Salt in the Soil: Monitoring Irrigated Corn in a Dry Year

A White Crust Between the Rows

1Sorrel Valley is a farm community where most corn fields are watered with irrigation from a reservoir in the hills. The valley and its numbers are made up, but they are based on real farm regions. In 2024, the valley had its driest year in decades: only 180 mm of rain fell, compared with about 400 mm in a usual year. Mrs. Greta Halvorsen, who farms 60 hectares of corn, noticed a thin white crust on the soil between her rows and pale, stunted corn in the low parts of one field. She asked Mr. Ezequiel Barros's agriculture class at Sorrel Valley Middle School to help her find out what was happening.

2Water from rivers and reservoirs carries small amounts of dissolved salts. When water evaporates from the soil surface or is taken up by plant roots, most of the salt stays behind in the soil. Rainwater has almost no dissolved salt, and a soaking rain can carry salt down below the roots. When too much salt collects near the roots, many crops have trouble taking in water. Corn usually grows well when the soil salt reading stays below 2.0 dS/m, and yields are often lower when the reading climbs above about 3.0.

3Students Wesley Tran and Amina Yusupova used a soil salt meter, which measures how easily electricity passes through moist soil. More dissolved salt lets electricity pass more easily, so the reading is higher. Readings are in decisiemens per meter (dS/m). Each spring before planting and each fall after harvest, they took readings 15 cm deep at 10 spots in Field 1 and at 10 spots in the pasture next door, which is never irrigated. Mrs. Halvorsen had taken the same readings in the two years before. A test of the reservoir water itself read about 1.0 dS/m, and rainwater read close to 0.

4Because the reservoir was low, the water district warned that next year each farm may use at most 450 mm of irrigation water for the season. To get ready, Mrs. Halvorsen had tried four watering methods in 2024, on four strips of Field 1 that had the same corn variety, planting date and fertilizer. Strip A was watered the usual way, in furrows (small ditches between the rows), with bare soil. Strip B was watered in furrows, with straw spread over the soil between the rows. Strip C was watered through drip tubes that let water drip slowly at the base of each plant, with straw over the soil. Strip D was watered in furrows, with an extra deep watering in spring to wash salt down below the roots. The Field 1 readings in Table 1 come from Strip A.