Variable-rate nitrogen from balance + NDVI — variable_rate

Bridges the constant-rate output of N_balance() / calculate_N_fertilization() or scheda_N() with the spatial NDVI-based variable-rate estimators of NFert (estimate_N_rate_from_calibration_curve() and estimate_N_rate_from_holland_schepers()).

Usage

variable_rate_N(
  ndvi_raster,
  n_dose,
  method = c("calibration", "holland"),
  envelope = 0.25,
  minN = NULL,
  maxN = NULL,
  mas_cap = NULL,
  plot = FALSE
)

Arguments

ndvi_raster: A raster::RasterLayer with NDVI (0–1), or a RasterBrick / RasterStack (layer NDVI if present, otherwise the first layer).
n_dose: Numeric. Field-average N dose (kg/ha) from N_balance() + calculate_N_fertilization(), scheda_N()$dose_final, or any custom target.
method: Either "calibration" (default, two-point linear) or "holland" (Holland & Schepers sufficiency index).
envelope: Numeric in (0, 1). Half-amplitude of the dose range around n_dose for the calibration method. Default 0.25 (i.e. minN = 0.75 * n_dose, maxN = 1.25 * n_dose).
minN, maxN: Optional explicit override of the envelope (in kg/ha).
mas_cap: Numeric or NULL. If provided, caps the per-pixel rate so it never exceeds the MAS limit (e.g. from get_MAS()).
plot: Logical, passed to the underlying estimator. Default FALSE.

Value

A list with:

rate_raster: RasterLayer of N rate per pixel (kg/ha).
mean_kg_ha: Mean rate over non-NA pixels (kg/ha).
min_kg_ha, max_kg_ha: Min and max rate over non-NA pixels.
n_dose_input: The input field-average N dose.
method: The method used.

Details

Two integration patterns are supported:

Calibration curve (method = "calibration"): the agronomic balance determines the target field-average dose; the function then derives a minN/maxN envelope around it (default +/- 25%) and applies the linear NDVI-driven calibration (low NDVI = more N, high NDVI = less N).
Holland & Schepers (method = "holland"): the agronomic balance is used as the base N rate of the H&S sufficiency-index algorithm.

The output preserves the field-average dose computed by the balance: the raster of variable rates integrates (mean) to approximately the input n_dose, ensuring the agronomic constraint (MAS, ZVN) is respected.

Examples

if (FALSE) { # \dontrun{
library(raster)
library(NFert)
# NDVI raster
data(s2.rast)
ndvi <- s2.rast

# 1) Compute agronomic dose with the balance
bal <- N_balance(expected_yield_tons_ha = 6,
                 crop = "Grano duro (pianta intera)",
                 ccp  = "Spring-summer crop 100-130 days",
                 sand = 15.5, clay = 18.5,
                 Ntot = 1.5, SOM = 2, CN = 7.73,
                 oxygen_availability = "Normal",
                 winter_rain = 150, start_spring_rain = 0,
                 prev_crop = "Maize stalks removed",
                 source = "None", fertorg_frequency = "every year",
                 location = "Plain adjacent to urbanized areas",
                 forg_quantity = 0)
n_dose <- calculate_N_fertilization(bal)   # ~142 kg/ha

# 2) Spatialise via NDVI calibration
vr <- variable_rate_N(ndvi, n_dose = n_dose, method = "calibration",
                      mas_cap = get_MAS("Grano duro (pianta intera)")$N_max)
raster::plot(vr$rate_raster)
vr$mean_kg_ha
} # }