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The crust that can form on powdery silt loam and some sandy loam soils rivals concrete. At least it can seem that way to irrigators farming on this type of land. The result is a crust so hard that when it dries, it limits water infiltration into the soil.
Some innovative Agricultural Research Service scientists not only have discovered what causes the crust (the impact of droplets from irrigation sprinklers) but also have devised a ready-fit nozzle system that promises to reduce crust occurrence. The system allows water applications to be adjusted during the season to suit growing conditions. “Sprinkler heads can be inexpensively and easily modified to adjust water volume and also the force with which the water droplets hit the soil surface,” says Gary Lehrsch, an ARS soil scientist who led a five-year research project on the topic at the Northwest Irrigation and Soils Research Laboratory in Kimberly, Idaho.
The cause of the crusting is the kinetic energy caused by water drops hitting soil. Certain soils — like powdery silt loam, which is low in organic matter — are especially prone to crusting. That particular soil is prevalent in south-central Idaho where this research was conducted. The soil crusting reduces water infiltration and can greatly impact seedling emergence. The ARS research team found, for example, that sugar beet seedling emergence increased 6.5% when droplet energy was reduced 50% by a change in sprinkler type.
Lehrsch also found that the impact from water droplets delivered by certain sprinkler types increased crusting of freshly tilled silt loam by 18% and increased the soil's water-filled pore space by 35% after just one irrigation in freshly tilled silt loam. Infiltration through certain small soil pores was reduced by almost 500%.
Lehrsch and his team set about devising a sprinkler head system that could be modified so that irrigation had either low- or high-droplet energies.
They field-tested a variety of nylon plates (shown below) to determine their droplet energy as well as application rate. That effort involved measuring soil aggregate stability and surface soil penetration resistance (a measure of crust strength) four days after the first postplant irrigation and up to 14 days after the last irrigation.
Gary Lehrsch | firstname.lastname@example.org
The ARS nozzle design consists of a sprinkler head body that can house a variety of different spray plates. These different plates alter droplet size to suit soil conditions. The sprinkler body attaches to either side of a dual-nozzle unit. One side of the dual-nozzle unit applies less water (usually early in the growing season), while the other side applies more water (later in the season).