Olla irrigation has been present for thousands of years. Ancient documentation from various cultures spread across several continents proves that clay pot irrigation has been one of the most successful, long lasting irrigation methods ever. Chinese texts that are well over 2000 years old mention clay pot irrigation. The Romans used ollas. Olla irrigation can be found in the Middle East, India, and Central and South America. It’s clear that olla irrigation is wildly successful across the planet; this begs the question, how does it work? What is the science behind the function? Read further to find out.
Very simply put, ollas slowly leak out water to feed the plants. However, the process is a lot more delicate and refined than that. Ollas are made of low-fire and unglazed clay; as a result, they are porous. The olla wall is filled with miniscule holes that allow a liquid, like water to slowly move through the wall of the olla. The movement of the water across the olla wall is stimulated by a water concentration gradient. A concentration gradient describes how much of a substance is on one side of a barrier compared to the other side. The soil water potential determines how strong the gradient is. The term soil water potential describes how much water the soil is prepared to absorb. If the soil is very dry, it has a high soil water potential and a more extreme gradient, which will pull more water from the olla. The concentration gradient of the water of the soil vs. the olla powers water movement due to the desire to reach equilibrium. Nature strives to be in equilibrium, meaning that everything is balanced. All in all, the soil outside the olla sucks at the water so that it seeps through the olla wall and into the soil.
The olla naturally operates under atmospheric pressure, meaning that it works under that natural conditions of the surrounding air and environment, and does not need any man-made forces. Because only forces of nature are regulating the olla’s water release, it is a very delicate and sensitive system. Water will only be drawn from the olla until the olla has reached its field capacity. This means the soil has sufficient water for the plants, but there is still plenty of space in the soil pores. Traditional surface irrigation can result in saturation, when the soil is overwatered and no soil pores with air space remain. This causes anaerobic conditions and is detrimental to the soil. Use of ollas also prevents soil from reaching its wilting point, where there is too little water in the soil for the plants to survive.
Water from the olla can reach several inches away from the olla due to soil capillary action. Water molecules are attracted to other molecules more strongly than they are attracted to each other, and stick to the other particles. The water molecules crawl along, sticking to other particles, until there are no more water molecules to spread. Plant roots naturally sense the moisture and grow in that direction.
Because the irrigation occurs below the surface, evaporation and run off are not an issue. Evaporation occurs when water molecules on the surface of the soil turn into their gaseous phase and leave the soil – then the plants cannot use them. Runoff occurs when the soil cannot absorb water quickly enough and the excess water flows off the surface and somewhere else, where the target plant cannot reach it.