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Objective 1. In high-phosphorus soils, identify which
pools
of P are
available for release to spring-sown vegetable crops.
We
tested P release on 31 soils representing
high-phosphorus vegetable soils from throughout the Northeast, using 10
different measures of P availability and 3 additional soil
characterizations. Some are highly correlated (i.e. Bray-1, Bray-2 and
Mehlich-3), but others measure different pools of soil P. Principal
component analysis was applied to identify characteristics of soils
that predicted release of P by bicarbonate. These soils were well
described with three components, and there was good representation
throughout that descriptive space. PC1 was generally associated with
the amount of phosphorus, PC2 with soil texture, and PC3 with pH and
soil organic matter. The first two components were associated with
response to bicarbonate. PC1 removed the confounding effect of variable
soil phosphorus, while PC2 contained most of the predictive power.
Therefore consideration of soil texture alone followed.
The most strongly bicarbonate-responsive soils were those high in sand
(>50%). The water-extractable phosphate was three to six times
as high following incubation with 50 µmol bicarbonate per
gram of soil (approx 5 lb/ac when banded). A separate analysis of the
sensitivity to pH and organic matter showed that pH plays no role, and
the response in low organic-matter soil was explained already by their
high sand content. The most responsive soils are characteristic of the
Delmarva Peninsula, South Jersey, Long Island and Cape Cod. The first
three areas are major vegetable producers with locally specific
conditions that produce a high ongoing phosphorus load. It is for these
locations that further investigation of the bicarbonate technique might
be worthwhile.
The P leaching potential (CaCl2-extractable
P) was
evaluated relative to that available for season-long growth
(Morgan-extractable P). The leaching potential was substantially higher
at every level of phosphorus than that found on dairy farms in Delaware
Co, NY. (Kleinman, et al. Soil Science 165:943–950.)
The time in spring when phosphorus becomes available was assessed by
collecting soil every 10 days from thaw through mid-summer. In a survey
of 11 bean fields, immediately available phosphorus (water extraction)
did not change in a consistent pattern with time. Most stayed constant
with some showing a steady increase and some a steady decrease. All
were high throughout. If the concentration of dissolved phosphate in
the soil solution were the limiting factor causing phosphorus-limited
growth under these conditions, the early values should have been low,
and the later values at least adequate. Later plantings in these soils
are not phosphorus-limited . This result is evidence that the
limitation is more likely to be in the root function than in soil
processes.
The higher leaching potential of vegetable soils
than in previously
studied soils is an indication that Northeast vegetable growers are in
particular need of phosphate management procedures that reduce risk
while preserving crop productivity.
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Objective 2. Test specific treatments that may make P
available without further increasing soil P
We tested several potential methods that
have promise based on grower experience, knowledge of soil chemistry,
and knowledge of rhizosphere biology. We evaluated and eliminated
three: citric acid (phytotoxicity), P-releasing microbes (management
concerns) and buckwheat cover crop (ineffective). Bicarbonate
application continued to show promise and received the most attention.
Phosphorus response There are no data from replicated trials
demonstrating that modern higher-yielding varieties respond to starter
phosphorus in high-P soils. We tested two varieties, Hystyle and Zeus,
at 7 locations. The response, an average yield increase of 20%, was
statistically significant in only 2 sites. However all sites had a
positive response of >10%. A 10% yield penalty will make
eliminating starter P uneconomical, so a substitute treatment is
needed.
Bicarbonate
in furrow Bicarbonate as an alternative to starter P was
tested at 5 sites in 2001. The rate selected was low (2 lb/ac), and the
response was slight. We showed that a rate of 5 to 8 lb/ac will be
safe, although rates over 10 lb/ac reduced stands. We tested the
response to higher rates in 2002, including placing higher rates in a
band 2" away from the seed. Snap beans differ from crops such as field
corn in that they do respond to starter phosphorus in cold soil, even
at very high soil phosphorus. The explanation may lie in a temperature
responsive phosphate-uptake mechanism. Since beans are
chilling-sensitive, it would not be unexpected for this
membrane-associated process to be less functional at the chilling
temperatures that exist in these cold soils. Additional phosphorus
would only have an effect if the concentration were high enough
(>20 mM) to allow some passive uptake. Sufficiently high
concentrations may only be obtained in the soil solution near pellets
of superphosphate.
