Hello. I just acquired two 40 foot date palms that are growing in west Sonoma County CA in heavy clay soil. The ground water is eight to -ten feet deep. The bottom palm fronds are yellow so I think they need fertilizing and maybe water? Any tips or suggestions would be appreciated

Greetings pictures would help. But if they have just been transplanted into the ground from what you’re saying. There would be stress involved gor them and a few leaves going yellow on the bottom leaves one would expect that to happen. Water with seaweed extract each watering for now. When you see some good growth happening then you can start to apply some fertiliser. Let the settle into there new home before you really start to push them with fertiliser. The heavy clay soil will gor a time act as large container until they tuck in and get there roots established, and with the amount of nutrients locked up in clay a large palm should be able to get into those nutrients once established.

@Larry Martin welcome to Palmtalk! Yellow tips on older palm fronds is fairly common, usually caused by a mild Magnesium deficiency. Magnesium is mobile in plant tissue. When the palm can't get enough from the soil (or fertilizer) then it'll "eat" the oldest fronds to power new growth. Some "palm" type fertilizer similar to PalmGain should help. Most include 2-4% Magnesium as part of the mix. If the deficiency is severe then some extra granular Magnesium Sulfate will help, or Epsom Salts. On my palms with consistent yellow tips (usually after winter) I sprinkle ~2 handfuls of magnesium sulfate in around a 10 foot diameter around the base. This helps to prevent yellowing, or makes it slow down. Keep in mind that once a section is yellow, it'll never turn back green. Plants don't "heal."