What is this strange ash-like substance in my garden soil?

Hi Vicky welcome to goy - as you say this all over garden so lets hope nothing fungal .Sid v. good with soil conditions.This
garden new to you ? Garden lime powder used to be commonly used to break up heavy clay soils esp on vegetable patches.Not so good for planting shrubs and flowers as those that like acidic clay soil not so keen on lime - alkaline.You could try a soil test kit.Put some soil free of' 'ash' to test - then test ash without soil.Soil will react to show acid if ash is lime this will react as alkaline.Hope this helps!