The Soil Bundle

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I. Introduction

Soils are one of Earth’s essential natural resources yet they are often taken for granted. Most people do not realize that soils are a living, breathing world supporting nearly all terrestrial life. They hold nutrients and water for plants and animals; they filter and clean water that flows through soils. They also influence the amount of water that recharges the groundwater. The amount of water in soil is known as soil moisture and it plays a very important role in predicting the type of plants that will grow in a given area, the occurrence of floods or droughts, and in predicting the weather (soil moisture can play a large role in cloud formation).

Soils and their function within an ecosystem vary greatly from one location to another as a result of many factors, including differences in climate, animal and plant life, and the type and age of the soil.

The purpose of the Soil Bundle protocol is to provide greater knowledge on the relationship between soil characteristics and their function for different ecosystems.

II. List of the GLOBE Protocols included in the bundle

GLOBE Pedosphere (soil) Protocols

  • Bulk density
  • Soil Fertility
  • Soil Infiltration
  • Soil Moisture
  • Soil temperature
  • Soil pH

 

GLOBE Atmospheric Protocols

  • Air temperature
  • Relative humidity
  • Precipitation

 

GLOBE Land Cover Protocols

  • Land cover

 

III. Science Background

Soils are essential for life. They provide the medium for plant growth, habitat for many insects and other organisms, act as a filtration system for surface water, and serve as carbon reservoirs. In addition, the amount of water that is stored in the top layer of the soil (soil moisture) plays a critical role in predicting floods, droughts, plant growth, and the weather. Changes in soil health and soil moisture can severely impact life on Earth.

Only 25 percent of the earth's surface is made up of soil and only 10 percent of that soil can be used to grow food. The contamination and loss of healthy soils and the drying of soils due to increasing temperatures can affect food production and ecosystem function. It is therefore of extreme importance to manage and protect soils wisely.

Measurements of soil characteristics are critical to identify factors that influence the health of the soil and impact ecosystem function, as well as hydrological, meteorological, and carbon storage processes. Because soils can be very heterogeneous, the role of citizen science to characterize them is very important. We have bundled the soil protocol in order to better understand the interaction of soils on ecosystem and atmospheric processes and vice versa.

 

Table 1:Selected thematic areas related to soils along with the respective GLOBE protocol.
Thematic Category GLOBE Protocol Scientific Importance
Influence of soil moisture on weather

Atmospheric Protocols:

air temperature, relative humidity

Pedosphere Protocols:

soil moisture, soil temperature
The amount of water in the soil has the potential to evaporate (air temp. driven) and when it does it can play a large role in cloud formation, air temperature, and relative humidity. You can make important cloud observations through the GLOBE Cloud App.
Influence of weather on soil moisture

Atmospheric Protocol:

precipitation

Pedosphere Protocol:

soil moisture
Soils moisture increases after a rain event and then gradually dries out from evaporation and transpiration. The soils closer to the surface usually dry out faster than deeper soils and it may take days to weeks for the wilting point(minimal soil moisture required by the plant in order to not wilt)to be reached.
Influence of bulk density and soil texture on soil moisture and infiltration

Pedosphere Protocols:

bulk density, soil moisture, soil infiltration
How soils dry at different depths depends on the properties of the soil in each horizon. Different types of soils and the compaction of the soil will influence infiltration.
Influence of soil health on land cover and soil moisture

Pedosphere Protocols:

soil fertility, soil ph, soil moisture

Land Cover Protocol:

land cover
Soil health or the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans has a strong influence on land cover and plant growth.
Influence of land cover on soil moisture and soil temperature

Pedosphere Protocols:

soil moisture, soil temperature

Land Cover Protocol:

land cover
Land cover infuences soil properties like compaction, texture, fertility, organic material, which influence soil moisture retention.

 

III. Student Research Questions

Below are examples of research questions:

1. How do high and low amounts of soil moisture influence air temperature and relative humidity?

2. After a given amount of rain, how long is the dry-out period of the soil and how does this dry-out period differ for different soils?

3. How does high and low bulk density influence soil moisture and infiltration?

4. What is the difference in plant growth between a health and a non-healthy soil?

5. What kind of plants to do you find in undisturbed soils vs. disturbed soils?

6. How does soil moisture differ for different land cover types?

 

IV. A Case Study:

Different soils with the same porosity and the same amount of water present can vary significantly in the value of Soil Water Content, and understanding whether the values measured are reasonable or not is easier if the soil characterization protocols have been done for a horizon.

Soils are expected to show an increase in water content after a rain or during snowmelt, if the soil is not frozen or saturated. Soils gradually dry out during times with little or no precipitation. How the soil dries at different depths depends on the properties of the soil in each horizon. In some cases, water enters the soil from below, when the water table rises. The water content in these soils may be more variable lower in the soil profile than at the surface.

