This is clearly in the sand texture class. More specifically, it is medium sand because the particle size range is predominately in the middle of the 2 - 0.05 mm size range for sand. The dropper is adding 10% HCl. Since the sand effervesces (fizzes) vigorously, it indicates that the sand contains much calcium carbonate. Most of the calcium carbonate appears to be in the form of bits of shells from the freshwater snails and mussels that live in the river and surrounding tributaries. A fairly intact shell is visible left of the center. Some of the calcium carbonate is also from the calcareous glacial till that has contributed sediment to the river. Since the sand is calcareous, the pH will be around 7.8 or higher.
The sand here was deposited by water as the river flooded and overflowed its banks. The sand was suspended in the rapidly flowing water in the main channel, but when the river overflowed, the velocity and turbulence of the water decreased. Recall from a previous lab how quickly sand settled out when you did the texture analysis by the hydrometer method. After only 40 seconds, the sand settled out below the bottom of the hydrometer. The same principle applies here. The sand settles out close to the channel of the river, while the silt and clay gets carried by the water to quieter, slack water areas further from the main channel. Often these are at the very edges of the floodplain, but right here those areas have been filled in by human activities. There is, however, a slack water area not far from this sandy area and we'll look at it next. Watch the video to see where we are going. (Sorry about the jitter. It definitely needs some image stabilization!)
We are now in a low, slack water area that has very different soil. Go to
this link to look around. As you can see, we are in a wooded area that is about 10 feet (~3 meters) lower than where we were before. When the river floods, the water is not moving very quickly through this area, so finer material settles out here. The soil is very different here as shown by this video.
The soil here is much finer and occurs as clumps of soil called peds. The soil is dry, but the peds can be broken up easily with your fingers. After moistening the soil, its texture by feel was determined to be silt loam. This is very different from the sand we just looked at and illustrates how quickly soils can change in a floodplain depending on where different materials sedimented out. The 10% HCl test shows less effervesce (bubbling) than for the sand, but there is some, so this soil is calcareous with a pH around 7.8 or higher.
The parent material of the soils here on the flood plain is alluvium, material that has been deposited by running water in fairly recent time. You have seen how different the surface soil can be over just a short distance. The same thing occurs with depth. Alluvium is often stratified, meaning it consists of layers or strata of sandy material alternating with layers of silty or clayey material. Ideally, we would have a pit to look at the stratification, but digging a pit in a park is likely to attract more attention than we would like.
Since new sediment is added to floodplains regularly, sediment builds up more rapidly than soil horizons can form. As a result, soils on floodplains tend to be very young and usually have only an A horizon on top of C material. These young soils with no pedogenic development other than a light colored surface horizon are classified as Entisols. At the suborder level, they are Fluvents, Entisols on floodplains.
Many of the soils on the Wabash River floodplain in this area have dark surface horizons. The accumulation of organic matter can occur fairly quickly, particularly when the parent material is calcareous as it is here. They are still young, weakly developed soils, but since they have dark surface horizons, they are classified as Mollisols instead of Entisols.
Soil Fertility, Drainage Classes, Hazards
Soils on floodplains tend to be very fertile because the alluvium on the floodplains is derived mainly from fertile surface horizons eroded from soils on the surrounding uplands of the watershed. Like the sandy soil we just looked at, many of the soils in the floodplain of the Wabash River and its tributaries are well drained. You can see this for yourself by going to Soil Explorer and switching back and forth between the Dominant Soil Parent Materials map and the Natural Soil Drainage Classes map. Very sandy areas, like we have seen here, have very low water holding capacity and can be droughty, but the silty and clayey soils that predominate in most floodplains have adequate water holding capacity. The major constraint to agriculture in floodplains is flooding. Sooner or later a crop will be lost to flooding, so flood insurance is a must. Despite potential flooding, high value crops like corn and soybean are grown all along the Wabash River floodplain, which you can see for yourself when driving along South River Road towards the US 231 bypass.
Trees grow well on floodplain soils. Cottonwood, sycamore, and maple are the major tree species here in Tapawingo Park and all along the Wabash River. In fact, the trees are very important in protecting the soil at the edge of the channel against erosion as you can
see here right at the edge of the channel. The initial view is to the south where you can see how the roots of a large silver maple tree are holding the soil in place. When the river is higher, the tree is right at the water's edge. Turn around and look north and you can see other trees doing the same. If these trees were removed or destroyed, the channel could begin to erode into Tapawingo Park and endanger the bridges and other infrastructure to the south. The trees, shrubs and other perennial plants adjacent to a stream or river are
riparian forest buffers. Riparian forest buffers provide many other ecosystem services in addition to stabilizing stream banks as seen here.
This concludes the virtual field trip to Tapawingo Park.