Oh, man! I hate these science fair projects that compare fertilizers. I judged the botany section of a regional science fair for 20 years and have seen this very experiment done several times. These aren't the worst projects, but let's just say they are hard to control. Why? Because once the grass starts to grow you will have roots in the soil. Roots are organic matter that holds water. The more roots the more water retention and the less you have to water them. Each container will have different water needs based on how well the grass is growing. Nobody thinks of that and you end up with soggy soil (dead grass) in one bin and dry soil (dead grass) in another. The only possible way to water them is to measure the soil moisture. You might be able to do that by putting the containers on a scale, but then again, you will have plant mass adding to the weight of the container. You might be able to use a wick system where the water wicks from a container into the soil.

Rye is the only grass that will sprout fast enough for you at this time of year. I am assuming you have until November to turn this in, right?

We have a guy here in town who does organic testing. His business is making compost and soil blends. He's one of the largest in the state, but his preferred soil for testing is Wal-Mart generic potting soil. Why? Because anyone anywhere can get it and try his experiments without thinking he is using only the best money can buy. He also likes it because it is pretty sterile. One problem he has found is that plants tend to get disease in that soil if you do not add compost to it. So to that generic potting soil he adds a little compost. Sometimes, depending on what he's testing, he'll add some special crushed rock powders, but that is getting more exotic than you need. He blends the compost in, but I would prefer to see him put it right on top of the soil. Again, why? Because that is where Mother Nature puts Her compost. She never blends it into the soil. But to his point, Mother Nature never starts with sterile soil either. So either way, just do it the same for all of them.

The fertilizer application rate is anywhere from 10 pounds per 1,000 square feet to 80 pounds per 1,000 square feet. Pick a number and stick with it. Make it easy on yourself. You have 0.422 square feet of surface to fertilize (are you sure the litter box is only 4 inches wide???). 30 pounds per 1,000 square feet translates to 0.2 ounces. That's not much. Similarly if you top dress with compost, the normal app rate is 700 pounds (1 cubic yard) per 1,000 square feet. That translates to 4.7 (5) ounces at your dimensions. Of course your son will have to verify those numbers as part of his project.

If you do this project with all the considerations I have presented taken into account, you should do well. Another consideration is this. Count each grass plant as an individual plant. If you count the seeds that go in, the plants that sprout, and the plants that survive, and use those numbers instead of simply saying you had 4 bins, you will be able to develop actual statistics for the charts. Judges love to see statistics graphs that are more than one number tall. It is far far better to have 100 plants in the the soy bean test than 1 bin. Then do your statistics based on the number of seeds, number of sprouts, and number of surviving plants.

Oh you are soooo good! Thank you so much for the response and all the very helpful information! It wasn't our first choice for the project, or even our second, but they gave him grief over a project involving bacteria and then over one using teeth. The project is not due until February.

Yes, the litter box receptacles are 4" wide. They collect the 'stuff' that the automated litterbox rakes into them. I was going to poke holes for drainage. I supposed if we want to wick, we can create self watering containers like I use with my tomatoes; a receptacle within a receptacle that maintains a constant supply of water with tubes down into that water supply containing soil that wick water into the growing area. Think that would work okay?

Thank you, thank you, thank ever so much!

Yes that wick system should work.

Glad I could help.

Worst project I ever had to judge was a girl who put 1 plant in front of her TV and one plant on top of her TV. She did not water either one. After a week the one on top of the TV was dried up and crispy. The one in front was sort of healthy looking. THEREFORE, it is better to watch TV than not to watch TV.

Sad but true.

"Worst project I ever had to judge was a girl who put 1 plant in front of her TV and one plant on top of her TV. She did not water either one. After a week the one on top of the TV was dried up and crispy. The one in front was sort of healthy looking. THEREFORE, it is better to watch TV than not to watch TV. "

So this one houseplant got coffee in the morning and beer in the evening.

The other houseplant didn't get any coffee or beer.

The first plant thrived and the second one died.

Ergo, coffee and beer are good.