With the masterful craftsmanship of one Scott Chenoweth, we now have a window into the world beneath our feet. The root window is a thin wooden planter box with one side made up of clear plexiglass. The planter is divided into two sections so that two different soil treatments can be compared side by side. Below you can see radish roots growing behind the plexiglass panel.
The radish in the above picture were grown in very rich soil. Looking to make the most fertile growing medium possible, I created a mixture of compost, worm castings and some soil from my veggie beds. In the other compartment of the planter I used the poorest bagged soil product I could find. This "Top Soil Plus" product was just what I was looking for. Its only two ingredients are sandy loam and forest humus. I know that the sandy loam almost certainly came from a loam pit, a place where deposits of sandy soil are mined. This material would be devoid of plant life and extremely low in soil biology. I suspect the second material, the "forest humus" they are talking about here is some sort of byproduct from the logging industry. It looked like very fine wood particles, not really humus. The fact that the bag instructs gardeners to "use where needed to fill in low spots in lawns and gardens," makes it sounds like the company doesn't have many illusions as to the usefulness and fertility of this product.
Radish were planted in the two different soils at exactly the same time. The same seed was used and the same amount of water applied to each side. The difference between the radishes in the two soils was striking both above ground and below ground. To the rigt you can see how few visible roots there are in the poor soil compared to the first photo taken of the rich soil. These two photos were taken at the same time, each photo is of right below the soil line.
After the radish had set seed the soil was removed from the root window and put onto a board with screws to hold the roots in place. The soil was then washed away so that only the large roots would remain behind. One thing that is interesting to note is that the poor top soil product washed away with ease. The fertile soil high in organic matter on the other hand was an enormous pain to wash away. There were so many roots that it was a single spongy mass. Water would flow through without taking away much soil. I had to blast the fertile soil at point blank range with extremely high pressure and even then it took quite a while to get it to wash away.
You can see in the picture below that the radish in the poor soil on the left grew spindly stalks, did not develop large edible portions, and formed very thin roots. The radish on the right however grew thick edible portions, large networks of roots, and thick stalks with dark green leaves.
This picture below shows how the radish on the right have much darker green leaves than the radish on the left, most probably because of higher levels of plant available nitrogen in the fertile soil on the right. I don't think the radish on the left in the poor soil would have gotten anywhere near as big as they did had I not given both sides a few doses of animal urine. It felt like as soon as I applied the diluted urine the radish on the left immediately responded with a growth spurt. Not very scientific to muddle with the experiment like that but I was curious to see if anything would happen.
Below you can see how the roots grew over time in the two windows. The fertile soil is first and the poor soil is below. (Unfortunately the animation starts as soon as the file loads on your browser so they usually aren't synced). You'll probably be able to notice the three week gap when I was slammed at work and neglected to take any pictures. This is when the roots fatten up instantaneously.
Here are some slower versions of the same animation. Below is the radish in the poor soil.
And here is the radish in the fertile soil.
One thing the surprised me was just how quickly the tap root on the radish reached the very bottom of the root window. Within one week of the root starting to noticeably elongate it was already at the bottom of the two foot deep window. Clearly if given the chance the roots would have gone down much deeper. In Root Development of Vegetable Crops by Weaver and Bruner, the variety of radish they examined grew roots down to 7 feet deep.