Andrew Ross explains how much water our crops need in warm weather – possibly much more than you thought!

Water meters help us to discover the condition of the soil and new solar-charged irrigation is taking some of the load – but we still depend heavily on stakeholders with watering cans!

The irrigation project

Mike Lucas has recently completed Phase 1 of our irrigation project. It is all run on solar energy and draws water from the pond. A panel collects the sun’s energy which is stored in a battery for night use or cloudy day use. One of our pumps is capable of pumping 200 cu m of water per month into our IBCs at a slow but steady rate. The other pump feeds the standpipes situated along the Roman Road. Hopefully, gone are the days of using a petrol burning engine and lugging heavy and leaky standpipes (although they remain in reserve).

Mike will be beginning some experimental work on Phase 2 this summer, using timers attached to the IBCs and drip feeds to small areas of appropriate crops.

Rainfall, evapotranspiration, and watering

We are now entering the time of year when evaporation from the surface of the soil and from plants generally exceeds the rainfall and so we need to make up the difference with irrigation. (1 mm of rain is equal to 1 litre of rainfall or watering on a square metre).  

Last week at Highbridge Community Farm, around 25 litres of water evaporated from each square meter of vegetated land!

With normal June weather, we can expect around 92 litres of water to be lost from each square metre over the whole month. If the weather is hot, dry and windy, this might even be more! So, if we should have a dry June, July and August like 2022 with a total of only 60 mm rain, we will have to do a lot of watering. Our watering cans hold 10 litres.  If no rain falls in June, each square metre of actively growing plants will need around 9 or 10 watering cans of water over the month – that is 2 or 3 cans of water per square metre per week.    

We have added a lot of organic matter to our soil which acts like a sponge holding water. The top 30 cm of soil will hold about 90 litres per square metre when at its field capacity. As the soil dries out, plants grow less well and stop growing when the amount of water in the top 30 cm falls to about 60 litres. By the time the soil has only 30 litres of water left in, it most of the plants will have died!

Watering your crops

  • Please do water your crops thoroughly, particularly if little or no rain has fallen. It isn’t sufficient just to wet the surface. You need to see an increase in the moisture levels down to 10 cm and ideally to as far as the roots grow down and to where you find that the soil is already wet.

This is a moisture meter. Use this before and after watering. Holding the metal sensor, push it into the soil to a depth of 5 cm and read the gauge. Push it in about 10 cm and read again. After watering, the readings at 5cm and 10 cm should be at least a MOIST 6 to a WET 8 on your scale.   

  • For most crops (except salads), it is better to water a bed thoroughly with 2 or 3 cans of water per square metre once a week than to sprinkle a can over a whole bed every day as most of that water will not infiltrate more than 2 or 3 cm. This is because deeper roots will not receive any water and a lot of the surface moisture will simply evaporate away very quickly.  
  • Ideally, water in the evening (or early morning) to give plants the opportunity to take up as much water as possible. Water the soil, not the plants. 

Water is a limited resource. Use it wisely!