Using silicon fertilizers to improve soil phosphorus availability and uptake by winter wheat in high-phosphorus soils

Using silicon fertilizers to improve soil phosphorus availability and uptake by winter wheat in high-phosphorus soils

Summary

Historical application of manure to agricultural lands in Delmarva Peninsula has led to accumulation of soil test phosphorus (STP) to levels that far exceed agronomic optimum, which is linked to water quality degradation in sensitive water bodies. Farmers growing small grains on Delmarva often apply starter P fertilizers (despite excessive STP) to address early season P deficiencies. Fertilization of soils with silicon (Si) is promising as a best management practice that can enhance crop P uptake, eliminate fall starter-P applications for small grains, and improve their yields. The purpose of this project is to evaluate the effects of Si fertilizer type and rate on soil P dynamics and winter wheat yield and P uptake by conducting a pot study and a corresponding field trial. We hypothesize that Si fertilization increase soluble P in soils and enhance early season P uptake by winter wheat, which will reduce the need of starter P and promote more rapid draw-down of STP during a typical grain crop rotation. Preliminary results from our pot study indicated that Si application improved soil P availability; but the effect depended on the source and rate of applied Si. Specifically, calcium silicate (CaSiO₃; a commercial liming material containing Si) at optimum liming rate only significantly increased the water extractable P in soil compared with no Si control in the first one or three month after application, while silicic acid at a total Si rate of 2 Mg ha^-1 consistently increased the water extractable P in soil for the entire growing season. Our results suggest that CaSiO₃ is a promising Si source that improve P availability in soils at the early growing season of winter wheat. However, we did not find a significant increase in winter wheat yield following Si application in our pot study. A field study is currently underway to assess the effects of CaSiO₃ on yield on a larger scale.

Objectives/Performance Targets

The objective of this proposal is to evaluate fall Si fertilization to enhance soil P availability and uptake by winter wheat from legacy (excessive soil test) P soils. We proposed a pot study and a corresponding field trial to evaluate fall silicon fertilization effects on soil P availability and P uptake by winter wheat grown on legacy P soils. The pot study was designed to determine the effects of Si fertilizer sources and Si rate on soil P dynamics and winter wheat response (e.g., yield, P uptake). The field trial extends the pot study to determine the utility of applying locally available Si sources to winter wheat under field conditions. The overall goal of the proposed Si fertilization research is to develop a novel BMP that can help farmers reduce the environmental risks associated with legacy P soils, increase crop uptake of P to speed draw down of soil test P levels, and enhance winter wheat yields.

The pot study was completed this year as proposed. Specifically, we have:

1. Maintained a winter wheat crop in pots (microplots) during the 2015-2016 growing season,
2. Completed monthly soil sampling,
3. Determined the chemical and physical soil properties,
4. Harvested wheat and calculated the yield of wheat for each pot,
5. Dried and grind plant sample in preparation for chemical analysis.

The field trial has been initiated in late Fall 2016 as proposed. Specifically, we have:

1. Identified one legacy P impacted agricultural fields (acidic soils and Mehlich 3 soil test P concentration in excessive range) on the Delmarva Peninsula,
2. Conducted analysis of initial soil properties,
3. Applied Si and lime treatments in a randomized complete block design,
4. Seeded winter wheat on the farm.

Accomplishments/Milestones

Pot Study

Pot Maintenance

Fifty-four pots were arranged outdoors in a pot-in-pot field plot at the University of Delaware (UD) Newark research farm in a completely randomized design with three replicates. Air-dried soil from each of the three “legacy P” sites was added to 11.4-L plastic pots to achieve a bulk density of 1.33 g cm^-3. Detailed properties of the three legacy soils were described in Table 1 in 2015 annual report. Silicon fertilizers were mixed thoroughly into each of the soils in as follows: (1) locally available calcium silicate (CaSiO₃; Agrowsil), at the rate required to raise pH to optimum growth of winter wheat based on results of the initial soil test; (2) silicic acid at the same total Si rate as in treatment 1; (3) silicic acid at two times the total Si rate as in treatment 1; (4) silicic gel/amorphous silica at a total Si rate of 2 Mg ha^-1; and 5) silicic acid at a total Si rate of 2 Mg ha^-1. All Si sources were analyzed for total Si content by X-ray fluorescence prior to application. An unamended control (treatment 6) was included for each soil type. The estimated actual Si application rate of each treatment for each soil is shown in Table 1. Calcitic lime (CaCO₃) was applied to all pots, except those receiving the agronomic CaSiO₃ treatment (Treatment 1), to achieve agronomic optimum pH for winter wheat at rates (1.5 to 2.5 ton ac^-1) based on the initial soil test. As described in 2015 annual report, each pot was applied with treatment and seeded with wheat in November 2015. Pot was then transferred outdoor at the UD Newark research farm on 1 Dec. 2015 to be grown under ambient conditions through July 2016 (approximately 38 weeks). Regular fertility was maintained for each pot based on initial soil test (described in 2015 annual report). Nitrogen fertilizer was again applied on 1 April 2016 and 2 May 2016 at an N rate of 67 and 34 kg ha^-1 respectively.

