Introduction

Accurate, timely, and representative in-situ observations across large areas have always been needed to report statistics on land use, land cover, and the environment. Precise geo-located in-situ information is also indispensable to train and validate algorithms that characterize the Earth’s surface based on remotely sensed observations. Comprehensive and thematically rich in-situ data can lead to better classifiers and more accurate multi-temporal land surface mapping.

The Land Use/Cover Area frame Survey (LUCAS) in the European Union (EU) was set-up exactly to provide such statistical information2. It represents a triennial in-situ land cover and land use data collection exercise that extends over the whole of the EU’s territory Lucas Dataset

LUCAS collects information on land cover and land use, agro-environmental variables, soil, and grassland. The surveys also provide spatial information to analyse the mutual influences between agriculture, environment, and countryside, such as irrigation and land management. For this exercise, I will focus on the soil monitoring module. LUCAS Soil is one of the world’s largest and most comprehensive, harmonized continental-scale soil databases to our knowledge because of the range of properties analysed. Furthermore, it is an open-access tool, with data freely available from the European Soil Data Centre (ESDAC).

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Why Soil is so important?

Soil is critical for terrestrial life: acting as a water filter, nutrient giver, and habitat for billions of organisms that make up a diverse ecosystem.

Schematic diagram of soil functions. This diagram is part of an uncommented “infographics” on soil functions put together by the FAO, with the subtitle “Soils deliver ecosystem services that enable life on earth.”

When it’s healthy, it provides nutrients that feed our crops, supplies humans with antibiotics that fight diseases,and its self-sustaining cycle can regenerate for decades. Healthy soil helps to mitigate climate change by acting as a carbon sink – meaning it stores vast quantities of carbon dioxide and other greenhouse gasses (GHGs) that would otherwise be released. In fact, soil is the second largest carbon sink after the ocean, capturing more CO2 than forests and holding three times more carbon than the Earth’s atmosphere.

Carbon Stocks: Volume of Carbon Stocks in different ecosystems

Why now?

Soil and Climate Change

Climate change has had a significant impact on soil health globally. Increasingly extreme weather has led to more droughts as well as more flooding. Rising sea levels can carry contaminants, like salt, into soil, as well as cause erosion. While the battle against a shifting climate presents challenge enough on its own, modern agriculture is degrading the planet’s soil at an accelerated rate.

Since soil degradation has been increasing worldwide and since it is not a renewable resource, we have realized it requires constant monitoring so that degradation can be prevented and sustainable management can be ensured.

Soil pH

Soils can be naturally acid or alkaline, and this can be measured by testing their pH value.

Having the correct pH is important for healthy plant growth. Soil pH affects the amount of nutrients and chemicals that are soluble in soil water, and therefore the amount of nutrients available to plants. Some nutrients are more available under acid conditions while others are more available under alkaline conditions. Being aware of the long-term effects of different soil management practices on soil pH is also important. Research has demonstrated that land use, land cover and some agricultural practices significantly alter soil pH.

What is pH?

A pH value is actually a measure of hydrogen ion concentration. Because hydrogen ion concentration varies over a wide range, a logarithmic scale (pH) is used: for a pH decrease of 1, the acidity increases by a factor of 10.

It is a ‘reverse’ scale in that a very acid soil has a low pH and a high hydrogen ion concentration. Therefore, at high (alkaline) pH values, the hydrogen ion concentration is low.

Soils can be classified according to their pH value:

  • 6.5 to 7.5—neutral
  • over 7.5—alkaline
  • less than 6.5—acidic,
  • and soils with pH less than 5.5 are considered strongly acidic.

Most soils have pH values between 3.5 and 10.

