This tutorial is intended to show how to deploy the
`mod_alcc()`

function for estimating critical soil test
values using the Modified Arcsine-Log Calibration Curve, originally
introduced by Dyson & Conyers (2013) and modified by Correndo et
al. (2017). This function produces the estimation of critical soil test
values (CSTV) for a target relative yield (`ry`

) with
confidence intervals at adjustable confidence levels. Behind the scenes,
the `mod_alcc()`

is based on the standardized major axis
(SMA), a bivariate regression model that assumes that both axis are
random variables (Warton et al., 2006).

- Load your dataframe with soil test value (
`stv`

) and relative yield (`ry`

) data. - Specify the following arguments into the function
`mod_alcc()`

`data`

(optional)`stv`

(soil test value) and`ry`

(relative yield) columns or vectors,`target`

of relative yield (e.g. 90%),desired

`confidence`

level (e.g. 0.95 for 1 - alpha(0.05)).- Used for the estimation of critical soil test value (CSTV) lower and upper limits.

`tidy`

TRUE (produces a data.frame with results) or FALSE-default- (store results as list),`plot`

TRUE (produces a ggplot as main output) or FALSE (default; no plot, only results as data.frame or list)

- Run and check results.
- Check residuals plot, and warnings related to potential leverage points.
- Adjust curve plots as desired with additional
`ggplot2`

functions.

Suggested packages

```
# Install if needed
library(ggplot2) # Plots
library(dplyr) # Data wrangling
library(tidyr) # Data wrangling
library(purrr) # Mapping
```

This is a basic example using three different datasets:

`mod_alcc()`

RY target = 90%, confidence level = 0.95, replace with your desired values

`tidy`

= FALSEIt returns a LIST (may be more efficient for multiple fits at once)

```
# Using dataframe argument, tidy = FALSE -> return a LIST
mod_alcc(data = corr_df, ry = RY, stv = STK, target=90, confidence = 0.95,
tidy = TRUE)
#> Warning: One or more original RY values exceeded 100%. All RY values greater
#> than 100% have been capped to 100%.
#> Warning: 2 STV points exceeded the CSTV for 100% of RY.
#> Risk of leverage. You may consider a sensitivity analysis by removing extreme points,
#> re-run the mod_alcc(), and check results.
#> Warning: 2 STV points exceeded two-times (2x)
#> the CSTV for 90% of RY. Risk of leverage. You may consider a sensitivity analysis by
#> removing extreme points, re-run the mod_alcc(), and check results.
#> # A tibble: 1 × 18
#> n r RMSE_alcc AIC_alcc AIC_sma BIC_sma p_value confidence target CSTV
#> <int> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 24 0.728 10.3 186. 17.0 22.9 5.57e-5 0.95 90 76.9
#> # ℹ 8 more variables: LL <dbl>, UL <dbl>, CSTV90 <dbl>, n.90x2 <int>,
#> # CSTV100 <dbl>, n.100 <int>, Curve <list>, SMA <list>
```

`tidy`

= TRUEIt returns a data.frame (more organized results)

```
# Using dataframe argument, tidy = FALSE -> return a LIST
mod_alcc(data = corr_df, ry = RY, stv = STK, target=90, confidence = 0.95, tidy = TRUE)
#> Warning: One or more original RY values exceeded 100%. All RY values greater
#> than 100% have been capped to 100%.
#> Warning: 2 STV points exceeded the CSTV for 100% of RY.
#> Risk of leverage. You may consider a sensitivity analysis by removing extreme points,
#> re-run the mod_alcc(), and check results.
#> Warning: 2 STV points exceeded two-times (2x)
#> the CSTV for 90% of RY. Risk of leverage. You may consider a sensitivity analysis by
#> removing extreme points, re-run the mod_alcc(), and check results.
#> # A tibble: 1 × 18
#> n r RMSE_alcc AIC_alcc AIC_sma BIC_sma p_value confidence target CSTV
#> <int> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 24 0.728 10.3 186. 17.0 22.9 5.57e-5 0.95 90 76.9
#> # ℹ 8 more variables: LL <dbl>, UL <dbl>, CSTV90 <dbl>, n.90x2 <int>,
#> # CSTV100 <dbl>, n.100 <int>, Curve <list>, SMA <list>
```

You can call `stv`

and `ry`

vectors using the
`$`

.

