Leaching Requirement & Flush Salts, Hold Yield
Computes leaching fraction
Enter irrigation-water salinity, crop tolerance and net irrigation to get the leaching fraction, the gross water depth and the extra leaching depth needed to flush salts.
Leaching requirement
Next: apply 109 mm instead of 100 mm — the extra 8.8 mm drains past the roots and carries accumulated salts with it.
LR = ECw / (5·ECe − ECw). Use the crop's salinity threshold ECe; higher water salinity or a more sensitive crop means a bigger leaching fraction.
Leaching requirement — key facts
- Leaching fraction
- ECw ÷ (5·ECe − ECw)
- Gross water
- net ÷ (1 − LR)
- Leaching depth
- gross − net
- Saltier water
- Needs more leaching
- Sensitive crops
- Low ECe; higher LR
- Too little
- Salts build up; yield falls
- Needs
- Good drainage to work
- Privacy
- Runs in your browser; nothing uploaded
When water is salty, you must over-irrigate to wash salts out
Crops take up almost pure water and leave the dissolved salts behind, so every irrigation with salty water concentrates salt in the root zone. To stop it building to yield-limiting levels, a fraction of the applied water must drain below the roots and carry those salts away — the leaching requirement. The saltier your supply and the more sensitive your crop, the larger that fraction has to be.
This tool computes the leaching fraction, the gross water depth to apply, and the extra leaching depth from your water salinity (ECw), crop tolerance (ECe) and net irrigation. Apply the gross depth each cycle and you hold the salt balance steady — provided drainage lets the salty water escape. Pair it with the SAR, Soil Salinity, RSC & Gypsum and Subsurface Drainage tools for complete water-quality management.
Hold the salt balance
Flush salts before they limit yield.
Size the extra water
Know the gross depth and leaching depth.
Match the crop
Tolerance sets how much you must leach.
Avoid over-leaching
Leach just enough; save water and nutrients.
Frequently Asked Questions
What is the leaching requirement?+
When irrigation water carries dissolved salts, those salts stay behind as the crop takes up pure water, concentrating in the root zone. The leaching requirement is the fraction of applied water that must pass below the roots to wash the excess salts away and keep the root-zone salinity safe for the crop.
How is the leaching fraction calculated?+
A common form is LR = ECw ÷ (5 × ECe − ECw), where ECw is the salinity of the irrigation water and ECe is the salinity the crop can tolerate in the saturated soil extract. Higher ECw or a more salt-sensitive crop (lower ECe) raises the fraction of water you must leach.
How does the leaching fraction change the water I apply?+
Gross water = net irrigation ÷ (1 − LR). The net depth is what the crop uses; you apply extra so that a fraction drains past the roots. For example, a net 50 mm with a leaching fraction of 0.15 means a gross depth of 50 ÷ 0.85 ≈ 59 mm, of which about 9 mm leaches.
What is the leaching depth?+
The leaching depth is the extra water — the gross depth minus the net irrigation — that moves below the root zone carrying salts with it. It is the price of using saltier water: the more saline the supply, the larger this depth must be to keep salts from building up to yield-limiting levels.
What is ECw versus ECe?+
ECw is the electrical conductivity (salinity) of the irrigation water itself. ECe is the salinity of the soil's saturated paste extract — the value crops actually respond to and the basis of salt-tolerance thresholds. The leaching requirement links the salinity coming in (ECw) to the salinity you can allow in the soil (ECe).
What happens if I leach too little?+
Salts accumulate in the root zone, the soil ECe climbs, and the crop takes up water with more effort (osmotic stress). Yields fall, leaf tips burn on sensitive species, and over seasons the land can become saline and hard to reclaim. Meeting the leaching requirement each cycle prevents this slow build-up.
Can I leach too much?+
Yes — leaching more than required wastes water and energy, leaches nitrate and other mobile nutrients below the roots, and can raise water tables and worsen drainage problems. The goal is to leach just enough to hold root-zone salinity in check, which is exactly the fraction this tool finds.
Does leaching need good drainage?+
Absolutely. Leaching only works if the salty water can actually drain away below the root zone. On poorly drained soils the leached water and its salts sit in the profile or rise back up, so leaching must be paired with adequate natural or installed drainage — see the Subsurface Drainage Spacing tool.
How does crop salt tolerance affect the answer?+
Salt-tolerant crops (such as barley or date palm) tolerate a high ECe, so they need only a small leaching fraction even with saline water. Sensitive crops (such as beans or many vegetables) need a low ECe, which forces a larger leaching fraction. Matching the crop's tolerance to your water quality is key to sustainable irrigation.
How do I use this with my normal schedule?+
First work out the net irrigation depth your crop needs from ETc or the soil-moisture deficit, then apply the leaching factor here to get the gross depth to pump. Doing this each irrigation keeps the salt balance steady. Pair it with the SAR, Soil Salinity and RSC & Gypsum tools for full water-quality management.