Fine material such as mud or clay carried and deposited by streams is called alluvium. There are three types of alluvial deposits: floodplains, alluvial fans and deltas. Great alluvial valleys and deltas include the Nile, the Mississippi, the Amazon, and other large river valleys, many of them the homes of early civilizations. Some of these soils are supplemented annually by a thin covering of fresh alluvium from eroding rocks which is added to the surface during periods of high water in the river.
Once adequate drainage is established, leaching will help remove the salts. Irrigation water should be applied well in excess of the amount needed by the crop. If the irrigation water itself contains harmful salts to any degree, apply extra-large quantities to make sure these salts, too, wash down. Don’t conserve water at the expense of soil conservation! Each field should be carefully leveled, of course, so the water will enter the soil uniformly.
Most alluvium is carried and deposited during floods because it is at this period that erosion is most active and the carrying capacity of streams is at a maximum. When a flooding stream overflows its banks, its carrying power is suddenly reduced as the flow area increases and velocity decreases. This causes the coarse sands and gravels to settle along the bank where they sometimes form conspicuous ridges called natural levees. As the water reaches the flood- plains of the valley, the rate of flow is slow enough to permit the silt to settle. Finally the water is left in quiet pools, from which it seeps away or evaporates, leaving the fine clay. Levees are characterized by good internal drainage during periods of low water, whereas floodplains exhibit poor internal drainage.
If excessive irrigation or other factors raise the groundwater level, sodium will rise with it and be left in the surface soil when the water evaporates. This causes black alkali soil, most damaging to crops because it has poor structure and puddles and packs easily. White alkali soils are similarly formed, but they contain less sodium and do not pack.
Besides building up the soil’s structure, organic matter exerts a “buffer” action, lessening the toxicity of the strong salts to plants. It also lowers the pH and improves the soil’s capacity to supply available nitrogen.
Streams flowing from hills or mountains into dry valleys or basins drop their sediments in a fan like deposit as the water spreads out. These alluvial fans are usually coarse textured, being composed of sands and gravels, are well or excessively drained, and low in organic matter. Floodplains as well as deltas are, in general, rich in plant nutrients and comparatively high in organic matter content. Special crops such as vegetables and fruits frequently are grown on the latter formations because the soil warms up quickly and their good drainage and coarse texture permit free root development.
Delta soils are formed when the fine sediment carried by a current is not deposited in the floodplain but flows into the body of water to which the smaller stream is tributary. Insufficient current and wave action allows the material to accumulate, beginning formation of a delta, which is enlarged as more material accumulates. Deltas are extremely fertile when subject to flood control and drainage.
Drainage is very important for reclaiming alkali soils, especially where the water table is high. The soil should be permeable to a depth well below the root zone. The groundwater level should never come nearer the surface than at least ten feet. In the case of hard pans, subsoiling is a must to insure good drainage. Tile drains are sometimes necessary.
Viewed 77 times by 30 viewers