The `tidy`

argument still applies for controlling the
output type

```
fit_vectors_tidy <- mod_alcc(ry = corr_df$RY,
stv = corr_df$STK,
target = 90,
confidence = 0.95)
#> Warning: One or more original RY values exceeded 100%. All RY values greater
#> than 100% have been capped to 100%.
#> Warning: 2 STV points exceeded the CSTV for 100% of RY.
#> Risk of leverage. You may consider a sensitivity analysis by removing extreme points,
#> re-run the mod_alcc(), and check results.
#> Warning: 2 STV points exceeded two-times (2x)
#> the CSTV for 90% of RY. Risk of leverage. You may consider a sensitivity analysis by
#> removing extreme points, re-run the mod_alcc(), and check results.
fit_vectors_list <- mod_alcc(ry = corr_df$RY,
stv = corr_df$STK,
target = 90,
confidence = 0.95,
tidy = FALSE)
#> Warning: One or more original RY values exceeded 100%. All RY values greater
#> than 100% have been capped to 100%.
#> Warning: 2 STV points exceeded the CSTV for 100% of RY.
#> Risk of leverage. You may consider a sensitivity analysis by removing extreme points,
#> re-run the mod_alcc(), and check results.
#> Warning: 2 STV points exceeded two-times (2x)
#> the CSTV for 90% of RY. Risk of leverage. You may consider a sensitivity analysis by
#> removing extreme points, re-run the mod_alcc(), and check results.
```

```
# Example 1. Fake dataset manually created
data_1 <- data.frame("RY" = c(65,80,85,88,90,94,93,96,97,95,98,100,99,99,100),
"STV" = c(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15))
# Example 2. Native fake dataset from soiltestcorr package
data_2 <- soiltestcorr::data_test
# Example 3. Native dataset from soiltestcorr package, Freitas et al. (1966), used by Cate & Nelson (1971)
data_3 <- soiltestcorr::freitas1966 %>%
rename(STV = STK)
data.all <- bind_rows(data_1, data_2, data_3, .id = "id")
```

Note: the `stv`

column needs to have the same name for all
datasets if binding rows.

`map()`

```
# Run multiple examples at once with purrr::map()
data.all %>%
nest(data = c("STV", "RY")) %>%
mutate(model = map(data, ~ mod_alcc(stv = .$STV, ry = .$RY, target = 90))) %>%
unnest(model)
#> Warning: There were 9 warnings in `mutate()`.
#> The first warning was:
#> ℹ In argument: `model = map(data, ~mod_alcc(stv = .$STV, ry = .$RY, target =
#> 90))`.
#> Caused by warning:
#> ! You have not specified the confidence level.
#> Please, modify if your desired confidence is different than the default (0.95)
#> ℹ Run `dplyr::last_dplyr_warnings()` to see the 8 remaining warnings.
#> # A tibble: 3 × 20
#> id data n r RMSE_alcc AIC_alcc AIC_sma BIC_sma p_value
#> <chr> <list> <int> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 1 <tibble> 15 0.968 1.45 59.6 -23.5 -20.0 3.30e- 9
#> 2 2 <tibble> 137 0.716 10.5 1038. -1.04 13.6 7.31e-23
#> 3 3 <tibble> 24 0.728 10.3 186. 17.0 22.9 5.57e- 5
#> # ℹ 11 more variables: confidence <dbl>, target <dbl>, CSTV <dbl>, LL <dbl>,
#> # UL <dbl>, CSTV90 <dbl>, n.90x2 <int>, CSTV100 <dbl>, n.100 <int>,
#> # Curve <list>, SMA <list>
```

Note: the output table is still generated despite warnings regarding confidence level and leverage points.

`group_modify()`

Alternatively, with `group_modify`

, nested data is not
required. However, it still requires a grouping variable (in this case,
`id`

) to identify each dataset. `group_map()`

may
also be used, though `list_rbind()`

is required to return a
tidy data frame of the model results instead of a list.

```
data.all %>%
group_by(id) %>%
group_modify(~ mod_alcc(data = ., STV, RY, target = 90, confidence = 0.95))
#> Warning: One or more original RY values exceeded 100%. All RY values greater
#> than 100% have been capped to 100%.
#> # A tibble: 3 × 19
#> # Groups: id [3]
#> id n r RMSE_alcc AIC_alcc AIC_sma BIC_sma p_value confidence
#> <chr> <int> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 1 15 0.892 1.79 66.1 -82.7 -79.1 7.84e- 6 0.95
#> 2 2 137 0.569 11.6 1067. -290. -275. 4.14e-13 0.95
#> 3 3 24 0.708 20.0 218. -5.01 0.885 1.10e- 4 0.95
#> # ℹ 10 more variables: target <dbl>, CSTV <dbl>, LL <dbl>, UL <dbl>,
#> # CSTV90 <dbl>, n.90x2 <int>, CSTV100 <dbl>, n.100 <int>, Curve <list>,
#> # SMA <list>
```

Bootstrapping is a suitable method for obtaining confidence intervals for parameters or derived quantities. Bootstrapping is a resampling technique (with replacement) that draws samples from the original data with the same size. If you have groups within your data, you can specify grouping variables as arguments in order to maintain, within each resample, the same proportion of observations than in the original dataset.

