heat air with nichrome wire
Let's assume you can surround your 4-hectare field with R-1 (SI units: 1 K-m2/W) insulation (e.g., approximately 1 inch of foam board insulation), top, bottom and sides. That's a total of 81600 m2 of heat-transfer area. You want to raise the temperature by 6&deg;F, which is 3.33 K, so the heat flux is over 270 kW.

You'll need almost 1200A @ 230V, or about 11A per row. That would be a resistance of 230/11 = about 21&Omega; per row. You could get this as a single strand of 3.6mm Nichrome (roughly AWG 7) or as, say, 8 strands of 1.27 mm copper (AWG 16).