Soil Sampling and Analysis

We changed our soil sampling frequency from biweekly to every month to reduce the disturbance to plant roots due to the small size of the pots and destructive nature of the soil sampling. Thus, soil samples were randomly collected from each pot using a 10 mL syringe (5 to 10 samples from each pot) every month until grain harvest. Soil samples were analyzed for water extractable P (WEP) by spectrophotometer (Self-Davis et al., 2009) and 0.5 M acetic acid extractable P (AA-P) and Si (AA-Si) by inductively coupled plasma optical emission spectroscopy (ICP-OES; Heckman and Wolf, 2011) (waiting results from Soil Testing Lab). Additional soil samples collected from each pot at 30 d (early growth) and harvest were analyzed for pH, organic matter, and Mehlich 3 P, Si, Al, Fe, K, Ca, and Mg, using standard soil testing procedures (NECC-1312, 2011); results are pending from the UD Soil Testing Lab. Sequential extraction of P and Si was also completed on the 30 d and harvest samples to evaluate changes in the distribution of P and Si among operational soil pools (e.g., shifts from insoluble to plant available and vice versa; Sui et al., 1999). In brief, 0.5 g of each soil were sequentially extracted with (1) water extractable P (30 mL deionized, shake for 16 hours, centrifuge, and decant solution); (2) labile P (30 mL 0.5 M NaHCO₃, shake for 16 hours, centrifuge, and decant solution); (3) moderately labile P (30 mL 0.1 M NaOH, shake for 16 hours, centrifuge, and decant solution); and (4) readily insoluble P (30 mL 0.1 M HCl, shake for 16 hours, centrifuge, and decant solution). All fractionation extracts were analyzed for P, Si, Al, Fe, K, Ca, and Mg by ICP-OES; results are pending. Soil samples taken at harvest were also analyzed for P adsorption characteristics to evaluate changes in P sorption capacity of soils following Si addition. Briefly, 2 g of air-dried soils were equilibrated with 30 mL of P solution (as KH₂PO4 dissolved in 0.01 M CaCl₂, CaCl₂ being used to maintain a constant ionic environment) at P concentrations of 0, 0.1, 1, 5, 10, 35, 50 mg L^-1. Samples were shaken for 24 h, centrifuged for 10 min, and filtered through 0.45-µm filters. All the extracts were analyzed for P using ICP-OES; results are pending. Phosphorus sorption data will be used to construct Langmuir isotherms to estimate soil maximum sorption capacity of P.

Our preliminary soil results indicated that, in general, soil WEP concentrations increased initially, peaked in February, and gradually decreased over time in all three soils during the winter wheat growing season (Figure 1). The decrease in WEP concentrations coincided with the warming temperature, which is likely related to crop uptake of P during the rapid spring growth of wheat. As early as 4 d after Si application (20 Nov. 2015), WEP concentrations increased (beyond the liming effect) compared to the no Si control for many of the treatments (Figure 1). For example, application of AgrowSil at optimum lime rate (about 0.40 and 0.67 Mg ha^-1 total Si for Ft. Mott-Henlopen and Ingleside- Hammonton, and Mullica-Berryland soil series respectively) and silicic acid at 2 Mg ha^-1 total Si rate significantly increased WEP in all three soils compared with no Si control (data not shown). Application of silica gel at the 2 Mg ha^-1 total Si rate also significantly increased WEP concentrations when applied to the Ft. Mott-Henlopen compared with no Si controls; no significant increase in WEP was noted for the Ingleside- Hammonton and Mullica-Berryland soil series (data not shown). However, the effect of AgrowSil application on WEP appeared to be temporary; no significant difference in WEP was noted in soils amended with AgrowSil at optimum lime rate about 40 d after Si incorporation in Ft. Mott-Henlopen, Ingleside-Hammonton, and 107 d after Si incorporation in Mullica-Berryland (data not shown). Application of silicic acid at the 2 Mg ha^-1 total Si rate significantly increased WEP over the time in all three soil series when compared to the unamended control (Figure 1). Silicic acid at two times AgrowSil liming rate and silica gel at 2 Mg ha^-1 total Si also sometimes increased WEP compared with no Si control; but the increase in WEP was lower compared with treatment received silicic acid at a total Si rate of 2 Mg ha^-1 (data not shown). Application of Si did improved soil P availability; but the effect depended on the source and rate of applied Si.

Plant Sampling and Analysis

Wheat was harvested on 24 June 2016. At harvest, whole aboveground grass tissues were cut at the soil surface. Roots samples were later hand-separated from the soils from each pot. Wheat berries were hand harvested to approximate yield (Murphy, 1993). All dried aboveground tissue (wheat berries, stems, leaves, and roots) were ground using a Wiley mill. Samples of plant tissues (stem and leaves together), wheat berries will be analyzed separately for Si and P by ICP-OES following microwave-assisted HNO₃/H₂O₂ digestion (Seyfferth and Fendorf, 2012). While Si treatment resulted in significant effects in WEP, there was no significant difference in yield of winter wheat grown in any of the soils (Table 2). Tissue analysis of Si and P content is pending.