However, most soils in Europe range between 4.0 (very acid) and 8.5 (very alkaline). The acceptable pH range for a productive agricultural land is about 5.5 to 7.5, with a pH of 6.0 to 6.5 preferred by most of the crops.

pH and Land use

Comparison of soil pH under forest and agricultural land use

The distribution of pH values in relation to land use showed that forest soils have lower pH values, while agricultural present the higher mean values. This is probably due to liming, a traditional procedure in preparing soil for planting. It is the application of calcium- and magnesium-rich materials to soil in various forms, including marl, chalk, limestone, or hydrated lime. The primary reason to apply agricultural lime is to correct the high levels of acidity in the soil. Acid soils reduces plant growth by inhibiting the intake of major plant nutrients -nitrogen, phosphorus and potassium. Some plants, for example legumes, will not grow in highly acidic soils. However, it is important to remember that the optimum pH is not the same for all crops or soils.

pH and Land Cover

Comparison of soil pH under different land cover

Soils formed under prairie (grasses) tend to be less acid than soils formed under forests. The roots of growing plants produce CO2 and small amounts of organic acids which increase soils acidity. Moreover, the residue from coniferous(evergreen) trees is more acidic than from deciduous (broadleaf) ones.

What is a Bradleaved woodland?

Broadleaved woodland is characterized by trees which do not have needles (this is why they tend to have higher ph values). Their leaves are broad and vary in shape, and most of them are deciduous.

Tipical Broadleaved forest

Broadleaved woods differ, depending on location. The soil, altitude and any nearby water can influence the species that thrive there. No two broadleaf woodlands are the same. The geology and soil will impact the species present, and the age of a wood will greatly impact its character.

Where can you find Broadleaved woodland in Europe?

Below an heat map showing a density of distribution in Europe. Please zoom in to have a better understanding of the distribution on your area of interest.

Measuring Soil pH

Soil pH can be measured in the field using a test kit or by sending a sample to a laboratory for more accurate results. * Soil pH in the field can be measured using a simple test kit based on a colour-card method available from agricultural supply stores called the Raupach soil pH kit. The kit gives the soil pH on the water scale and should be used only as a guide to soil pH. * Analysis in a laboratory provides the most accurate measurement of soil pH. It is the best basis we can have when deciding whether or not to start an acid soil management strategy such as liming.

The two main laboratory methods use extraction in water or in calcium chloride.

Soil pH in water

This is the easiest way of measuring soil pH. ISO 10390:2005 specifies an instrumental method for the routine determination of pH using a glass electrode in a 1:5 (volume fraction) suspension of soil in distilled water (pH in H2O). Results are expressed in pH(w).

Soil pH in calcium chloride

This is the standard method of measuring soil pH. An air-dry soil sample is mixed with five times its weight of a dilute concentration (0.01M) of calcium chloride (CaCl2), shaken for 1 hour and the pH is measured using an electrode. The results are usually expressed as pH(CaCl2).

The pH(CaCl2) test is the more accurate of the two pH tests, as it reflects what the plant experiences in the soil. The values of pH(CaCl2) are normally lower than pH(w) by 0.5 to 0.9.

A useful, but not consistently accurate, conversion is to subtract 0.8 from the pH(w) to obtain a pH(CaCl2) value. Let’s see if this conversion is valid for this database.

First, we need to check if we have a linear relationship between the two extractions methods:

regression = lm(pH_CaCl2~pH_H2O, data = LUCAS)
plot(pH_CaCl2~pH_H2O, data = LUCAS, 
     main = "Linear Model", 
     xlab = "pH in the soil solution", 
     ylab = "pH in water",
     pch = 19)
abline(regression, col = "red")

it seems we have a good correlations.

## 
## Call:
## lm(formula = pH_CaCl2 ~ pH_H2O, data = LUCAS)
## 
## Residuals:
##      Min       1Q   Median       3Q      Max 
## -3.04481 -0.14578 -0.00179  0.14993  3.05357 
## 
## Coefficients:
##              Estimate Std. Error t value Pr(>|t|)    
## (Intercept) -0.841809   0.007924  -106.2   <2e-16 ***
## pH_H2O       1.046134   0.001239   844.5   <2e-16 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 0.2252 on 18981 degrees of freedom
##   (1 observation deleted due to missingness)
## Multiple R-squared:  0.9741, Adjusted R-squared:  0.9741 
## F-statistic: 7.132e+05 on 1 and 18981 DF,  p-value: < 2.2e-16

According to the coefficients of this LUCAS data-based prediction model, the rule appears to be accurate.

\[ pH(CaCl2) = 1.05 pH(w) -0.85 \]