Function to calculate the S index (Dexter, 2004) for evaluating the soil
physical quality based on the *Water Retention Curve* (van Genuchten, 1980).

* S = - n * (θ_S - θ_R) * (1 + 1/m)^{-(1 + m)} *

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`theta_R` |
the residual water content. |

`theta_S` |
the water content at saturation. |

`alpha` |
a scale parameter of the van Genuchten's formula. |

`n` |
a shape parameter in van Genuchten's formula. |

`m` |
a shape parameter in van Genuchten's Formula. Default is |

`vcov` |
optional (default is |

`nsim` |
the number of Monte Carlo simulations; default is 999. It is used only if |

`conf.level` |
the confidence level; default is 0.95. It is used only if |

`graph` |
logical; if TRUE (defaul), the soil water retention curve is plotted. |

`...` |
further graphical arguments. |

A list of

`h_i ` |
the modulus of the water potential at the inflection point. |

`theta_i ` |
the water content at the inflection point. |

`S.index ` |
the modulus of the S index. |

`PhysicalQuality ` |
A character indicating the soil physical quality, as proposed by Dexter (2004). |

`simCI ` |
the simulated confidence interval. It is stored only if |

`conf.level ` |
the confidence level for the simulated confidence interval. It is stored only if |

Anderson Rodrigo da Silva <anderson.agro@hotmail.com>

Dexter, A. R. (2004). Soil physical quality Part I. Theory,
effects of soil texture, density, and organic matter,
and effects on root growth. *Geoderma*, 120:201-214.

Genuchten, M. T. van. (1980). A closed form equation for predicting the
hydraulic conductivity of unsaturated soils. *Soil Science Society of
America Journal*, 44:892-898.

Mualem, Y. (1976). A new model for predicting the hydraulic conductivity
of unsaturated porous media, *Water Resource Research*, 12:513-522.

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