Water Retention

Gravel Water Retention

Water retention in different types of soils is one of those concepts where figuring out the process for the experiment is every bit as important as the actual results. I want the kids to come up with a way to test this on their own and be able to measure their results. They generally come up with lots of great ways to allow soil to absorb water, but they can struggle a bit when trying to make the amount of water absorbed measurable. The discussion of our plan oftentimes takes longer than our actual experiment. It's worth it, though, because it really makes them use their noggins! Eventually they come up with this...

To test water retention of different types of soils, we use a graduated cylinder, panty hose (which the kids think is hilarious!) and different types of soil. We put each individual type of soil (200ml) into the panty hose, SLOWLY add 200 ml of water, and figure our how much water was retained by the soil by looking at the amount of water than fell into the beaker. The kids are always surprised by how much water the sand retains!

Sand Water Retention

Top Soil Water Retention

Soil Texture

The students are responsible for knowing the properties of soil in 4th grade - color, texture, water retention, and plant growth. We began by looking at color and texture. The students observed top soil, clay, sand, silt and gravel. They added a bit of water to each soil type to help them really feel the texture. They had to determine if the soils make a ball, and if they are grainy, silky, or sticky. Then they predicted what the texture would be like when different soils were mixed together, such as clay and sand, before actually mixing them together. We had a bit of a water spill on one of the plates, so we used that opportunity to revisit erosion. They were able to see the soil being carried away by the water, while the rocks stayed in their place. We discussed the fact that rocks take much longer to erode than soil.

Adding water to our soil

Water eroding the soil

Divisibility Rules

This week we listed out multiples of many numbers, and used these lists to help us come up with some divisibility rules. We thought knowing these rules might help us when determining factors of larger numbers. 

Breaking Apart Numbers in Multiplication

What we did this week was go through our basic facts and determine which ones we know quickly, and the ones with which we still need some practice. We made a list of those we're still working on, and came up with ways to "get" to that answer. For example, if 9 x 12 is still a struggle, we could think of it as (9 x 10) + (9 x 2). We made the arrays to go with the problem, which made visualizing the strategy a little easier.

If a student can visualize breaking apart a number for basic multiplication, it's going to make his life much easier when we get to 2-digit by 2-digit multiplication. It's so important for the kids to understand the place value involved in larger multiplication, and also to learn to make their numbers work for them. Flexibility with numbers is a HUGE concept to grasp, and it has arguably the greatest impact on the success of a mathematician!!!!!!

How Water Can Change the Shape of Rocks

To test how water can change the shape of rocks, we had to get a little creative. Obviously we don't have the kind of time to watch weathering, erosion, and deposition in real-time. :) In order to speed process up a bit, we used limestone rocks that had been soaking in water overnight. This made them very soft.

We weighed the mass of our rocks, which was 54g. We also observed the rocks' edges were very jagged and sharp. We put our rocks into a glass jar with about 1/2 cup of water. We shook our jar for 3 minutes, removed our rocks, and checked their mass again. This time, the mass was 46g, and the edges of the rocks were smoother and more rounded. We put our rocks back into the water and shook again for 6 minutes. This time, when we measured the mass, it was35g. By now, the rocks were completely smooth, and much smaller. The water was completely full of sediment by this point as well.

We discussed how this is related to a river rushing over pebbles. They become more and more smooth, and will eventually become smaller in size.

After 3 minutes of shaking

After 6 minutes of shaking

Changes to our Land

This week we began to discuss changes to land. First, we talked about what kinds of changes can happen to our land. We discussed the meaning of weathering, erosion, and deposition. In 4th grade we focus on changes caused by wind, water, and ice, and do investigations that show how each can affect the land. 


We started with our investigation of wind. The kids made a sand dune in their box lids, trying to make the dune come to a point as much as possible. They then predicted what would happen when the wind (the kids blowing through straws) hit the sand. Most thought the sand would move to the other end of the box, forming a new dune. When they blew the wind, however, they notices the sand really just spread all over the box. We discussed why this happened, focusing on the size of the sand particles in relation to where they landed. We also talked about why the particles didn't form another dune. The kids came to the conclusion is was because the box was too slippery, so the particles just blew around all over the box lid. We did notice a stronger stream of air was needed to move the larger particles. We finished up by looking at some really cool pictures of wind erosion. We discussed how wind can cause erosion:  by picking up the particles (just like our experiment) and slamming them into objects such as rocks. Eventually, the rock is going to wear down from repeatedly being hit by sediment.