marking paint

Is there a non-toxic marking paint I can use?  Or other method?
I want to space out where I will plant my fruit trees and want to use a post hole digger to break up the clay and make sure there are no boulders within the top 12 to 18 inches.  
After the holes are dug I can put in expanded shale, gypsum and some EM.

you can use chalk. if you can find a sack of the stuff they use at ball parks that might be ideal

Bruce, great idea.  Since it is made out of crushed limestone I may just use hydrated lime since I have a small bag now.  I know it will wash away after a rain but that is no big deal.  thanks again.

John Seymour in his book self sufficiency  says
"Very good paints can be made from a mixture of sour milk, hydrated (slaked) lime, and any coloured earth pigment that you can find. The lime and the sour milk must have neutralized each other, and this can be tested with litmus paper: if the paper turns red add more lime, if it turns blue add more sour milk. The pigment that you add to this is any strongly coloured earth, sediment or clay. Dig it out and boil it in water several times, each time in new water. Strain off the water and dry- the sediment in a warm place. Pulverize it as finely as you can and store. Mix this powder with the milk-lime mixture until you get the colour you want. Keen paint-makers keep an eye out as they travel about the countryside for any colourful looking earth or clay, and grab it when they see it. "

if you want something a bit more holdfast than chalk

Another great idea. THanks

I'm looking for something for marking that will last through the winter as I have a meadow I have cleared of invasives that I am planning to replant with a number of native grasses, wildflowers and bulbs, so I'm interested in your thread.  Up until I saw this, I was thinking about using a pile of large tree limbs I've accumulated.  Would the lime and sour milk paint last through a rainy winter or is there another concoction someone could recommend?   Thanks!

Maybe some bamboo would work for your application.  Either laying down or stuck in the ground.  I need something I can drive over.