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.

Taking care of your sub-soils

Soils hold from 60 to 300 mm water per metre depth depending on texture. Whatever the soil, if roots go deep enough, crops will have plenty of water for high yields. For example with the Yield Enhancement Network competition, a crop of rye entered ingrown on a coarse sand with pebbles and it won! We estimate that it did so by taking water all the way down to 3 metres and more!

Table 1.    Soil Water (mm) available for winter or spring cereals according to soil texture, rooting depth & subsoil structural conditions.

Soil Texture Winter Cereals rooting to 1.5m Spring Cereals rooting to 1.0m
Subsoil conditions: Average Good Average Good
Loamy medium sand over sand 90 90 65 65
Clay loam over clay 140 230 100 160
Silt loam to depth 230 270 160 180

 

Despite the current focus on topsoil health, it’s actually the subsoil that most affects the water supplies for crops. Not only rooting depth but subsoil structure is hugely important here (Table 1). Good, deep subsoils are essential to provide the resilience that farms need to sustain their crops through tricky years as well as through the occasional good year.

Weather data (see tables at the end of Crop Action) show that, across England, 2022 had the driest March since 2012, and now April has also been dry (rainfall ranged from 63% of average in the north to 38% in the east; see maps here) so we already have Soil Moisture Deficits (SMDs) of 50 mm in East Anglia. This means that some lighter soils have already lost much of their available water, which will be affecting your worries (and prayers!), depending on how much water you think your soils hold (see Table 1).

We don’t yet know whether this will be a good year but, with current events, we are pretty certain that crops will be more valuable this year than ever! So, it is probably worth reconsidering the value of supplying extra water to crops, whether by irrigation, deeper rooting or praying or dancing for rain! Table 2 shows the results of our calculations.

Table 2. The value of extra water supplied to deficient crops, estimated from typical proportions of biomass that are harvested (harvest index), typical efficiencies of water use, water in sold produce, and guesstimated future crop prices for 2022 harvest.

Crop Harvest index (crop yield, % total biomass) Water Use, mm/tonne crop biomass/ha Estimated 2022 Crop Price, £/tonne Water in sold produce Approx. 2022 Water Value, £/ha per mm
Sunflowers 43% 25 £850 9% £16
Oilseed Rape 35% 25 £850 9% £13
Milling Wheat 51% 20 £350 15% £11
Potatoes 80% 20 £220 79% £42
Grain Maize 52% 18 £300 15% £10
Feed Wheat 51% 20 £310 15% £9
Malting Barley 51% 20 £300 15% £8
Field Beans 58% 25 £310 15% £8
Barley-feed 51% 20 £270 15% £8
Combining Peas 52% 25 £300 15% £7
Oats for milling 46% 20 £270 15% £7

Potatoes are the arable crop most commonly irrigated. However, current global events and current price expectations, especially for oilseeds, are making their irrigation more worthwhile, compared to potatoes.

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This advice was first published in our technical arable update Crop Action 28 April 2022 edition. Crop Action provides regular up to date tailored advice based on current weather conditions and crop development on topics including weed, pest, disease, nutrition and soil management.

To learn more about Crop Action, visit our Agricultural Publications webpage.

Alternatively contact Chloe Morgan (Joint editor) on 01623 848332 or email chloe.morgan@adas.co.uk