Add nitrate and ammonium nitrogen parameters to fertilizer form

[coderabbitai]

Description

Add support for nitrate nitrogen (p_no3_rt) and ammonium nitrogen (p_nh4_rt) parameters to the fertilizer form default values.

Context

This was suggested by @gerardhros in the context of expanding fertilizer parameter support in the form schema and default values.

Current State

The fertilizer form currently includes various nitrogen-related parameters like:

• p_n_rt (total nitrogen)
• p_n_if (inorganic nitrogen fraction)
• p_n_of (organic nitrogen fraction)
• p_n_wc (nitrogen working coefficient)

Requested Enhancement

Add the following parameters:

• p_no3_rt - Nitrate nitrogen content
• p_nh4_rt - Ammonium nitrogen content

These should be added to:

1. The fertilizer form schema
2. The default values in the form initialization
3. Any related parameter descriptions

References

• PR: Add comprehensive support for fertilizer application methods and metadata #168
• Comment: Add comprehensive support for fertilizer application methods and metadata #168 (comment)
• Requested by: @gerardhros