Closing the phosphorus cycle on vegetable farms: releasing soil-bound phosphorus to support springtime seedling growth

Vegetables are often grown on high-phosphorus soils, but additional phosphorus is often required for early sowings. We have demonstrated that an new granular potassium bicarbonate can release phosphate quickly in such soils, and potentially replace the phosphorus addition. We are developing a protocol for using this material, determining on which vegetable soil types if is usable, as well as investigating high-phosphorus cover crop residue and seed-applied phosphorus as alternatives for supplying needed phosphorus until mineralization from soil is sufficient.

Team Members:

Thomas Björkman, Cornell University
Steve Reiners, Cornell University
John Howell, University of Massachusetts
Joseph Heckman, Rutgers University

Funding:

Northeast SARE (USDA)
NYS Vegetable Research Foundation
Pennsylvania Vegetable Mkt. & Res Program
Church & Dwight (in kind)

Project duration:

1 Sept 1999 to 31 November 2002

Objectives:

In high-phosphorus soils, identify which pools of P are available for release to spring-sown vegetable crops.
Test specific treatments that may make P available without further increasing soil P.

Major conclusions:

Vegetable farms with high phosphosrus soils need to be more concerned than other farms about phosphorus leaching, and need good management methods to meet crop P needs without aggravating the phosphorus load.
Snap beans planted early in the season respond substantially to starter phosphorus, even when soil test phosphorus is high or very high.
Bicarbonate can make some soil-bound phosphorus available to plants.
Potassium bicarbonate is effective, inexpensive, and least phytotoxic of the common bicarbonates.
The most promising soils for replacing starter phosphorus with bicarbonate are sandy soils, such as on Long Island, South Jersey and the Delmarva Peninsula.
Bicarbonate was not able to provide enough phosphate to early snap beans in the heavier soils of western New York or central Pennsylvania for it to be used as a replacement for starter phosphorus.

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.

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.

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.

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.

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.