I am planning to set up a drip irrigation system for my tomatoes.

How many drippers (or emitters) will I need for pots with an approximate volume of 9 litre (2.4 US gal) in order to achieve an evenly moist soil?

Or: does two emitters for each pot sound reasonable?

I have used watering clay cones in the past, however only one for each pot and the pots were smaller. After a while I realized that the roots had grown very strongly only around the watering cones but not in the rest of the soil, since it was unevenly wet.

Therefore I I would like to know if a drip system with 2 emitters in 9 litre pots will produce an evenly distributed moisture. I have searched the web but haven't found any site covering this issue explicitly.


There could be a few factors at work here: capillary action, water adhesion and flow rate. Taking the latter first, if there is not enough water going into the soil then clearly it will only wet a small volume, so one thing to try is to increase the flow rate either as you suggest by doubling up on the emitters or greater throughput of water.

The main reason that water is able to move sideways in soil is through capillary action which allows water to move against gravity. So critically the soil medium has to be homogeneous, that is completely uniformly endowed with channels of air pockets through which the water can travel. What can happen is that a seedling is brought on in a special seed mixture and then moved to a different size pot using a different type of soil, for example a peat based seed mixture and then a regular garden soil for the larger pot.

For water to move around in the whole pot it has to be evenly moist - once a region of the pot becomes dry it is then very hard to re-wet it. The reason for this is that water can only pass in volume through open channels by sticking to other water molecules, not dry foreign particles from which water is repelled. First we have to make water adhere to the soil, then more water can cohere to the water that already adheres to the particles. Water that coheres is available to plant roots, but water that adheres is not because water that adheres binds very tightly to the particles.

Recommendations for number of emitters and flow rate depend very much on the stage of the crop, temperature and relative humidity and so on. At the height of the season here in hot, dry conditions I can be applying 2,3,4 litres of water on each plant, but at seedling stage of course a lot less than that.

  • This answer solved my problem. Its key statement is that the soil must be evenly moist at the offset, otherwise it will not work as desired. I suggest to start a new paragraph with this statement in order to bring it out. Further, as stated, the homogeneity of the soil is a crucial prerequisite. (Many thanks.)
    – Will
    Mar 5 at 5:48

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