Impacts
and Management of Soil Acidity under Direct Seed Systems
- Status and Effects on Crop Production
R. L
Mahler
Professor of Soil Science
University of Idaho
Soil acidity has
long been recognized as a primary factor affecting crop yields throughout
the world. Yield reductions because of excess soil acidity occur primarily
in the midwestern, southern, and eastern regions of the USA. Agricultural
soils west of the Cascades in Washington and Oregon are also acid.
An acid soil has
a pH below 7.0. For agricultural purposes, a soil with a pH below 6.6
is considered acid. The term acid is usually applied to the plow layer
(upper 8 to 10 inches of the soil profile) and does not imply anything
about the pH of the subsoil (soil below the plow layer).
Soil acidity occurs
when levels of exchangeable aluminum (Al+++) and hydrogen (H+) on clay
particles (cation exchange sites) in the soil are high compared to levels
of calcium (Ca++), magnesium (Mg++), and potassium (K+). Excessive levels
of H+ and Al+++ on clay particles in the soil have been shown to reduce
wheat, alfalfa, and pea yields by more than 80 percent in parts of the
USA. Application of lime to soils corrects acidity problems.
Soil pH Change
in the Region
Soils in eastern
Washington and northern Idaho have become increasingly acidified over
the last 40 years. This acidification is due to the long-term use of high
rates of ammonium-based nitrogen fertilizers. In 1960 less than 15 percent
of agricultural soils (surface 12 inches) in all eastern Washington counties
had soil pH values below 6.0 (Fig. 1). Between 15 and 45 percent of the
agricultural soils in Kootenai, Nez Perce, Lewis, Idaho, Adams, and Valley
counties in northern Idaho were less than 6.0 in 1960 while less than
15 percent of the soils were less than 6.0 in the other northern Idaho
counties. By 1972, a higher percentage of soils were less than 6.0 in
almost all eastern Washington and northern Idaho counties compared to
1960 (Fig. 1). By 1980, a majority of soils had pH values less than 6.0
in Whitman County, Washington and in Latah, Nez Perce, Lewis, Idaho, and
Valley counties in Idaho.
Fig. 1. Soil pH
shifts in northern Idaho and eastern Washington between 1960 and 1980.
A survey of agricultural
fields in northern Idaho in 1995 found that 9 percent of fields had pH
values above 6.0 (Table 1). Conversely, 21 percent of the fields had pH
values less than 5.2. This 1995 survey suggested that average pH values
in agricultural fields in the region have continued to decline since the
1980 survey.
The soil pH decline
since 1960 can be traced to historical nitrogen fertilizer use. As an
illustration, a field on the Idaho-Washington border in a cereal-legume
rotation can be used to illustrate historical N fertilizer use (Fig. 2).
Prior to 1940, no commercial N fertilizer was used on the cereal crop
on this field. When World War II ended, munition plants were converted
to produce cheap N fertilizers resulting in N applications of about 30
lbs/acre in 1948. In the 1950s, new higher yielding varieties of wheats
were introduced, which again increased N application rates up to near
60 lbs/acre by 1960. Higher yielding varieties and increased management
intensity further increased N application rates up to 105 lbs/acre by
1975. Nitrogen application rates have been level for the last 25 years.
Fig. 2. Historical
N fertilizer use on a field located on the Idaho-Washington border near
Moscow, Idaho.
If you sum up the
total amount of N fertilizer used on this sample field assuming that N
is applied only in alternate years to the cereal crop, by 1995 over 1,800
pounds of N had been applied to this field (Fig. 3). Between 1885 (first
year of agricultural production) and 1960 only 290 pounds of N had been
applied to this field. Consequently, the pH value observed in 1960 was
similar to the value probably observed in 1885. Between 1960 and 1980
the soil pH value on this field declined steeply as an additional 870
pounds of N was applied-nearly three times the amount of N (870 vs. 290)
applied in the previous 75 years (1885-1960). Soil pH values continued
to decline after 1980 as additional N was added. Consequently, it is apparent
that there is a direct link between N (ammonium-based N fertilizers) and
soil pH decline.
Fig. 3. Sum total
amounts of N applied to a typical field on the Idaho-Washington border
near Moscow, Idaho.
Specific Crops
The relationship
of soil pH to the potential yield of crops commonly grown in northern
Idaho is shown in Table 2. In general, legume crops are most sensitive
to acid soil conditions while cereal and grass crops are more tolerant
to low soil pH.
