How to measure root depth to assess drought-resilience in crops
This heatwave is a timely reminder of the crucial role water plays in crop performance and the importance of sufficient rooting depth to secure it
On average, each tonne of crop biomass requires around 20 mm of water per hectare. For high-yielding cereals, their water demand can reach 300–400 mm between spring and mid-summer, far exceeding the rainfall typically received during this period in most regions. Without supplementary irrigation, the rest must come from the soil.
The importance of subsoils
As we explained in an older article – To irrigate or not to irrigate – soils are capable of holding between 60 and 300 mm of water per metre depth, depending on texture. However, this is only helpful if crops can reach it. While much attention in recent years has focused on topsoil health, it is the subsoil that often limits the amount of water available to the crop.
Compacted or poorly structured subsoils will restrict root growth – preventing crops from tapping into deeper reserves. In contrast, well-structured soils with cracks and pores allow roots to explore further, improving drought resilience. This is also the reason fields with good soil structure tend to retain their green canopies for longer.
How to calculate crop rooting depth
It’s possible to estimate how deep your crops need to root with a couple of calculations.
First, we have to make some assumptions on crop water use:
- Maize requires 18mm/tonne
- Oilseed and legumes require 25mm/tonne
- All other crops and vegetation require 20mm/tonne
With your selected crop, three factors then come into play:
- Your field’s subsoil water holding capacity in mm per metre depth, adjusted for stone content. See our table below.
| Sub-soil texture | Water held, mm per metre depth |
|---|---|
| Sands | 50 |
| Loamy sands | 100 |
| Fine sands, silts and clays | 150 |
| Organic soils | 200 |
| Peaty soils | 250 |
| Marine silts | 300 |
2. April to July rainfall in mm (measure this on farm or look it up online)
3. Final crop biomass in t/ha.
Calculate final crop biomass using this formula: Dry yield (in t/ha) ÷ Harvest Index = Final crop biomass.
The table shows the harvest index of different crops.
| Crop | Harvest index |
|---|---|
| Wheat | 0.52 |
| Barley | 0.51 |
| Oats | 0.46 |
| Oilseed rape | 0.35 |
| Peas | 0.52 |
| Beans | 0.58 |
| Forage maize | 0.90 |
| Potatoes | 0.80 |
With all this information to hand, calculate the required rooting depth using this formula:
((Final crop biomass x crop water need) – April to July rainfall) ÷ your subsoil’s water holding capacity = the root depth required for your crop
A practical example
Field X:
- Has a silty clay with 7% stones capable of holding 140mm of water
- Recorded 186mm of rainfall between April and July
- Produced winter wheat with a final crop biomass of 18 t/ha
…and given that wheat requires 20mm/tonne of water…
((18 x 20) – 186) ÷ 140 = 1.24.
To have sufficient water, the crop on field X must grow roots that go down 1.24m.
If you keep repeating these sums year on year (your soil’s water-holding capacity will be the same every year), a trend will emerge. Patience is key as this may take several years, depending on the number of wet and dry summers.
Make improving sub-soil condition your ‘long-term’ game plan
Our weather is becoming increasingly unpredictable, and unfortunately, rainfall alone is rarely sufficient to meet the needs of crops. Improving subsoil condition is a slow and difficult process, but continually working towards improving your sub-soils will deliver benefits both for your crop productivity and long-term resilience.