description_osi_phosphor.Rmd

---
title: "Description of osi_phosphor"
author: 'Elise van Eynde & Gerard Ros'
date: "`r Sys.Date()`"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Description_osi_phosphor}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

This document explains how the phosphorus indicator is calculated for each country and provides references


## Netherlands:



## Belgium (Flanders): 

## France:

Principle: The P-olsen is evaluated based on (1) the crop needs (high need, moderate need, low need) which is mentioned in the osi_crops table (2) the region (3) the soil type 

Two thresholds exist     (1) Timp = soil supplies the crop need, above this concentration no fertilizer response
                         (2) Trenf = below this threshold, soil needs to be replenished, fertilizer recommendation > supply 

The logistic function is based on the P Olsen evaluation in comparison with the export by the plant: score 0 when supply = 0; score 1 when supply = export
The growth rate and offset are based on the threshold values, the growth rate near the maximum is set to 1. 

See page 25 in reference. !!! Threshold values are given in mg/kg P2O5, conversion is done by =(Treshold/(141.943x10^3))x2x30.97*10^3

Reference: Comifer, 2019, La fertilisation P-K-Mg; Les bases due raisonnement