Salinity & Leaching & Salt In, Yield Out

The leaching fraction is the share of applied irrigation water that drains past the root zone instead of being used by the crop. That drainage carries dissolved salt away, keeping the soil from getting saltier and saltier over time. A leaching fraction of 0.15 means 15% of the water you apply must pass through to maintain a salt balance the crop can tolerate.

The leaching requirement is the minimum leaching fraction needed to hold the root-zone salinity at the crop's tolerable limit. This tool uses the Rhoades form LR = ECiw / (5·ECe* − ECiw), where ECiw is your irrigation-water salinity and ECe* is the crop's salinity threshold. If the water is too salty for the crop to leach to its limit, the requirement is capped near 0.95.

Maas and Hoffman found that crop yield stays at 100% up to a crop-specific soil-salinity threshold (ECe in dS/m) and then falls in a straight line by a fixed slope, in percent per dS/m, above it: Yr = 100 − slope·(ECe − threshold), floored at zero. The tool finds the steady-state root-zone ECe from your water salinity and leaching fraction, then reads the yield off this line for the crop you pick.

ECe is the electrical conductivity of the saturated soil-paste extract — the salinity the roots actually experience, in deciSiemens per metre (dS/m). ECiw is the electrical conductivity of the irrigation water you apply. Because water is taken up but salt is left behind, the root zone concentrates: the steady-state ECe is roughly ECiw ÷ (5·leaching fraction).

Dissolved salts lower the osmotic potential of soil water, so the plant must spend more energy to pull water in — effectively a drought even in moist soil. Some ions are also directly toxic. Above the crop's threshold, every extra dS/m costs a fixed percentage of yield, which is exactly the slope in the table on this page.

The tool turns the leaching fraction into depth: total applied = net crop need ÷ (1 − leaching fraction), and the extra is the difference. So if your crop needs 100 mm net and the leaching fraction is 0.2, you apply 125 mm, of which 25 mm is the leaching water. Higher water salinity demands a higher fraction and more extra depth.

It is the yield you can expect as a percentage of the crop's full potential, given the steady-state root-zone salinity your water and leaching produce. 100% means salinity is not limiting; 85% means you would lose about 15% to salt stress at that ECe. It is a planning figure to compare water sources, crops and leaching strategies.

From the Maas-Hoffman table here, barley, cotton, sugarbeet, date palm and bermuda grass are among the most tolerant, with thresholds of 6–8 dS/m. Beans, carrots, onions, strawberries and most stone fruit and citrus are sensitive, with thresholds near or below 1.5 dS/m. Matching crop tolerance to your water salinity is often cheaper than leaching.

Yes. By default the tool uses the leaching requirement, but you can enter the fraction you will actually run. A lower fraction saves water but raises root-zone salinity and may cut yield; a higher fraction protects yield but uses more water and can waterlog poorly drained soils. The tool shows the yield trade-off for whatever fraction you choose.

It comes from the FAO-29 steady-state relation ECe ≈ ECiw ÷ (5·leaching fraction). The 5 reflects a typical water-uptake distribution down the root zone (a 40%-30%-20%-10% extraction pattern). It is an accepted planning constant; very different irrigation or drainage conditions can shift it, so treat the result as a sound estimate rather than an exact field value.

Critically. Leaching only works if the extra water can actually drain below the root zone — on a poorly drained or shallow-water-table soil that water perches and the salt stays. If drainage is the limit, the answer may be subsurface drains rather than more water. The leaching figures here assume the soil can pass the drainage fraction freely.

It is a solid steady-state planning estimate from published Maas-Hoffman tolerances and the FAO-29 leaching relations. Real fields vary with rainfall, soil variability, drainage and how evenly water is applied, so treat the yield and leaching depth as a guide. Confirm with a soil ECe test and a water analysis, and adjust the leaching fraction as you monitor.