If it rains, some of the rainfall is expected to soak or infiltrate into the ground and increase soil moisture. This infiltration starts happening immediately and can continue for several hours if water continues to be available from a steady rain or puddles. If infiltration continues until all the pore space is filled, then the soil becomes saturated. Most soils drain rapidly, usually within hours or days. The field capacity of a soil is the amount of water a soil will hold without downward drainage or redistribution.

As the ground dries from evaporation and transpiration, soil moisture decreases slowly, with the soils closer to the surface usually drying faster than deeper soils. Soil moisture decreases from field capacity to a water content known as the wilting point, (the point at which the soil holds the water too tightly for plants to take it up). Depending on the soil properties, soil temperature, air temperature, and relative humidity, it may take from days to weeks for the wilting point to be reached. A general picture of how soil water content changes in a single horizon with time is illustrated in Figure 1, however, there are times when the actual data do not follow this pattern.

 

Figure 1: Example of soil wetting and dry-down period

 

Moisture content is affected by rainfall variation and soil properties. In a soil profile some horizons retain more water and have a greater porosity than others, affecting the flow of water from one horizon to another.

For example, if a sandy horizon is located above a clayey horizon, water moving through the sandy horizon will enter the clayey horizon very slowly because of the difference between the large pores in the sandy soil and the very small pores in the clayey soil. The small pores act as a tight layer that only lets water move gradually, so that the sandy soil may actually be much wetter at a given time than the clay.

Examining graphs of data collected at three locations will help demonstrate the process to determine whether data are valid or not. The following graphs are used for this demonstration: Valdres, Norway (61.13 N, 8.59 E): Figure SO-GR-2, Stowe, Vermont, USA (44.48 N, 72.708 W): Figure SO-GR-3 and Norfork Arkansas, USA (36.19 N, 92.26 W): Figure SO-GR-4. Each data set includes rainfall, new snow rain equivalent, and soil moisture.

For the first two schools, the classes chose to take weekly measurements for three months. In this case, the protocol calls for taking measurements during periods when the soil moisture is changing. The students in Valdres, Norway knew from experience that melting winter snow would result in wet soils initially, then drying out gradually as summer approaches. Of course, near-surface soil moisture can also increase during spring rains (as happened on May 28 and later in July).

The students in Stowe, Vermont decided to monitor their soil moisture as it changed from dry summer conditions to wet fall conditions. Again, the near-surface soil moisture appears more variable, drying significantly for a short period early in October 2001. Conversely, the deeper 10 cm soil moisture shows fewer extreme changes.

The class in Norfork, Arkansas decided to take monthly measurements for 12 months to investigate the seasonal cycle of soil moisture in their area. Despite having precipitation off and on throughout the year, the soil moisture shows a gradual dry-down, particularly at the surface. The soil moisture at 10 cm shows less variation for most of the year.

All three of these are interesting data sets. Comparison with precipitation has helpedexplain some of the variability while applying basic climatic knowledge has helped explain some of the longer-term trends. Knowing the soil characterization properties (texture, bulk and particle density, etc.) helps scientists and students understand more about how water is moving or stored in the soil.

 

What do scientists look for in the data?

Generally, scientists want to understand how water cycles through the local or regional environment. For example, they want to understand how precipitation and melting snow relate to increases in the water levels of streams, rivers, and lakes. Soil moisture measurements help to understand these processes. When soil moisture measurements are available for a whole profile, they can be used to predict floods, droughts, or the optimal timing for crop irrigation. Scientists also use soil moisture data with soil temperature, relative humidity and land cover data, to estimate the rate at which water is returned to the atmosphere through evaporation and transpiration.

Phenology scientists look at the effect of soil moisture on the annual cycles of plants, such as trees and annual grasses. In some forested regions, tree growth begins in the spring when the soil becomes moist and then stops during the summer when the soil becomes dry.Scientistsare interested in soil moisture changes over time. They are also interested in examining the regional or spatial patterns of soil moisture changes. Scientists focus on patterns rather than the absolute values of the measurements because soil moisture is a function of precipitation, soil texture, infiltration rate and local weather conditions.

Scientists would like to know the soil water content over large areas and ultimately, they hope to use remote sensing data from satellites like the SMAP mission to help measure this. Ground-based soil moisture data are required in order to develop and assess the methods for estimating soil moisture from satellites. By contributing to GLOBE’s soil moisture campaign, students are helping with this exciting scientific advance.

 

 

 

Acknowledgements: Special thanks to all the members of the GLOBE Science Working Group and Brian Campbell of NASA Wallop for improving the quality of the work.

Compiled by: Dr. Erika Podest, Mr. Olawale Oluwafemi (Femi) and Dr. Rebecca Boger

Edited by: Dr. Dixon Butler and Dr. Costantinos Cartalis