Cotton Salinity, Sodicity, and a Gypsum Recommendation
A 120-acre furrow-irrigated cotton field in the Texas High Plains has been in continuous cotton for six years. The 0-12 in. soil test shows ECe 3.8 dS/m, ESP 12%, CEC 18 cmol/kg, pH 8.1, and 1.2% organic matter. Irrigation water is from a well with EC 2.2 dS/m and SAR 8. The grower fertigates 120 lb N/ac and applies 40 lb P2O5/ac. Stand is uniform, but infiltration in the lower end of the field has slowed and water ponds for several hours. Cotton yield last year was 850 lb lint/ac, near the county average. The grower has been told he must apply 3 tons/ac of gypsum immediately and switch to a more salt-tolerant crop. He asks whether that is justified and what he should do this season.
1. Interpret the soil and water tests for cotton and decide whether 3 tons/ac of gypsum is justified.
Read the worked answer
Cotton is salt-tolerant: yield loss begins around ECe 7.7 dS/m, and the soil ECe is 3.8 dS/m, about half the threshold. Salinity alone does not justify a 3-ton gypsum application. The ESP of 12% is moderate sodicity, and the water SAR of 8 can worsen surface sealing, especially with furrow irrigation and low organic matter. Gypsum need can be estimated for reducing ESP from 12 to 8: CEC 18 x (0.12 - 0.08) = 0.72 cmol/kg, and 1 cmol/kg in 0-12 in. is roughly 1.5-1.6 tons gypsum/ac, so about 1.1 tons/ac. But gypsum is only warranted if infiltration is actually limited by sodicity. Check aggregate stability, crusting, and whether ponding follows the furrow path. Three tons/ac is excessive and could leach Mg and K.
2. Recommend a practical irrigation and amendment plan for this season.
Read the worked answer
First, separate the ponding cause: measure infiltration in the lower end, check for compaction or a broken furrow, and compare with the upper end. If sodicity is the cause, apply 1 ton/ac of gypsum and incorporate it or dissolve it through irrigation; do not apply 3 tons. Use a leaching fraction of about 10-15% rather than a heavy leaching event, because the water EC of 2.2 dS/m is moderate and cotton can tolerate the salinity. Irrigate in shorter sets, add residue or a cover crop to protect the surface, and avoid working the soil wet. Monitor ECe and ESP after harvest. Keep the current cotton crop; switching to a more salt-tolerant crop is not supported by ECe 3.8 dS/m. Fertigation at 120 lb N/ac is reasonable; do not increase N to compensate for salinity.
3. The grower says the well water SAR of 8 will ruin the field and wants to blend with canal water of EC 0.4 dS/m and SAR 1. What is the agronomic value of blending?
Read the worked answer
Blending lowers both EC and SAR of the applied water. If half the water is canal water, the blend has roughly EC 1.3 dS/m and SAR near 4.5, which reduces sodicity risk and improves infiltration. But the field's soil ESP is already 12%, so the blend does not replace the need to check infiltration and possibly apply 1 ton/ac of gypsum. Blending also changes the leaching fraction: lower EC water is more effective at leaching salts, but the total salt load still depends on volume. The practical benefit is reduced surface sealing and a safer margin for germination; the risk is that growers assume blending solves a soil problem. Test the blend and monitor ESP yearly. Do not blend just to allow more irrigation; keep the 10-15% leaching fraction.