Series 19 Nutrient Cycling VIII Phosphorus Cycle with Pools and Fluxes

Summary

This lecture from Cornell University delves into the phosphorus cycle, a crucial aspect of soil fertility and plant nutrition. The speaker explains the different pools of phosphorus in the environment, including ocean sediments, soils, and biota. Emphasis is placed on the complexities of phosphorus availability, which is highly influenced by soil pH and its interaction with calcium, iron, and aluminum. Fertilizer use, microbial activity, and the role of organic and inorganic phosphorus forms are key topics. The lecture provides insights into managing phosphorus to ensure effective plant uptake and sustainable agricultural practices.

Takeaways

• 😀 Phosphorus is an essential macronutrient for plants, crucial for energy production, DNA, RNA, and cell membranes.
• 😀 Phosphorus is found in small amounts in plant biomass, but it is vital for many biological functions despite its low concentration.
• 😀 The primary sources of phosphorus in the environment are ocean sediments, soils, and mineral rocks (such as phosphate rock).
• 😀 Soil is the main pool of phosphorus that plants rely on for growth, but much of it is bound in forms that plants cannot easily access.
• 😀 Phosphorus can exist in various forms in the soil, such as organic phosphorus, calcium-bound phosphorus, and iron/aluminum-bound phosphorus.
• 😀 Phosphorus is primarily available to plants in its inorganic form, which can change depending on the pH of the soil.
• 😀 Soil pH significantly affects phosphorus availability—at low pH (acidic), phosphorus binds to iron and aluminum; at high pH (alkaline), it binds to calcium, making it unavailable.
• 😀 In agricultural practices, phosphorus is often added to the soil as fertilizer, but only 10-15% of the phosphorus added is taken up by plants.
• 😀 The phosphorus cycle in the soil involves complex fluxes, including uptake by plants, immobilization in organic matter, and mineralization back into the soil solution.
• 😀 Phosphorus availability to plants can be influenced by both biological processes (such as microbial activity) and chemical reactions (such as calcium binding in alkaline soils).

Q & A

Why is phosphorus considered a macronutrient despite being present in small quantities in plant biomass?

Phosphorus is a macronutrient because it plays an essential role in key biological processes such as energy transfer (ATP), DNA and RNA formation, and cell membrane structure, even though it is only present in small amounts in plant biomass.

What are the main phosphorus pools in the environment, and where is most of the phosphorus located?

The major phosphorus pools in the environment include ocean sediments, soils, and mineral rocks. Most of the phosphorus is located in ocean sediments and soils, with soil being the primary pool for plant availability.

How does phosphorus availability to plants change with soil pH?

Phosphorus availability is highest when soil pH is near neutral (around 6.5). In acidic soils (low pH), phosphorus binds to iron and aluminum, making it unavailable to plants. In alkaline soils (high pH), phosphorus binds to calcium, also reducing its availability.

What are the forms of phosphorus that are available for plant uptake?

Phosphorus must be in an inorganic form to be taken up by plants. Common forms include phosphate ions (H2PO4-) and phosphate ions (PO4^3-) in the soil solution, which are accessible to plants.

Why is phosphorus often unavailable to plants, even if it is present in soil?

Phosphorus is often unavailable because it is bound in insoluble forms in the soil, such as organic phosphorus or inorganic phosphorus bound to iron, aluminum, or calcium, which plants cannot absorb easily.

What role do fertilizers play in phosphorus availability, and how efficient are they?

Fertilizers are used to add phosphorus to the soil, making it more available to plants. However, only 10-15% of the phosphorus applied through fertilizers is typically taken up by plants, with the rest either remaining in the soil or leaching away.

How do microbial populations interact with phosphorus in the soil?

Microbial populations can immobilize phosphorus in the soil by taking it up into their biomass. When these organisms die or are decomposed, the phosphorus is released back into the soil, potentially becoming available again to plants.

What happens to phosphorus when it is bound with calcium in alkaline soils?

When phosphorus binds with calcium in alkaline soils, it forms calcium phosphate, which is typically insoluble and unavailable for plant uptake. This can limit the availability of phosphorus in such soils.

Why is phosphorus considered a non-renewable resource in the context of fertilizers?

Phosphorus is considered a non-renewable resource because the primary source of phosphorus for fertilizers comes from mined rock phosphate, a finite resource that takes millions of years to form.

What is the primary factor influencing the fluxes of phosphorus in the soil, and how does it affect soil fertility?

The primary factor influencing phosphorus fluxes in the soil is the soil pH, as it controls the solubility and binding of phosphorus to various minerals. Understanding these fluxes is crucial for managing soil fertility and ensuring that phosphorus remains available for plant uptake.