Electrical – Nichrome wire not heating up in circuit

OK, you have 20VAC from the transformer, which is then full-bridge rectified to give (1.414*20) - (0.7*2)=26.9V (peak volts - diode drops), and passed through D2 (now down to 26.2V) into a Zener diode D3, which is a 9.1V diode! D3 has to pull enough current for T1.1's winding resistance to drop 17 volts. (some of this droop is provided by R118 and L8 as well.)

Toasty!

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°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 21Ω 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).