Soil temperature should be upwards of 12 degrees for spuds, but they will lie there waiting for it to warm up. A bit like me in the mornings..........
I plant them deep, cover them with soil then a heavy dose of sheep poo over the surface, not dug in. I do this after it has rained. One of the things with spuds is that even though, they need good drainage, you also have to watch out for water stress. If they dry out at any stage it will retard their growth. Mulching with thick straw helps in many ways to retain the soil at a good moisture level, but will also encourage the spuds to grow nearer the surface so be judicial about how thick you put it.
Different soils will hold moisture for different periods of time and you can only really gauge it by knowing your own climate. Poor growth is more likely to be caused by the soil drying out as much as any other cause. Keeping a potato crop at optimum moisture levels for the growing season requires management and is not a "plant them and leave them" situation. There are crucial stages where you can manage water over and above what falls from the sky.
A few tips from a long time Murphy:
Avoid watering on planting to try to get them started. If the soil is dry before planting, give the ground a good soaking at the base of your planting rows. Plant them then leave them, obviously making sure they don't dry out entirely.
If the crop is watered either just before or just after the shoots emerge, you can cause them to rot or get affected by fungus that is not obvious. until you see the plants wilting.
As the spuds are setting tubers you need to encourage plenty tubers by watering more to cool the ground than to feed the plant. If the soil gets over 20 degrees, the plant actually reabsorbs the setting tubers, so if you get a hot few days water to keep soil temperature down.
As the spuds are bilking up, i.e. growing, much will yield will depend on avoiding stress. Even slight moisture stress at this stage will reduce your yield and this will occur 'in the dark' well before you get any ideas that they may be struggling. ( Severe stress at this stage will cause distorted tubers). One of the things that can occur is cracking. This happens as the tubers dry out and start to form skins, then get more water that brings on an additional growth spurt. You should avoid them drying out followed by heavy watering to avoid this.
In essence, while the tubers are growing and getting big, their final size depends entirely on soil moisture. If the crop is watered until the death of the tops, you will get large tubers but with some varieties risk hollow hearts. So don't overdo the watering at the final stages, unless you are growing a variety like Kennebec or Nicola for baking or chipping. However, don't let them dry out entirely, if you are growing over a hot period, because the hot soil will deplete both the keeping and cooking quality of the spuds. Some varieties, such as Sebago, develop tuber disorders if allowed to finish too dry.
I always water lightly for a few days before harvesting to soften the soil. This lets the kids get lovely and dirty before I send them home whistling
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