Field Trial

Treatment Adjustment

To better compare the effect of Si application on soil P availability compared with traditional starter P application, we added a treatment with starter P fertilizer in addition to our proposed treatments. The final Si treatments are:

• Standard fertility (no P) without calcitic lime
• Standard fertility (no P) with calcitic lime (1.5 ton ac^-1)
• Standard fertility + starter P (50 lbs ac^-1) + calcitic lime (1.5 ton ac^-1)
• Standard fertility (no P) + Agrowsil (1.5 ton ac^-1 or about 0.5 Mg Si ha^-1)

Soil Selection and Initial Analysis

We initially proposed the use of a legacy P impacted agricultural field with excessive soil test P level (Mehlich3 P concentration = 300-500 mg kg^-1) and require lime to manage pH (pH < 6) on Delmarva Peninsula for the field trial. We were unable to locate a soil with proposed soil test P concentrations with pH lower than 6. The acidic soil pH (<6) was important for this study because one of the Si source (AgrowSil) is a liming material. Therefore, we selected soils with a suitable pH and the best range of soil test P as possible. The initial soil properties of selected soil are shown in Table 3.

Twelve strip plots (4.57 m × 30.5 m) were set up on a cooperator farm at University of Delaware Carvel Research and Education Center in Georgetown, DE. Soil samples were collected randomly to a depth of 15 cm from each plot on 1 November 2016 (prior to planting) using a soil auger. Soils will be air-dried, ground, passed through 2-mm screen, and analyzed for, pH, Adams-Evans pH, organic matter, WEP, AA-P, AA-Si, and soil test P, Si, Al, Fe, K, Ca, and Mg (Mehlich 3 extraction) as described previously. Treatments were applied to each plot on 2 November 2016, and were seeded with winter wheat on 3 November 2016. Both calcitic lime and AgrowSil were applied for the target pH of 6.0 according to initial soil test. Potassium fertilizer was applied in late October at a rate of 110 lbs ac^-1. Nitrogen fertilizer (UAN 30-0-0) was applied on 22 November 2016 at a rate of 32.5 lbs ac^‑1.

We plan to collect additional soil samples from each plot before dormancy (late fall), at spring greenup (late Feb/early Mar), and post-harvest (late July/ early Aug) and analyze for the same parameters as described previously. We also plan to collect aboveground plant biomass in late fall and pre-harvest from three to four areas (1 m x 1 m) within each plot. Samples will be dried, weighed, digested, and analyzed for total Si and P as described previously.

Table 1-2016 Annual Report

Table 2-2016 Annual Report

Table 3-2016 Annual Report

Figure 1-2016 Annual Report

Reference-2016

Impacts and Contributions/Outcomes

Grain farmers in areas of intensive animal production, like the Delmarva Peninsula, face significant nutrient management issues related to historical application of manure to agricultural lands. Application of poultry litter to meet crop N requirements resulted in over-application of P, and ultimately to accumulation of P (often to levels that are several times the agronomic optimum) and saturation in many agricultural soils on Delmarva. This accumulated or “legacy” P acts as a continuous source of dissolved P from agricultural soils during runoff and/or leaching events, which can negatively impact water quality in sensitive water bodies like the Chesapeake Bay. As a result, it is important for farmers to carefully manage legacy P soils to reduce the risk of P losses that lead to water quality degradation. In the meantime, many farmers (and results of some regional research) tout the benefits of starter P fertilizers to winter wheat grown on excessive STP soils to combat early season P deficiency and ensure good fall tillering, which is vital for maximizing yield. Early season P deficiency is related to cool soil temperatures at the time winter wheat is planted and the fact that significantly amount of P are strongly bonded with iron (Fe) and aluminum (Al) in acid legacy P soils. However, applications of P fertilizer to legacy P soils are often considered taboo because they may further enrich soils with P. We anticipate this project will provide valuable information to help grain farmers better manage “legacy P” soils (e.g., reduce risk of P losses, speed drawdown of soil P stores) and improve yields of small grains (e.g. improved crop uptake of P, increased resistance to drought and other abiotic stresses). This project will also provide information to help Delmarva farmers to increase grain production to support a growing poultry industry, close the regional P cycle by eliminating the need for grain importation, and meet state and federally mandated nutrient load reductions. Dissemination of project findings to nutrient management/crop consultants will expand the number of farmers who might benefit from Si fertilization of small grains. Moreover, the results of this research should be useful to policy makers who develop/implement nutrient management policy and cost-share programs. We anticipate that the practice of Si fertilization will become a regionally accepted practice in the USDA-NRCS nutrient management standard (Code 590). In addition, results of my research will also be applicable to farmers in other areas of intensive animal production.

Collaborators: