Preparation is the key to growing vegetables using hay bales in place of soil. Seeds can be planted on top of the hay or make small hole and add some potting soil. There are various different views on whether to use straw (stems of grains) or hay (grass that has been cut and dried). Recommendations for using hay here. Staw is more likely to produce sprouts from leftover grain seeds.
Day 1 - 3 Water and keep bales wet for 10 days
Day 4 - Pour 5 ounces (ten tablespoons) of ammonium nitrate fertilizer onto bales (Miracle Gro)
Day 7 - Pour five tablespoons of ammonium nitrate fertilizer on top
Day 10 - Pour 1 cup of fertilizer with an nitrogen, phosphorus and potassium ratio of 13-13-13 (Spectrum, Jobe, Greenleaf)
Day 11 - Put seedlings on top of the bale by driving the hay/straw apart using a trowel. For plants, make an indentation and add some potting soil before planting.
Note: Bales held together with synthetic twine are less likely to ball apart. Keep bales moist as they will dry out quickly.