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Impacts
and Management of Soil Acidity under Direct Seed Systems
- Strategies for Management
Greg
Schwab, David Bezdicek, Ian Wildey, and Mary Fauci
Washington State University, Pullman
Introduction
Decreasing
soil pH of agricultural land in the Pacific Northwest (PNW) as a result
of nitrogen fertilizer applications has been well documented  (Mahler
et al, 1985). In the majority of the country, management of acid soils
is simply applying the recommended rate of agricultural limestone every
four to six years. In the Palouse, the cost of limestone is so high that
alternative management strategies can be cost effective. In direct-seed
cropping systems, acid soil is a greater problem because lime must be
leached into the profile rather than using primary tillage for mixing.
For these reasons, it is especially important that growers here in the
PNW limit inputs of acidity through crop and fertilizer management techniques
and thereby reduce the need for lime. To effectively manage soil pH, one
must first have a basic understanding of soil pH and a thorough understanding
of the factors causing soil acidity. Then this knowledge can be applied
to growers' unique cropping systems.
Soil pH
There is an inverse
relationship between soil pH and acidity; the lower the soil pH the more
acid the soil. Soil pH is important because the availability of nutrients
essential for plant growth as well as elements toxic to plant growth are
directly related to pH (Figure 1). For example, phosphorus availability
decreases dramatically from pH 6.0 to 5.0 because of the formation of
insoluble iron and aluminum complexes. Conversely, the availability of
soluble aluminum, which is toxic to plant roots, increases as soil pH
decreases. Most agricultural crops have maximum yield when soil pH is
between 5.5 and 7.0, while some crops like wheat can tolerate soil pH
as low as 5.2. There are many factors that contribute to increasing soil
acidity. For most agricultural systems, nitrogen fertilization (also some
phosphorus and sulfur fertilizers) and nitrogen fixation account for the
majority of the acidity produced in a given growing season. Other factors
include removal of basic cations (calcium, potassium, and magnesium) with
the harvested portion of the crop, as well as, cation losses associated
with nitrate leaching. In conventional tillage cropping systems, the acidity
produced is uniformly mixed into the surface layer during the primary
tillage operation. In a direct-seed cropping system, however, horizontal
layers of acid soil are formed as a result of acid deposition from fertilizer
placement and plant growth. Because there is no primary tillage, the acidity
of this layer builds with successive years of direct-seeding and eventually
will begin to affect plant growth. There are two ways to manage acid soils:
(1) reduce the amount of acid (hydrogen ions) being added to the system
and (2) apply lime to neutralize acidity and thus increase soil pH.
Reduction of Acid
Inputs
Fertilizers that
contain ammonium (NH4+) or N that is converted to ammonium (like anhydrous
ammonia or urea) after application are considered acid forming. The acid
is produced when the NH4 is converted to NO3 by soil microbes (as discussed
in the previous paper). In addition to nitrogen fertilizers, some phosphorus
and sulfur fertilizers also form acid when applied. Because there are
many different fertilizer formulations, a wide range in resulting acidity
or basicity exists (Table 1). By selecting a fertilizer that produce less
acidity, the grower can reduce the need for future lime application. For
example, anhydrous ammonia (82-0-0) and aqua ammonia (20-0-0) are the
main nitrogen fertilizers used for wheat production in the Pacific northwest,
because of their low price and high percentage of nitrogen per unit weight.
Phosphorus and sulfur are commonly applied as a starter of ammonium-phosphate-sulfate
(16-20-0). In the high precipitation zone, growers planting direct seeded
wheat commonly apply 100 lbs/a as 82-0-0 and 16 lbs/a as 16-20-0. This
fertilizer combination requires approximately 300 lbs/a of pure CaCO3
yearly to completely neutralize the hydrogen ions produced (Mortvedt et
al., 1999). If a combination of UAN (urea ammonium nitrate, 32-0-0) was
used for N instead of anhydrous and a mixture of triple superphosphate
and potassium sulfate was used as the starter, then the fertilizer acid
addition would have only required 90 lbs of CaCO3 for neutralization.
In addition to fertilizer formulation, the fertilizer requirement of the
crop produced has a large impact on the acid inputs.
Because of the price
premium for the hard red wheat classes, many growers are choosing this
market class over the more traditional soft white market class. Hard red
wheat generally requires 1 additional pound of N per bushel to obtain
the desired 14% protein. Depending on N source, this could amount to an
acid input requiring an additional 180 lbs of CaCO3 for neutralization.
In a conventional tillage system, this acidity would be mixed into the
entire plow layer and its effects would be buffered by the bulk soil,
but in the direct seeding system, the acidity remains in the seed zone
and builds year after year. In the high rainfall region, soils with pH
below 4.8 (in the 2-4 inch layer) have been observed in many fields with
a history of direct seeding and/or a history of high N application rates.
