Dry springs are becoming the norm these past few years. However, with crop prices higher than ever before, this spring in particular is of critical importance.
In this article, we evaluate (in £s) the rain or water being irrigating crop by crop, and show the importance of each extra mm rain, each extra centimetre of rooting depth, or each irrigated mm.
Impact on crop growth
Crops need water to grow. No water today = no growth today. Soils hold water that supplies crop growth every day, until it is exhausted, or until it rains.
Luckily, for us in the UK…
(a) All soils refill with water every winter (with very rare exceptions) right to the bottom (if you know where that is!),
(b) Sometimes it rains in summer!
The more that crops grow the more water they need, it’s proportional. Most crops have very similar demands for water. Each bit of biomass they grow, they need about 200 times that in water. This means for every 5 grammes of crop growth, they need a litre of water. Every tonne per hectare needs 20 mm (or a bit more if they make oil instead of starch, or if they fix their own nitrogen).
With average sunshine, potential biomass growth through May, June, July, and August is about 20 tonnes, so 400 mm water is needed to support this. Most crops cannot be kept fully green through all four months, but most farms would hope to maintain crops with full green canopies for around three of these months, so 300 mm is a more realistic target.
Very few farms can expect 300 mm rain through May to July or August, so the rest must come from somewhere, and without irrigation, it must be from the soil.