field
experience and trials we feel this is the most effective low cost method
of starting degraded dry lands on the way to recovery.
Studies in other areas of the West have found similar results. Barnes
(1950) found that pitting was superior to all other treatments for
improving short grass range. Soil pitting allowed 32% more sheep to be
carried per acre, with a 50% increase in perennial grass left at the end
of the year. Double the number of western wheatgrass plants existed 10
years after pitting. Pitting with an eccentric disk pitter increased
foliage production 32-68% on deep soils in Wyoming (Lang, 1958).
The results of the experiments conducted by Stern et al. (1992) at
Roodeplaat, South Africa on a silty clay loam showed that pitting was
beneficial in reducing surface runoff, maintained higher plant
evapotranspiration rates and yields than the control treatment and
cultivation costs were low.
The effectiveness of the soil pits declines as they fill in with blown or
washed soils and debris. Pits in sandy soil in a high wind environment
were largely filled in after one season at Anza Borrego Desert State Park
(Bainbridge, unpub.). The increased fertility of the pits due to the
capture of fines, litter, seeds, and microsymbionts may still provide a
significant advantage for plant establishment.
Pitting should be more commonly selected as the preferred method for
treating large areas of degraded lands and construction impacted soils.
Further Reading:
Bainbridge, D.A. 1997. Soil pitting; a technique to improve arid land
revegetation.
SDSU Soil Ecology and Restoration Group, Bulletin
#1, 12 p.
Barnes, O.K. 1950. Mechanical treatments on Wyoming rangeland. Journal of
Range Management 3(3):198-203.
Barnes, O.K. 1952. Pitting and other treatments in Wyoming Range Land.
Wyoming Agricultural Experiment Station, Bulletin
#318.
Barnes, O.K., D. Anderson and A. Heerwagon. 1958. Pitting for range
improvement in the Great Plains and the Southwest
Desert Regions. USDA
Prod. Research Report #23.
Gintzburger, G. 1987. The effect of soil pitting on establishment and
growth of
annual Medicago spp. on degraded rangeland in
Western Australia. Australian
Rangeland Journal 9(1):49-52.
Oliveira, C.A.S., R.J. Hanks, and U. Shani. 1987. Infiltration and runoff
as
affected by pitting, mulching and sprinkler
irrigation. Irrigation Science
8(1):49-64.
Rauzi, F. 1968. Pitting and interseeding native shortgrass rangeland.
Wyoming
Agricultural Experiment Station Research Journal
17. 14 pages.
Slayback, R.D. and D.R. Cable. 1970. Larger pits aid reseeding of semiarid
rangeland. Journal of Range Management
23(4):333-335.
Stern, R., A.J. Van der Merwe, M.C. Laker and I. Shainberg. 1992. Effects
of soil
surface treatment on runoff and wheat yield under
irrigation. Agronomy
Journal 84(1):114-119.
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