On the 160 acre Cunningham Research Farm near Pullman, 185 soil samples
were collected to access the soil variability of a typical Palouse landscape
(Figure 2). The field has a history of continuous cereal cropping and
direct seeding. Results indicate more than 40% of the samples collected
had soil pH (in the 2-4 inch layer) below the critical level for wheat
(pH 5.2) established by Mahler and McDole (1987).
Figure 2. Soil pH
distribution at the 0-2, 2-4, and 4-6 inch depths for samples collected
on the Cunningham Farm.
In addition to fertilizer
applications, the plant itself can also have an effect on soil pH. As
a plant absorbs nutrients from the soil solution, it maintains electroneutrality
by releasing H+ (acid) when cations like NH4, Ca, K, and Mg are absorbed
and releasing HCO3- (base) when anions like NO3 and Cl are absorbed. Researchers
have found that this mechanism is important in the uptake of some micronutrients,
but under field conditions it has little effect on the soil pH because
the total uptake of cations and anions is nearly balanced (Barber, 1995).
Correcting Acid
Soils
Applications of CaCO3
and/or MgCO3 are used to raise soil pH. In most parts of the US, lime
is applied at a rate high enough to raise soil pH near neutral (pH 7)
once every 4-6 years. The application rate (typically 2-4 tons/acre) depends
on type and amount of clay in the soil, as well as the amount of organic
matter in the soil. For this reason, lime application rate should always
be determined based on a soil analysis. Because of the cost of limestone
in the PNW and problems associated with spreading that amount of material,
large scale lime application might not be feasible here. An alternative
application method would be to apply enough lime to neutralize the acidity
produced by the current years fertilizer application. With this approach,
lime could be placed in direct seed contact using the drill at the time
of planting, thus avoiding the need for tillage to incorporate the lime.
For direct seed cropping systems, seed placed lime is the ideal placement
because of the high seed zone acidity. Results from the first year of
a study conducted near Palouse, WA show that seed placed increased soil
pH in the seed zone for at least one treatment (Figure 3). A STEEP funded
project has been initiated this year to examine the long-term effectiveness
of seed placed lime in direct seeding cropping systems.
Figure 3. Effect of
pelletized lime (P lime) and pelletized sugar beet by-product (SB lime)
applied at rates of 100 and 200 lbs CCE/acre on soil pH with depth.
Liming materials
vary in their neutralizing capacity. For this reason, CaCO3 equivalent
(CCE) is used to compare the effectiveness of liming materials (much like
a fertilizer analysis is used to compare different N sources). Agricultural
limestone generally has a CCE of 65-75%, but products with CCE of over
90% are available. In addition to agricultural lime, some byproducts also
make good liming agents and can be less expensive. High quality lime is
used in the processing of sugar beets and drinking water, so in some areas
this is a readily available source. As with agricultural lime, an analysis
of the material is needed to determine the cost effectiveness. These by-products
are often fluid suspensions, so the amount of water in the material should
also be considered. Another form of lime that is available in the PNW
is pelletized lime. Pelletized lime is made by mixing a binding agent
with finely powdered limestone in order to ease handling and to assure
a more uniform application. Because the binding agent readily breaks down
when mixed with water, the CCE of the pelletized lime is the same as the
powdered material used to make it.
Conclusions
Except for a few
isolated saline and sodic areas, soils naturally become more acidic with
time. This process is accelerated when acid forming fertilizers are applied
and when basic cations are removed with the harvested portions of the
crop. Liming low value crops like wheat in the Pacific Northwest is not
profitable until soil pH begins to effect crop yield. Direct seeders are
in a unique situation because when crop yield begins to decline due to
low pH, it might be too late to correct without the use of primary tillage.
Research which is under way will help to answer many of the current questions
regarding crop response to lime applications. Meanwhile, growers (especially
direct seeders) should be considering alternative fertilizer formulations
when they are economically feasible to reduce the acid inputs and ultimately
reduce the need for lime applications.
References:
Barber, S.A. 1995.
Soil Nutrient Bioavailability: A Mechanistic Approach. John Wiley and
Sons, Inc. New York, NY.
Hawkes, G.R. 1980.
Western Fertilizer Handbook. The Interstate Printers and Publishers, Inc.
Danville, IL.
Mahler, R.L., A.R.
Halvorson, and F.E. Koehler. 1985. Long-term acidification of farmland
in northern Idaho and eastern Washington. Commun. in Soil Sci Plant Anal.
16:83-95.
Mahler, R.L. and
R.E. McDole. 1987. Effect of soil pH on crop yield in northern Idaho.
Agron J. 79:751-755.
Mortvedt, J.J., L.S.
Murphy, and R.H. Follett. 1999. Fertilizer Technology and Application.
Meister Publishing Co. Willoughby, OH.
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