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Ontario Grain Farmer Magazine

Enhancing soil health and phosphorus-use efficiency for crop production with management of organic amendments

Enhancing soil health and phosphorus use efficiency for crop production with management of organic amendments

Principal Investigator: Paul Voroney

Research Institution: University of Guelph (U of G)

Timeline: April 2019 – March 2024  

Objectives:

  • To determine ability of various organic amendments to complex with nutrients alone and in soils amended with organic amendments.
  • To determine the effects of various organic amendments on the chemistry of phosphorus (P) in calcareous soils.
  • To determine the availability of P to corn and soybeans in calcareous soils amended with various organic amendments relative to that of inorganic P fertilizer.
  • To determine the appropriate soil P test for local calcareous soils treated with various organic amendments compared to that with inorganic P fertilizers for maintaining grain yields, while minimizing risks of runoff P.

Impacts:

  • By providing detailed characteristics of organic matter from various organic amendments and its effects on soil fertility, especially related to soil P chemistry, provide better recommendations for an evidence-based policy to guide farmer’s decisions for soil P fertility management.
  • Produce a comprehensive evaluation of the use of different organic amendments for effective corn and soybean growth in Ontario’s different types of calcareous soils to improve recommendations for usage of organic amendments, which will result in reduced P fertilizer costs for farmers.
  • Provide guidance on the use of different soil P test methods for calcareous soils amended with various organic amendments and to estimate P losses from farmlands to provide more effective P management in agricultural soils.
  • Provide information on a mechanistic role of various organic matter on soil P chemistry to provide guidance to manage organic matter amendments to soils.

Scientific Summary:

Organic amendments provide both plant-available nutrients and organic matter (OM) to soils for growing corn and soybeans, enhancing soil physical and biological properties and fertility. However, these amendments can be an environmental concern when improperly managed, especially phosphorus (P). Phosphorus-enriched runoff from agricultural land continues to receive attention due to the critical impact P has on water quality. Organic amendments, which typically contain high concentrations of calcium and P in addition to OM, affect soil P chemistry by altering both the amounts and distribution of the various P forms. Recent studies have shown that the presence of OM delays formation of stable P forms in soils. If OM favors the formation of labile P forms over stable P forms in calcareous soils, soil P fixing can be delayed and therefore excess P fertilization should be avoided. While the benefits of OM on soil health are well known, the role of OM in governing soil P chemistry is not well understood.

This project sought to clarify the mechanistic role of OM on the chemical behavior of soil and fertilizer P by analyzing the capabilities of OM to complex with nutrients including P and determine if OM prevents plant available P from being transformed into stable plant unavailable P forms. It also investigated how various organic amendments affect corn and soybean growth and soil P chemistry compared to inorganic P fertilizers in both greenhouse and field settings. In addition, Ontario’s current soil P test in soils amended with organic amendments was examined for validation. The project also provided critical scientifically-based information for establishing effective P management strategies for agricultural grain production in Ontario, thereby leading to reduced fertilizer costs for farmers while improving soil health and minimizing the risks of P to water quality.

Results:

Objective 1: To determine ability of various organic amendments to complex with nutrients alone and in soils amended with organic amendments.

The core findings from this objective were:

  1. Humic acids (HA) in soils receiving liquid and composted swine manure were more humified, that is more stable, than those of the solid manure and the control (inorganic fertilizer).
  2. Carboxyl-rich aliphatic structures, derived from microbial activity, are the most important class of biomolecules contributing to the liquid and solid swine manure HA, whereas lignin-like structures, derived from crop residues, are the most important class of biomolecules in the HA extracted from soil receiving composted swine manure and in the control (inorganic fertilizer).
  3. Aliphatic structures susceptible to biodegradation were formed in the HA from the solid manure treatment whereas more biologically resistant aromatic structures were formed in the HA of the control (inorganic fertilizer).
  4. The HA derived from liquid swine manure amendments have the highest potential to enhance plant nutrient availability.

Objective 2: To determine the effects of various organic amendments on the chemistry of P in calcareous soils.

The core findings from this objective were:

  1. All organic amendments increased plant available P, with composted and solid swine manure being the highest.
  2. The organic matter associated with the manures helped to maintain soil P availability, that is, reduced conversion to plant unavailable forms.
  3. Liquid swine manure may be a better form of swine manure for use as a soil amendment.

Objective 3: To determine the availability of P to corn and soybeans in calcareous soils amended with various organic amendments relative to that of inorganic P fertilizer.

The core findings from this objective were:

  1. Most of the P in the organic amendments was as inorganic P, with source-separated organics and turkey litter compost primarily contained in Ca-P and Mg-P forms and associated with aluminum (AI) in the municipal biosolid pellets.
  2. Corn plant height was reduced in soil receiving organic amendments compared to fertilizer P for the first 45 d, but thereafter was the same; however, grain yields were lower.
  3. Future research should identify the cause of the organic amendment effects on early plant growth. The reduced early plant growth has been attributed to delayed nutrient release; however, there is potential for volatile organic acids contained in the organic amendments or produced during early stages of decomposition to be phytotoxic. Stimulation of plant root growth has also been observed.

Objective 4: To determine the appropriate soil P test for local calcareous soils treated with various organic amendments compared to that with inorganic P fertilizers for maintaining grain yields, while minimizing risks of runoff P.

The core findings from this objective were:

  1. The modified Morgan method is more convenient analytically to the Olsen P method, which requires an acid addition to the sample extract prior to analysis, whereas the ammonium acetate soil extract can be analyzed directly. Preliminary inspection of the data suggests that it extracts 1.8 times more soil inorganic P than the Olsen method. However, the relationship between the Olsen method and the modified Morgan method was not linear, likely reflecting differences in inorganic P extractability from organic amendments. Future research should compare the inorganic P extracted from organic amendments using these two methods.

Recommendations:

Organic amendments from various sources can provide sufficient plant available P for agricultural crop production. Depending on the source of the organic amendment and the costs associated with transport and field application, there is potential to eliminate use of conventional fertilizer P. Rate of application of organic amendments should be based on their potential P supply and not N supply.

Organic amendments enhance the chemistry of soil, increasing soil cation exchange capacity, increasing overall plant nutrient availability (N, K, micronutrients) and buffering changes in soil pH.

Organic amendments provide substantial inputs of organic C and N which stimulate soil microbial activity, thereby enhancing beneficial soil properties (i.e., aggregation of soil mineral particles, and promotion of soil health).

Organic amendments provide a significant input of organic C and N that contribute to formation of soil humic substances and sequestered organic C.

  1. Conduct regular soil P tests (every three years) to assess foil fertility status. Rates of fertilizer P input, either as conventional fertilizer or organic amendment, should be based on the soil P test and OMAFRA recommendations.
  2. Conduct regular measurements of the nutrient content of organic amendments: total N, total P.
  3. Allow a minimum of two weeks after organic amendment application before planting.

External Funding Partners:

Ontario Ministry of Agriculture, Food and Rural Affairs (2018) New Directions Research Program

We acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC).