An olla is an unglazed clay pot buried to its neck and kept full, which seeps water into the soil at a rate set by how dry the soil is, so it suits a few widely spaced plants. By the carrier guide’s own figures, a 1-gallon pot serving a 3-foot radius that empties in 2-5 days supplies only about 0.29-0.72 millimetres of water a day over its patch, far below the several millimetres a crop uses in hot weather. Calibrate each pot by marking the water level, cap the neck, and empty the pot before frost.
An olla is an unglazed, porous clay pot buried in the soil beside plants and filled with water, which seeps through the clay wall into the root zone.
The Soil Draws, And The Pot Supplies What The Soil Takes
Water moves from the pot through the pores of the clay into the soil. The drier the soil around it, the faster the water is drawn out; as the soil wets, the flow slows. The pot therefore supplies on demand, to a limit set by the clay wall’s permeability. Roots grow toward the wet zone.
The carrier guide (Drought Resilience & Water-Efficient Agriculture) says an olla gives “zero evaporation loss”. That is too strong: water can evaporate from the open neck, and from the damp soil near the surface if the wet zone reaches it. A cap and a mulch keep the loss small. A related guide, Clay Pot Irrigation, covers the method in more detail.
Worked Example: What One Olla Supplies
The carrier gives: a 1-gallon olla serves a 3-foot radius; refill every 2-5 days; about four ollas per 100 ft² bed.
- Capacity: 1 US gallon = 3.785 L.
- Area served: π × (3 ft)² = 28.3 ft² = 28.3 × 0.0929 = 2.63 m². Four ollas over 100 ft² (9.29 m²) is 9.29 ÷ 4 = 2.3 m² each, consistent.
- Supply if the pot empties in 5 days: 3.785 ÷ 5 = 0.76 L/day; in 2 days: 1.89 L/day.
- Depth over the patch: 0.76 ÷ 2.63 = 0.29 mm/day; 1.89 ÷ 2.63 = 0.72 mm/day (1 L over 1 m² is 1 mm).
The crop water use in hot dry weather is commonly several millimetres a day. FAO Irrigation and Drainage Paper 56 gives reference evapotranspiration in warm to hot climates of roughly 4-8 mm/day (quoted from memory; check it). The olla patch therefore receives a fraction of full demand. Ollas fit sparse plantings (the dry-farming spacing), cool seasons, or crops needing little water, or they must be larger or placed more closely.
What You Need
- Unglazed terracotta pots, 5 L or more, with the drainage hole sealed with a clay plug, wax or silicone, or two pots joined rim to rim.
- A lid (a flat stone or saucer).
- Rainwater or other soft water if you have it; hard water leaves deposits that clog the pores.
Method
Step 1: Test the clay. Drop water on the pot wall. It should darken and soak in; if it beads, the pot is glazed and will not seep.
Step 2: Seal the hole, and test for leaks.
Step 3: Bury the pot to the neck in loose soil, and plant 10-30 cm (4-12 inches, the carrier’s distance) away.
Step 4: Fill and cap.
Step 5: Calibrate. Mark the water level, return in 24 hours and measure the fall. Do this for a week and on hot and cool days to learn the rate in your soil.
Step 6: Refill before it empties, and refill with a cup, not a flood.
Step 7: Empty the pots and store them before a hard freeze. Water in the clay freezes and cracks it.
Diagnosing Problems
- No drop in water level — clogged pores, glazed pot or very wet soil.
- Pot empty in a day — a crack or an unsealed hole.
- Plants wilting despite full pots — the pot supplies too little for the crop; add pots, add a second watering method or thin the plants.
- Mosquitoes — the neck is uncapped.
Common Mistakes
- Using a glazed pot.
- Planting too far from the pot — outside the wet zone.
- Treating one pot as full irrigation for a thirsty crop.
What’s Next
- Learn next: Scheduling: When and How Much — matching supply to need
- Combine with: Wide Spacing for Root Competition — the planting that suits ollas