Choose crops with deep roots for dry farming, but first check that the soil lets roots go deep and that there is stored water down there to reach, because a hard layer, rock or a poor subsoil stops them. Look at the soil profile with an auger or a shallow pit, and look at the roots of last year’s crop. A deep-rooted crop also empties the subsoil, so the next crop may start with less water in a dry year than the soil could normally supply.
Deep-rooted crop selection is the choice of species whose roots reach moisture stored below the dry topsoil, matched to a soil that allows them to grow down and holds water at depth.
Roots Go Where The Soil And The Water Let Them
The carrier guide (Drought Resilience & Water-Efficient Agriculture) says to choose varieties that reach moisture 3-6 feet below the surface, so they survive when topsoil is dry. Whether a crop reaches that depth depends on the crop and on the soil. Compaction, a plough pan, rock, a cemented layer, a permanent water table, salty or very acid subsoil and dense clay can each stop roots short, however deep-rooted the species is in a loose soil.
Which Crops Root Deeply
The carrier credits deep roots to winter wheat (roots reach stored winter moisture), watermelon in sandy soils and Jerusalem artichoke; it lists sorghum, sweet potato and cassava for drought tolerance by other means or without naming the mechanism. Trees and perennial crops, such as the food forest’s canopy and sub-canopy trees (Canopy and Sub-Canopy Trees), reach deepest of all once established. Rooting depth for any crop varies with the soil, so treat a published depth as a hint.
What You Need
- A soil auger or a shovel.
- Last season’s crop roots, dug up after harvest.
- A record of rainfall and subsoil moisture from the previous season.
Method
Step 1: Read the profile. Auger or dig at several places in the field. Note topsoil depth, where the colour or texture changes, any hard or wet layers and where roots stop.
Step 2: Dig up the roots of a crop at the end of the season, from the side, and see how deep they went.
Step 3: Match crop to depth. Put the deep-rooted crops on the deepest soil and the shallow-rooted ones on the shallowest.
Step 4: Rotate with shallow-rooted crops so the subsoil can refill in a wet year, and do not plant two deep-rooted dry-season crops in a row after a dry winter.
Step 5: Break a hard layer if you have one, with a deep-working tool or deep-rooted cover crops (Cover Crops).
Common Mistakes
- Assuming roots go deep because the crop is “deep-rooted” — a hardpan stops them.
- Planting deep-rooted crops year after year in dry climates — the subsoil empties and does not refill.
- Going down a pit to look — see the safety note.
Safety
Look at roots from the side or with an auger, not from inside a deep pit
Do not climb into a pit deeper than 1.2 m, and do not work in an unsupported pit of any depth with the spoil piled at the edge or alone. Dig a stepped or sloped face, use an auger, or dig the plant out and trace the roots from above.
What’s Next
- Learn next: Drought-Tolerant Varieties — which species and varieties
- Combine with: Soil Structure — keeping the soil open to roots