Study Notes on Phosphorus

The chapter covers both phosphorus (P) dynamics in soil and the application of phosphorus fertilizers.
Focus on:
- Phosphorus cycle in soils
- Availability to plants
- Measurement techniques for phosphorus
- Issues related to phosphorus fixation in soils
- Sources of phosphorus and their characteristics

The cycle incorporates two main components:
- Mineral Part of Soil: Contributes inorganic phosphorus
- Organic Part of Soil: Releases phosphorus through mineralization
Phosphorus is taken up by plants mainly from the soil solution, typically in forms of:
- $H2PO4^-$ (dihydrogen phosphate)
- $HPO_4^{2-}$ (hydrogen phosphate)
The dominant form is affected by soil pH, determining phosphorus availability.

Availability is influenced by fixation, which is a major problem across different soils due to the following:
- Rapid absorption of phosphorus by soil particles.
- Processes of phosphorus desorption from soil.
When phosphorus becomes available through:
- Weathering of minerals in soil, leading to inorganic phosphorus.
- Mineralization of organic matter, releasing phosphorus into solution.

In laboratory conditions:
- Phosphorus is added to soil and incubated; comparisons are made against controls without added phosphorus.
- After incubation, concentrations are measured after extraction with water.
Observations: In coastal plain soils, about 15-20% of phosphorus is absorbed within a week. In Piedmont soils, this can rise to 50-60%.

The phosphorus in the soil solution must be replenished multiple times a day (approx. 10 times) due to plant uptake.
Common measurement errors exist in available phosphorus tests, as direct quantities measured do not represent actual availability.
The organic phosphorus is mineralized by microorganisms, leading to either:
- Immobilization: Microbial biomass trapping phosphorus, making it unavailable.
- Releasing: Organic matter minerals release phosphorus into the solution.

The fraction of phosphorus as $H2PO4^-$ or $HPO4^{2-}$ changes with soil pH between 7 and higher alkalinity; typically at pH 7, half is $H2PO4^-$ and half $HPO4^{2-}$ .
Required concentrations for plant absorption range from 0.003 to 0.3 mg P/L.
Different crops require varying concentrations of phosphorus:
- For instance, lettuce requires approx. 0.3 PPM, whereas wheat and corn need significantly lower concentrations.

Primary mechanism of phosphorus movement towards plant roots is diffusion.
The distance of effective diffusion is around 1.5 to 2 inches from the fertilizer source, primarily due to concentration gradients created by plant uptake.
Factors influencing diffusion include:
- Soil moisture
- Temperature
- Soil structure (tortuosity)

Fixation is defined as the retention of phosphorus in forms that are not available to plants. It arises from two processes:
- Absorption: Fast (days), where phosphorus attaches to soil particle surfaces.
- Precipitation: Slow (months), where phosphorus forms stable, insoluble compounds.
Some soils exhibit high fixation capacity due to presence of clay and iron/aluminum oxides which bond with phosphorus, rendering it unavailable.

The main sources of phosphorus typically include:
- Animal manures
- Phosphate rock fertilizers
After application, the phosphorus undergoes a sequence of transformations into more complex, less available forms.

Phosphorus Mineralization:
- Most organic phosphorus is in the form of inositol phosphates, predominant in grains.
Soil microorganisms release phosphatases, enzymes that help release available phosphorus from organic matter during mineralization.
Isotopic Dilution Studies: Used for quantitatively analyzing mineralization rates with radioactive isotopes (e.g., $^{32}P$ ) to measure mineralization more effectively without immediate absorption interference.

Organic matter plays a dynamic role in phosphorus availability:
- Coats soil minerals, reducing fixation.
Carbon to Phosphorus Ratios determine mineralization vs. immobilization:
- C:P < 200 indicates mineralization.
- C:P > 300 indicates immobilization of phosphorus.

Different types of manures lead to varying mineralization rates:
- Control soils (no fertilizers): 0.7 to 2.2 PPD (parts per day)
- Manured soils: 2.7 PPD
Important to understand the organic matter’s impact on regulating phosphorus availability and the microbiological requirements for decomposition.

Key factors affecting phosphorus fixation:
- Soil pH
- Presence of iron and aluminum oxides
- Degree of saturation with phosphorus affects how effective absorption is.
- Presence of organic acids from decomposing organic matter competes with phosphorus for adsorption sites.

Understanding the dynamics of phosphorus in soil involves integrating cycling, uptake, fixation, and chemical reactions.
The rapid absorption and slow precipitation create significant retention challenges for plant uptake.
Long-term fertility strategies should take into account the complex interactions between organic matter, pH, and soil dynamics to optimize phosphorus availability.