This function returns a table with as many rows as the resampling size (n) containing the results for each resample.

```
set.seed(123)
boot_alcc <- boot_mod_alcc(data = corr_df,
stv = STK, ry = RY,
target = 90, n = 500)
#> Warning: There were 1332 warnings in `dplyr::mutate()`.
#> The first warning was:
#> ℹ In argument: `model = map(...)`.
#> ℹ In group 1: `boot_id = 1`.
#> Caused by warning:
#> ! One or more original RY values exceeded 100%. All RY values greater
#> than 100% have been capped to 100%.
#> ℹ Run `dplyr::last_dplyr_warnings()` to see the 1331 remaining warnings.
boot_alcc %>% head(n = 5)
#> # A tibble: 5 × 14
#> boot_id n r RMSE_alcc AIC_alcc AIC_sma BIC_sma p_value target CSTV
#> <dbl> <int> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 1 24 0.508 12.4 195. 11.9 17.7 0.0112 90 81.9
#> 2 2 24 0.750 9.50 182. 7.68 13.6 0.0000240 90 61.8
#> 3 3 24 0.625 10.6 188. 10.6 16.5 0.00109 90 86.6
#> 4 4 24 0.688 8.96 179. 16.1 22.0 0.000204 90 72.4
#> 5 5 24 0.593 9.74 183. 13.3 19.2 0.00225 90 80.9
#> # ℹ 4 more variables: CSTV90 <dbl>, n.90x2 <int>, CSTV100 <dbl>, n.100 <int>
# CSTV Confidence Interval
quantile(boot_alcc$CSTV, probs = c(0.025, 0.5, 0.975))
#> 2.5% 50% 97.5%
#> 63.70241 77.17436 93.90476
# Plot
boot_alcc %>%
ggplot2::ggplot(aes(x = CSTV))+
geom_histogram(color = "grey25", fill = "steelblue", bins = 10)
```

We can generate a ggplot with the same `mod_alcc()`

function. We just need to specify the argument
`plot = TRUE`

.

As ggplot object, plots can be adjusted in several ways, such as modifying titles and axis scales.

Behind the scenes, the `mod_alcc()`

runs a Standardized
Major Axis regression (SMA).

We can extract the SMA fit and also check the residuals of this model out as follows:

```
fit_3 <- mod_alcc(data = corr_df, ry = RY, stv = STK, target = 90)
# Extract SMA regression fit and residuals from fit_3 (data_3, (Freitas et al., 1966))
SMA_freitas_1966 <- fit_3$SMA %>% as.data.frame()
SMA_freitas_1966 %>%
ggplot(aes(x = arc_RY, y = ln_STV))+
ggtitle("SMA Regression. Dataset 3")+
geom_point(shape=21, fill = "orange", size = 3, alpha = 0.75)+
#SMA Line
geom_path(aes(x=arc_RY, y = SMA_line, linetype = "SMA_fit"), linewidth = 1.5, col = "grey25")+
scale_linetype_manual(name="", values = c("solid"))+
#Critical value
geom_vline(xintercept = 0, col = "grey10", size = 1.25, linetype = "dashed")+
theme_bw()+
# Axis titles
labs(y = "ln_STV", y = "asin(sqrt(RY))-centered")
```

```
# Residuals plot
SMA_freitas_1966 %>%
ggplot(aes(x = fitted_axis, y = residuals))+
ggtitle("Residuals SMA. Dataset 3")+
geom_point(shape=21, fill = "orange", size = 3, alpha = 0.75)+
geom_hline(yintercept = 0, col = "grey10", linewidth = 1.25, linetype = "dashed")+
theme_bw()+
# Axis titles
labs(x = "Fitted Axis -SMA- (see Warton et al. 2006)", y = "Residuals (STV units)")
```

*Correndo, A.A., Salvagiotti, F., García, F.O. and Gutiérrez-Boem,
F.H., 2017. A modification of the arcsine–log calibration curve for
analysing soil test value–relative yield relationships. Crop and Pasture
Science, 68(3), pp.297-304. 10.1071/CP16444*

*Dyson, C.B., Conyers, M.K., 2013. Methodology for online
biometric analysis of soil test-crop response datasets. Crop &
Pasture Science 64: 435–441. 10.1071/CP13009*

*Warton, D.I., Wright, I.J., Falster, D.S., Westoby, M., 2006.
Bivariate line-fitting methods for allometry. Biol. Rev. Camb. Philos.
Soc. 81, 259–291. 10.1017/S1464793106007007*