Water Storage Systems to Suit Soil Conditions

Our lovely politicians have been busying themselves and we are now a little closer to knowing the extent of the water charges that every household is to incur each year. The results don’t make for good reading for those of us who use free tap water to irrigate our gardens. It certainly looks clear that water storage systems, both big and small will be needed to replace the once free flowing water.

Each garden is different and has different watering storage and watering requirements. We add water to plants to ensure that water is available to plant roots. Each soil is different and can hold different amounts of water. Water is held by soil in 3 ways…

• Locked in tiny particles – Unavailable
• Held in small pores – Available
• Sitting in Large pores – Available but will drain away – replaced by air

The amount of water & the type of water storage system that a garden needs is dependent on the following factors

The type of soil

Peat soils have a very high field capacity for holding water but also lock some water in tiny particles. The result is that more water in the soil is available to plant roots in these soils. Peat soils hold 90mm of available water in small pore spaces. This equates to 90l per 10m2.

Clay Loam soils have a high field capacity for water but also a lock a small amount of water in tiny particles. This means that at times of drought irrigation is important as water availability is low. Clay loam soils hold 55mm of available water in small pore spaces. This equates to 55l per 10m2

Course sands have both a low field capacity & low wilting point. These soils hence have low available water content. Course sandy soils hold 25mm of available water in small pore spaces. This equates to 25 l per 10m2

Weather conditions

Wind, evaporation & humidity effect water loss from soils. They also influence plant growth which leads to further loss of water from soil. The other side is rain, and this will obviously add water to the soil. Wind & evaporation reduce the amount of available water in the soil meaning further irrigation would be required. About 20mm of water can be absorbed from the soil’s surface daily in Warm & windy & low humidity conditions. This equates to 0.2litres over 10m2


Poor drainage in soils can be caused by compaction, impermeable layers, iron or clay pans, and high water tables. Where drainage is poor land can be unusable while where land is well drained irrigation and water storage may be required.

Cover crops

Different crops require water at different rates. Water loss from soils growing wheat can be about 50mm. Over 10m2 this equates to 0.5 Litres of water daily.

These factors combine to effect the soils water conditions and will advise you as to how much to water and what size water butt or water storage system you will require. Your Irrigation system should ensure that soil never drops below 50% of available water. In most cases rainfall will return soil water levels to and above field capacity (above field capacity the soil is considered saturated)

In times of drought your irrigation system will step in to replace the rainfall. Ideally you should never water less than 25mm of water at one time. This equates to 2.5L of water per 10m2. This should be applied daily to lawns, beds & vegetables in drought conditions, but the conditions stated above will determine the exact amount of water required.

For a vegetable garden of 3m x 10m, you would require a minimum of 75 Litres per day. Therefore to maintain a steady watering system you would certainly need a water butt with a minimum capacity of twice that.