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 areas of intensive animal production, like the Delmarva Peninsula, has led to accumulation of soil test phosphorus (P) to levels that far exceed agronomic optimum. The increased risk of P losses from these legacy P soils is linked to water quality degradation in sensitive water bodies, like the Chesapeake Bay. Farmers growing small grains on the Delmarva Peninsula often apply starter P fertilizers (despite excessive soil test P levels) 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 small grain 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 for starter P fertilizer and promote more rapid draw-down of soil test P during a typical grain crop rotation. We initiated a pot study to evaluate the effects of Si source and application rate on soil P availability and uptake by wheat. Preliminary results indicated that application of AgrowSil (commercial liming material containing Si) at optimum liming rate and silicic acid at a total Si rate of 2 Mg ha^-1 significantly increased the water extractable P in soil compared with no Si control. Initial results also suggested that AgrowSil is a promising locally-available Si source that can effectively release labile Si, thus improve P availability in soils. Results of this project will inform Si fertilization recommendations to reduce the environmental risk of grain production on legacy P soils and increase small grain yields and farm profits.

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 is 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 has been initiated this year as proposed. Specifically, we have:

1. 1. Identified three legacy P impacted agricultural fields (acidic and represent Mehlich 3 P concentrations from 200-500 mg kg^-1) on Delmarva Peninsula,

1. 1. Determined the initial chemical and physical soil properties,

1. 1. Initiated Si treatments by incorporating Si sources into the selected soils, and

1. 1. Seeded winter wheat in the pots.

Accomplishments/Milestones

Soil Selection and Initial Analysis

We initially proposed the use of three legacy P impacted agricultural fields (optimum: 50-100 mg P kg^-1; two to five times optimum: 200-500 mg P kg^-1, above five times optimum: >500 mg P kg^-1) on Delmarva Peninsula for the pot study. We were unable to locate a soil with soil test P concentrations that were six to ten times optimum (600-1000 mg kg^-1) and pH lower than 6 as proposed initially. 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. Three field sites were identified and about 208-liters of soils were sampled (top 20 cm) from each field using a shovel for use in the pot study.

Soils from each field were air dried and thoroughly mixed before transferring to pots. Subsamples of the three soils were ground and sieved through 2-mm screen for initial soil analysis. Initial soil samples were analyzed, in duplicate, for soil particle size (hydrometer method), pH (1:10 soil:deionized water), Adams-Evans buffer pH, organic matter (loss on ignition) using standard soil testing procedures (NECC-1312, 2011). Soil test P, Si, aluminum (Al), iron (Fe), potassium (K), magnesium (Mg), and calcium (Ca) concentrations were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) following Mehlich 3 extraction (NECC-1312, 2011). Initial soils were also analyzed for water extractable P (WEP) and Si (WESi) by spectrophotometer (Self-Davis et al., 2000) and 0.5 M acetic acid extractable P (AAP) and Si (AASi) by ICP-OES (Heckman and Wolf, 2011) (waiting results from Soil Testing Lab). Operational forms of P were determined by a modified Hedley sequential chemical fractionation (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. Selected initial soil properties were summarized in Table 1.

Treatment Adjustment

Based on the pH and buffer pH, commercial lime rates of 2.2-3.4 Mg ha^-1 (1-1.5 ton ac^-1) were suggested for the selected soils to provide optimum growth of winter wheat (pH = 6.0). Analysis by X-ray fluorescence on AgrowSil revealed that the total Si content was 12%. We were concerned that Si treatments based on application of AgrowSil (100% calcium carbonate equivalent) at the 2.2-3.4 Mg ha^-1 may not provide enough Si to affect plant growth and P solubility (only about 100 mg kg^-1). As a result, we increased the liming rate slightly to achieve a pH of 6.5 (4.5-5.6 Mg ha^-1 (2-2.5 ton ac^-1) based on 100 calcium carbonate equivalence) and Si treatments were changed as follow:

1) Locally available calcium silicate (AgrowSil; a lime alternative) at the rate required to raise pH to 6.5 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 same total Si rate as in treatment 1;

4) Silicic acid at a total Si rate of 2 Mg ha^-1;

5) Silicic gel/amorphous silica at a total Si rate of 2 Mg ha^-1;

6) Unamended control (no Si).

Calcitic lime (CaCO₃) was applied to all pots, except those receiving the agronomic CaSiO₃ treatment (treatment 1), to achieve pH 6.5 at rates based on the initial soil test. Nitrogen, potassium, and manganese (Table 2) were also applied to soils at University of Delaware recommended rates based on initial soil test (N based on target yield) to ensure optimum growth of winter wheat.

Pot Preparation and Soil Sampling

Lime, nutrient, and Si sources were thoroughly mixed into all three soils using a mixer and transferred to pots. Each pot was then thoroughly wetted on 16 Nov 2015 and left fallow for a week to allow activation of applied fertilizers and lime. Soils were sampled using a hand probe on 20 Nov 2015 to evaluate Si and P dynamics right after the application of Si. Soil subsamples were collected from three random locations in each pot and mixed to create a composite sample for each pot. Soil samples were then air-dried and sieve through 2-mm filter, and analyzed for WEP, WESi, AAP, and AASi, as described previously. Winter wheat was planted to achieve a seeding rate of 400 seeds m^-2 (equal to 25 seeds per pot) in greenhouse (to achieve quick and uniform germination) on 24 Nov 2015.

Our initial results (4 days after Si incorporation) indicated that Si application went beyond liming effect and increased WEP compared to the no Si control for many of the treatments (Table 3). Application of AgrowSil at optimum lime rate and silicic acid at 2 Mg Si ha^-1 significantly increased WEP in all three soils compared with no Si control. Application of silica gel at 2 Mg Si ha^-1 significantly increased WEP in Ft. Mott-Henlopen and Mullica-Berryland soils series compared with no Si controls; but had no significant effect on Ingleside- Hammonton soil series. We expect that the Si treatments will have more significant effects on soil P availability once more time is allowed for the Si to react with soil. The WESi indicated that, when applied at the same Si rate, silicic acid released slightly more Si compared with silica gel; while Agrowsil released about the same amount of Si as silicic acid. The commercial liming material (AgrowSil) seems to be an effective-releasing Si source.

• Table 1. Initial soil Properties of three legacy P soils.
• Table 2. Summary of N, K, and Mn applications to three legacy P soils before seeding of winter wheat
• Table 3. Effect of Si treatments on water extractable P and Si in three legacy soils sampled on 20 Nov 2015.
• Reference List

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 Fe and 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.

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