Phytotoxicity.
Greenhouse tests indicated that rates up to 10 mg/ seed
could be placed with the seed without affecting emergence, even 50
mg/seed has minimal effect. In the field, crop stand and growth was
substantially reduced at rates exceeding ~25 mg/seed (5 lb/ac) in the
furrow. Potassium bicarbonate placed 2 inches away from the seed was
non-toxic at 100 mg/seed (20 lb/ac). Using a soil-filled plexiglas
boxes, a 1000 mg dose placed 2 inches from the seed produced a sharp
zone without roots about 1 inch in diameter. There is a definite upper
limit to the rate of potassium bicarbonate that can be safely applied
to the snap-bean root zone. The safe rate for application in the seed
furrow may be too low to generate a sufficient phosphorus
concentration.
Phosphate
coatings. We tested coatings of zinc phosphate and calcium
phosphate at 10 mg/g seed, which is abut 3 times the amount present in
the seed. There was no significant yield response at either Geneva, NY
or Adelphia, NJ. Calcium phosphate showed a slight numerical increase
in yield at both sites, a slight increase in tissue P, and a
significant shift to smaller (more valuable) beans at
maturity.
Buckwheat
cover crop. Buckwheat partitions phosphorus into the
stem,
and the stems decompose rapidly over winter. We tested whether this
phosphorus would be available to bean plants in the spring. Strips of
buckwheat and oats (control) were sown in a high-phosphorus field in
late summer. Phosphorus in the cover crop and soil in each strip was
measured when the cover crop froze in the fall, when the field was
ready to till in the spring and at bean planting 2 weeks later. Growth
of the beans was measured in the second year. There was no significant
difference in rapidly-available (AEM) soil P among buckwheat, oat and
bare-ground. The bean growth was unaffected by cover crop treatment.
Thus there was no indication that buckwheat would provide the starter P
needed by bean seedlings.
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Impacts and Outcomes
- Vegetable soils in the Northeast are more
susceptible to phosphate
leaching than the field crop and dairy soils that have been studied
previously. Therefore, vegetable growers in high-risk locations and
high-P soils need to be aware of measures to reduce leaching.
- Soils
vary substantially in their ability to release phosphorus with small
amounts of bicarbonate. The release is not predicted by the phosphorus
measured by common soil test extractions.
- Beans (Zeus and Hystyle)
planted in mid May (the first 10 days of planting in the region)
responded to banded starter phosphorus, averaging about 20% yield
increase. The response was not predicted by the amount of phosphorus in
the soil, nor by the content of phosphorus in leaves of young
plants.
- We have identified granules of potassium
bicarbonate as a material that
has low phytotoxicity, is easy to meter in common planting equipment
and has have the ability to release substantial amounts of phosphate
from test soils representative of the entire Northeast. However,
seedling tissue P and yield were not increased with bicarbonate
application rates used.
- If soil processes, rather than root
physiology, limit P uptake in cold soils, further work in the sandy
soils of the vegetable-producing areas of the Atlantic seaboard is
worthwhile.
- This project has raised awareness among
vegetable growers
for the need to manage phosphorus with regard to the potential for
leaching. This target group will now be more receptive to phosphorus
management programs offered by NRCS and other agencies.
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Areas needing additional study
- The results of this work point strongly to a
temperature-sensitive root
process as being the limiting factor for snap beans. Only the very high
P concentration in a band of superphosphate can overcome this
limitation. Identification of this process is critical for further
progress. Our prediction is that a key step in P uptake by the roots is
markedly slowed by low temperature and becomes a limiting factor for P
uptake at 10°C even when the soil solution is raised to 10 mM
with bicarbonate treatment.
- Such investigation could lead to
selection
of snap bean varieties that are less temperature sensitive in this
process, and could therefore respond to the bicarbonate technique. Some
might even be identified that would, like field corn, use the abundant
P in these soils with no treatment at all.
