Wide spacing gives each plant a larger volume of soil to draw water from, and because spacing is a length, doubling it in both directions quadruples the area per plant. The carrier guide’s dry-farming spacing of 2-3 times normal is linear, so the area per plant rises four to nine times, and the number of plants per acre falls by the same factor. Per-plant yield rises less than per-plant area, so yield per acre falls; you buy water security with land.
Wide spacing in dry farming means planting fewer plants per unit of land so the stored soil water is shared among fewer roots and lasts through the season.
A Plant’s Water Share Is Proportional To Its Area
Stored soil water per square metre is fixed by the soil and the rain. Each plant gets the water in the ground it can reach, which is roughly proportional to the ground area it has to itself. Spacing is measured along a line, but the area per plant is the product of the row and in-row spacings, so a doubling of spacing in both directions multiplies area by 2 × 2 = 4, and a tripling by 3 × 3 = 9.
The carrier guide (Drought Resilience & Water-Efficient Agriculture) gives dry-farmed spacing at 2-3 times normal, with examples: wheat in rows 12-18 inches apart against 6-8 irrigated; tomatoes 5-6 feet apart against 2-3 feet; squash 8-10 feet apart against 3-4 feet. Checking the ratios: wheat rows are 12 ÷ 8 = 1.5 to 18 ÷ 6 = 3 times as far apart; tomatoes 5 ÷ 3 = 1.7 to 6 ÷ 2 = 3; squash 8 ÷ 4 = 2 to 10 ÷ 3 = 3.3. So “2-3×” is a loose summary of linear spacing.
Worked Example: Tomatoes
Illustrative inputs: stored plant-available water of 100 mm in the root zone (1 mm over 1 m² is 1 litre, so 100 litres per square metre); normal spacing 0.75 m square (about 2.5 ft), wide spacing 1.7 m square (about 5.5 ft).
- Area per plant: normal 0.75 × 0.75 = 0.5625 m²; wide 1.7 × 1.7 = 2.89 m². Ratio 2.89 ÷ 0.5625 = 5.1.
- Water per plant: normal 0.5625 × 100 = 56 L; wide 2.89 × 100 = 289 L.
- Plants per hectare: normal 10,000 ÷ 0.5625 = 17,778; wide 10,000 ÷ 2.89 = 3,460.
For the wide-spaced field to equal the yield per hectare of the close-spaced one, each plant would need to yield 5.1 times as much. Plants rarely do. This is the carrier’s “yields per plant increase, yields per acre decrease” trade-off, with a size. The 100 mm is an assumption; your soil and rain set the real number.
What You Need
- An estimate of your stored water per square metre (the squeeze and auger tests in Dry Farming Techniques).
- Seed enough for the planting (wide spacing uses less).
- A measured trial with two spacings in the same soil.
Method
Step 1: Plant a trial of two spacings of one crop, side by side, with the same care.
Step 2: Record water stress (wilting time), plant size and yield per plant and per square metre.
Step 3: Choose the spacing that keeps most plants alive in your poorest year.
Step 4: Compensate the loss of per-acre yield by planting a larger area, if land allows.
Common Mistakes
- Reading “2-3×” as area — it is length; area is four to nine times.
- Wide-spacing without weeding — the open ground fills with weeds that take the water.
- Wide-spacing a crop that needs a canopy — grain crops such as wheat may thin out into patchy stands.
What’s Next
- Learn next: Deep-Rooted Crop Selection — the other half of the water share
- Combine with: Scheduling: When and How Much — if you irrigate at all