# Why Do Very Few Plants Grow in Deserts? | DFRobot Science Lab Season 2 EP06

### Standards

NGSS

5-LS1-1 To expound and prove that plants mainly get nutrients from air and water.

### Overview

This lesson focuses on talking about the question“why do very few plants grow in deserts”, and leading students to find out the difference between sand and soil. It aims to help students explore the difference in water storage capacity between sand and soil through experiments, and understand that water in the soil is stored in soil pores through capillarity.

### Materials

Play and Learn with Boson Science Design Kit: https://www.dfrobot.com/product-2176.html

HARDWARE LIST
1 Boson Science Design Kit

Engage

Our general impression of deserts is that they are vast seas of sand, piled up into dunes, always very barren, with scarce plants. But in most plain areas on land, there are large areas of grassland, arable land, and forests, which breed a variety of plants. Have you ever wondered why there are so few plants in the desert?

We know that plants cannot grow without water, while deserts are often dry all year round. So, could it be that there are so few plants in the desert because of the lack of water? Let's make a bold assumption that if there is enough precipitation in the desert, can sand breed life? Can sand hold water like soil? Have a guess!

Explore

The experiment is divided into two parts. The first part is to observe sand particles and soil particles. In the second part, we will compare the difference in water storage capacity between sand and soil.

Part 1 Observation: What’s the Difference between Sand and Soil

We use a magnifying glass to observe sand particles and soil particles, compare the particle size and uniformity, and record the results in the table below.

### Activity Conclusion

The referable results are as follows:

Through the above observation, it can be found that the sand particles are large and uniform in size, while the soil particles are with both large and small size, and nonuniform in size.

### Conjecture

If the same amount of water is given to sand and soil, which one is more likely to hold water? Take a guess!

Part 2 Experimental Exploration: Which Holds Water better, Sand or Soil?

In order to investigate the water storage capacity of sand and soil, we can add equal amounts of water to sand and soil. Then use BOSON soil moisture sensor to observe changes in moisture of sand and soil. And we use the data to verify whose water storage capacity is better. The referable experimental table is as below.

### Materials

Prepare the following BOSON modules and other materials.

### Activity Step

1. Let's build an experimental circuit together to detect the soil moisture value.

2. After completing the circuit connection, set up the two experimental groups of sand and soil. The experimental process is as follows.

### Activity Conclusion

The referable data is as follows:

Observing the above table, we may find that the obtained data are not convenient for us to directly see the changes in soil moisture and sand moisture as time goes by. What should we do?

The data directly measured in the experiment is generally called the original data. In order to obtain the information we want, it is often necessary to process the original information. For example, in this experiment, we want to see the change in soil moisture as time goes by, so we can subtract the original data at two moments to get the change over a period of time. As the table below.

The results obtained are as follows, "+" means increase, "-" means decrease.

From the above table, it can be concluded that within 10 minutes after adding the same amount of water, the sand moisture increases by 552 and the soil moisture increases by 561, which are relatively close. Every 10 minutes thereafter, the moisture value of the sand decreases more than that of the soil. Especially after 12 hours, the moisture of the sand decreases by 114, while the soil one decreases by only 6. It shows that the water loss in the sand is faster than that in the soil.

Therefore, we can draw the conclusion: the water storage capacity of sand is weak, and that of soil is strong.

Sand cannot hold water, and consequently it cannot provide water for plants. Even if it rains heavily in the desert, the rainwater will quickly run away from the sand, and it is still difficult for plants to survive.

Science Background

In the experiment, we first observed the particle size of sand and soil, and then explored the water storage capacity of sand and soil. Why can the soil hold more water? Does this have anything to do with their particle size? How do we distinguish sand and soil? Let's learn together!

### How Does the Soil Hold Water?

In Part 1, we can see through a magnifying glass that the soil particles are nonuniform in size. Because of this, these soil particles are stacked together, leaving pores of different sizes between the soil particles. Most of the water in the soil is stored in these pores. This phenomenon is called capillarity. The capillarity allows water to overcome gravity and adhere to small pores.

If it rains heavily, the pores of the soil are already full of water, and the excess water will be drained from the ground surface, forming a stream, and flowing to low-lying lakes, ponds, and ditches. If there is a continuous drought and there is no water in the pores of the soil, the soil will become dry, cracked and caked.

### Why Cannot Sand Hold Water?

In Part 1, we can see through a magnifying glass that the sand particles are large and uniform. Because of this, the pores between the sand particles are also relatively large, and the capillarity generated is weak. Water will pass through the sand under gravity. This is why the sand cannot hold water.

### Learn Soil and Sand

Soil is a complex mixture of minerals, water, air, organic matter and countless creatures. The soil particles are of all sizes. Some scientists divide soil particles into three types: clay, silt and sand according to their diameter. So sand is actually a kind of soil!

According to the proportion of clay, silt and sand in the soil, the soil can be divided into clay, loam and sand soil.

Clay has small pores and strong capillarity, so it has a strong water storage capacity. Sandy soil has large pores and a weak capillarity, so the water storage capacity is weak. The water storage capacity of loam is between clay and sand, weaker than clay and stronger than sand.

Conclusion

In this program, starting from “why there are few plants in the desert”, we learned the difference in water storage capacity between sand and soil through observation and experimental exploration.

The project is going to an end, please remove cables, and put all the BOSON modules back into the kit.

Extend

1. What are the characteristics of the roots of desert plants? And why is it?

2. What is the climate like in the desert? Will this affect the water storage of sand?

3. Why does the land become a desert? How can we prevent desertification?