Watering sandy, loamy and clay soils: how long and how often.
The crop and the weather decide how much water your plants use each day. Your soil decides how often you must give it, and how fast. This guide shows how sandy, loamy and clay soils take in and hold water, how to judge soil moisture by feel, and how to set a drip run time for each.
Written for farmers, greenhouse and nursery growers, homeowners and estate and facility managers.
Find out which soil you have
Texture is the mix of sand, silt and clay in a soil. Farmers often call a sandy soil light, because it is easy to work, and a clay soil heavy. You cannot change the texture, so learn it before you set any watering times.
- Take a handful of soil from where the roots grow, not only from the surface.
- Wet it a little and rub it between your fingers.
- Sandy (coarse) soil feels gritty. The grains fall apart in your hand, even when moist.
- Loamy (medium) soil feels soft, like flour, when dry, and silky when wet.
- Clay (fine) soil sticks to your fingers when wet and forms a ball when pressed.
- Repeat in a few places. Many fields have sandy patches, and the plants there show stress first.
Sources: FAO (1985), Irrigation Water Management Training Manual No. 1: Introduction to Irrigation, chapter 2; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 3.
How each soil takes in and holds water
Two things matter. The infiltration rate is how fast water soaks into the soil, measured in mm of water per hour. The available water is how much water the soil holds that roots can take up: the water between field capacity (what a soaked soil still holds once it has stopped draining) and the wilting point (so dry that plants wilt and do not recover). Here it is given in mm of water per metre depth of soil, and 1 mm of water over 1 square metre is 1 litre.
Clay holds the most water in total, but it grips much of it too tightly for roots. In FAO 56's table, the water a crop can use from clay is about the same as from loam; FAO's older training manuals give clay somewhat more. Water also soaks in faster when the soil is dry than when it is already wet.
| Soil | Takes water in (mm per hour) | Water roots can use (mm per metre of soil) | What it means |
|---|---|---|---|
| Sand | Faster than sandy loam | 50 to 110 | Holds little: water little and often |
| Sandy loam | 20 to 30 | 110 to 150 | Takes water in quickly and holds a fair amount |
| Loam | 10 to 20 | 130 to 180 | Holds well and takes water in steadily: the easiest to water |
| Clay | 1 to 5 | 120 to 200 | Takes water in slowly: apply it slowly and less often |
Sources: FAO (1988), Irrigation Water Management Training Manual No. 5: Irrigation Methods, Annex 2, Table 7; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 7, Tables 19 and 20; FAO (1985), Irrigation Water Management Training Manual No. 1: Introduction to Irrigation, chapter 2.
What each soil needs
- Sandy soil: water little and often. It holds little, so the crop runs short quickly in hot weather, and any water beyond what the root zone can hold drains below the roots within hours. Water spreads sideways less in sand, so drippers closer together, or a higher dripper flow, are needed to wet a full strip along the row.
- Loamy soil: the easiest to manage. It holds enough water to go longer between waterings and takes it in at a steady rate.
- Clay soil: water slowly and less often. Water soaks in slowly, so it pools and runs off if it comes too fast. Clay also drains slowly, over two or three days after a soaking, and can stay waterlogged, which harms crops such as onion.
- Shallow soil over rock or a hard layer: treat it like sand. A shallow soil holds little water, even when it is clay.
Sources: FAO (1988), Irrigation Water Management Training Manual No. 5: Irrigation Methods, chapter 6; FAO (1985), Irrigation Water Management Training Manual No. 1: Introduction to Irrigation, chapter 2; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 3; NAERLS (2016), Extension Bulletin No. 239: Irrigated Onion Production and Management.
The same watering in three soils
- Sandy soil: water sinks fast and spreads little, so the damp patch is narrow and deep
- Loamy soil: the damp patch spreads about as far as it sinks
- Clay soil: water spreads wide and sinks slowly, so the damp patch is wide and shallow
- The emitter on the drip line, letting out a few drops at a time
- The edge of the damp soil after the same watering time
Source: FAO (1988), Irrigation Water Management Training Manual No. 5: Irrigation Methods, chapter 6.
Check the moisture by feel
The feel test from the USDA Natural Resources Conservation Service tells you how much of the soil's usable water is left. You need a spade or an auger and a few minutes.
- Dig or auger down through the root zone. Take a sample from the middle of each 30 cm layer, at three or more places in the field.
- Squeeze the sample firmly in one hand several times to form a ball.
- Bounce the ball in your hand. A very weak ball falls apart at the first bounce, a weak ball after two or three.
- Squeeze some soil out between your thumb and forefinger. If it makes a flat strip, called a ribbon, note it.
- Look at the colour and at any wet stain on your fingers, then compare with the table.
| Soil | 50 to 75 percent left: water vegetables now | 25 to 50 percent left: the crop is already short |
|---|---|---|
| Sand | Moist, makes a weak ball, darker colour, wet stain on fingers, no ribbon | Slightly moist, a very weak ball with clear finger marks, loose grains stay on fingers |
| Sandy loam | Moist, makes a ball with finger marks, very light stain, darker colour, does not stick | Slightly moist, a weak ball with finger marks, darker colour, no wet stain, grains break away |
| Loam | Moist, makes a ball, very light stain, darker colour, bends, makes a weak ribbon | Slightly moist, a weak ball with a rough surface, no wet stain, a few grains break away |
| Clay | Moist, a smooth ball with finger marks, light stain, ribbons between thumb and finger | Slightly moist, a weak ball, a few crumbs break away, no wet stain, clods flatten when pressed |
FAO lets tomato use about 40 percent of the soil's usable water before stress starts, and pepper and onion about 30 percent. So water as soon as the feel test drops into the 50 to 75 percent class, at its moist end for pepper and onion, and never wait for 25 to 50 percent.
Sources: USDA-NRCS (2001), Estimating Soil Moisture by Feel and Appearance: Program Aid 1619, copy hosted by Texas A&M AgriLife; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 8, Table 22.
How long and how often
The run time comes from the crop's daily need and the drip line's output. How often to run it comes from how much water the wetted soil can hold for the roots. This example uses the tomato figures from our crop guide.
How the tomato's daily need is worked outChoosing a drip line and dripper spacing
Sources: FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 7, Tables 19 and 20; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 8, Table 22; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, Annex I, Table 8; FAO (1988), Irrigation Water Management Training Manual No. 5: Irrigation Methods, chapter 6.
Signs the timing is wrong
- Water pools around the drippers or runs off the bed: the soil cannot take it in that fast. Split the run into shorter runs with a rest between them.
- The soil is wet deep down but dry between the plants: in sand, water sinks rather than spreads. Drippers closer together, or a higher dripper flow, wet a wider strip.
- Clay is still wet and sticky the day after watering: water less often.
- Leaves turn darker and dull, and growth slows: the crop is already short of water. By the time plants show it, they have waited too long, so go by the feel test, not the leaves.
- Young plants: their roots are shallow, so they need smaller, more frequent waterings than the example, rising as the crop grows.
Seven causes of drip irrigation failureWatering in the rainy season
Sources: FAO (1988), Irrigation Water Management Training Manual No. 5: Irrigation Methods, chapter 6; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 3; FAO (1985), Irrigation Water Management Training Manual No. 1: Introduction to Irrigation, chapter 2.
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Want watering times set for your soil?
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