Alfalfa -
The minimum soil pH for maximum growth of alfalfa is 5.7. Alfalfa yields
will decrease rapidly as field pH values decline. For further information,
see University of Idaho Current Information Series 447, Northern Idaho
Fertilizer Guide: Alfalfa.
Barley -
The minimum pH value for maximum spring barley production in northern
Idaho is 5.3. Research on spring barley has shown that yield declines
sharply as the soil pH falls below 5.2 (Fig. 4).
Fig. 4. The relationship
of soil pH to the percentage yield of spring barley grown in northern
Idaho.
Bluegrass Seed
- Bluegrass is relatively tolerant to acid soil conditions. Yield declines
in this seed crop have only been observed in fields with pH values less
than 5.2. Little yield data are available on soils with pH values less
than 5.0. Of the crops commonly grown in northern Idaho, bluegrass appears
to be the most tolerant to very acid soil conditions.
Lentils -
The minimum pH value for maximum lentil production in northern Idaho is
5.65 (Fig. 5). The relationship between soil pH and yield is strong. Differences
in pH tolerance among lentil varieties are not expected because, even
though lentils do have a fairly wide genetic diversity, all evolution
occurred in areas with high pH soils. Soil pH may affect lentils indirectly.
Soil pH values less than 5.65 adversely affect Rhizobium leguminosarum.
This may result in less nodulation which in turn adversely affects the
nitrogen nutrition of the lentil plants. This observation has been made
for other legumes.
Fig. 5. The relationship
of soil pH to the percentage yield of lentils grown in northern Idaho.
About 57 percent
of agricultural soils in northern Idaho have surface (12 inch) pH values
less than 5.65. Consequently, over half of the acres annually planted
to lentils in northern Idaho have pH values too low to produce maximum
yields. Data plotted in Fig. 5 indicate that lentil yield will be only
79, 65, and 50 percent of maximum yield potential at soil pH values of
5.4, 5.2, and 5.0, respectively.
Peas - The
minimum acceptable pH value for maximum spring pea yields was 5.52 (Fig.
6). At soil pH values of 5.3, 5.1, and 4.9, spring pea yields would be
only 77, 59, and 42 percent, respectively, of maximum yield potential.
About 48 percent of the acres annually planted to spring peas in northern
Idaho have pH values less than the minimum acceptable value for 100 percent
(maximum) yield.
Fig. 6. The relationship
of soil pH to the percentage yield of spring peas grown in northern Idaho.
As is true with
lentils, pea varieties show little difference in their response to decreasing
soil pH. Since peas evolved in neutral to alkaline soils, they logically
would be similar in tolerance to pH. Growers within the region have known
for several years that low soil pH values reduced both spring pea and
lentil yields. The low economic value of peas and lentils relative to
the cereal crop in the rotation has prevented corrective action, however.
Spring peas, like lentils, are legumes nodulated by R. leguminosarum.
Low soil pH apparently affects the pea plant indirectly by inhibiting
N2 fixation and causing inadequate N nutrition of the pea plants. Research
conducted near Cavendish in Clearwater County showed that populations
of nodulating bacteria are severely reduced under acid soil conditions.
Wheat - A
strong relationship between soil pH and winter wheat yields was observed
in field research conducted in northern Idaho (Fig. 7). The scatter in
Fig. 7 can be attributed to the different winter wheat varieties used
in the field experiments. The wheat varieties studied originated in different
states and different soil pH environments, unlike the peas and lentils.
Fig. 7. The relationship
of soil pH to the percentage yield of winter wheat grown in northern Idaho.
Minimum pH values
for maximum winter wheat yields range between 5.1 and 5.4, depending on
variety. Since the soil acidity present in northern Idaho soils developed
over a long period of time as a consequence of ammonium-based N fertilizers,
simple, cheap techniques to neutralize soil acidity will not work. The
only way to correct a soil acidity problem is to apply some type of lime
material. At least 80 percent of the soil in northern Idaho and eastern
Washington (Palouse) will someday require lime to produce maximum crop
yields.
Since the use of
liming materials means added production costs, a crop yield loss from
soil acidity of less than 20 percent may not be economical to correct.
Crop prices, as well as lime material costs, will impact future crop yields
produced in northern Idaho.
|