- In the Northeast,
bicarbonate release of phosphorus deserves further investigation on the
sandy soils of the Delmarva peninsula and southern New Jersey. Both of
these regions are severely impacted by high-phosphorus soil and are
prime producers of early-season snap beans.
- The availability of
granules that can be metered is likely to be a problem. Church and
Dwight Co. formerly had the facilities to manufacture the granule size
necessary for accurate metering and resistance to clumping in this
agricultural application, bu they no longer do. Although the cost of
such compounding is estimated to be less than $0.10 per pound, the
logistics of providing it for this use could be a barrier. Further work
is needed to identify a supplier and marketing structure that would
make this material easily available to farmers.
- There is a critical need for phosphorus
management guidelines that
allow growers respond to the demand for nutrient management plans. The
large yield reduction associated with eliminating starter phosphorus
would make early snap bean production untenable. The
higher-than-predicted risk of phosphorus leaching may make continued
high P inputs unacceptable.
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Materials and Methods
- Soil
characterization.
High-phosphorus Northeast vegetable soils were collected by contact
with farmers, extension staff and commercial consultants with local
knowledge. They were asked for 10 to 15 lb samples of soil from fields
with high-phosphorus and that could be planted to snap beans the
following spring. That synchronized the rotation factor and made the
soils comparable in their suitability. Thirty-one samples were
collected, representing all the significant areas for raising snap
beans. Sites were in Delaware, Massachusetts, New Hampshire, New
Jersey, New York and Pennsylvania. Soils were stored at 4° C
(simulating early spring) until used. The following soil analyses were
performed on each soil: Modified Morgan-P, Weak Bray-P, Strong Bray-P,
Water-extractable-P, CaCl2-extractable P,
Anion-exchange membrane-P,
HCl-extractable-P, Organic-P, pH, %sand, %clay, bicarbonate release of
P by water and by AEM. The soils were characterized by Principal
Component Analysis of all but the last two assays. The Principal
Components were then used as predictors for the two measures of
susceptibility to bicarbonate release.
- Hi-resolution
Field P
kinetics. Eleven high-P fields scheduled for snap beans
were selected in winter.
From precisely mapped sites (about 1/2 square meter per field) five
soil cores were removed about every 10 days from late April through mid
July (9 sampling dates). The soil cores were immediately frozen to stop
chemical and biological processes affecting P availability. These
samples were analyzed for immediately available P by water extraction
and by AEM.
- Field
trials of bicarbonate. The field trials were
designed with a no-P control, a banded
unlimited-P control (70 lb P2O5
/ ac) and a bicarbonate treatment (5
lb/ac). Trials were conducted at grower farms in Ontario Co, NY, the
Research Farm at NYSAES and at the NY Crop Research Farm (a
grower-owned facility in Batavia, NY). The grower sites were planted as
part of a commercial snap-bean field with the same seed and field
preparation, except for P. On the research farms, N and K were applied
by broadcasting at the rate recommended by the Cornell Nutrient
Analysis lab based on the soil tests. Plots were seeded with a Monosem
vacuum seeder that had a fertilizer banding unit for applying triple
superphosphate in a band 5 cm over and 5 cm below the seed zone, and a
granule metering box (aka "Gandy box") to dispense potassium
bicarbonate granules in the seed furrow in close proximity to the
seeds. The plant population and seed depth were adjusted to give an
optimal stand for each field based on moisture, temperature and
variety. Plots consisted of two rows, 20 feet long, with two untreated
rows between plots to eliminate effects of adjacent treatments. Six
replications were used in a randomized block design. Additional
experiments testing P and bicarbonate rates were similarly designed,
with only the number of treatments different. Additional experiments on
the effect of seed-applied P had the same design, but the P treatments
were applied to the seed, and the seeds were changed for each plot. the
sead treatment experiments were planted in Geneva, NY and in Adelphia,
NJ. The results were tested for significance by analysis of variance
and linear contrasts; the aggregate data were analyzed by t-test for a
difference in means.
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