I am so sorry to hear of your problems with your Ponderosa Pines. There are a number of factors that could be causing them to turn brown and die. First, we have been in a fairly severe drought situation for the past two years. Do the trees receive regular winter watering?  Since tree roots in our heavy clay soils are shallow, root damage can occur from activities such as rototilling, for example.

There are steps you can take to help reduce water stress in your Ponderosa Pines.  

First, prevent soil compaction caused by vehicle or animal traffic near trees, which can lead to poor water penetration, adequate aeration and damage fine roots, most of which lie within a foot of the soil surface.

Next, avoid direct damage to trees and roots by machinery and do not alter drainage patterns near established trees.

You should also reduce competing vegetation by thinning trees and undershrubs.

Applying a mulch to maintain soil moisture can also be helpful.

During dry weather, especially late spring and early summer when trees are generating new growth, apply water slowly so it penetrates to tree roots by using drip irrigation lines or soaker hoses. During prolonged droughty periods be sure to water your trees thoroughly once a week. This is true of newly planted trees.

Finally, do not fertilize during drought conditions. Fertilizers are salts and can damage plant roots under stress. They can also stimulate needle production increasing a tree's water requirements.

Here are some publications you may find useful:

https://extension.oregonstate.edu/forests/tree-care/why-are-my-trees-dying

http://msue.anr.msu.edu/news/why_are_my_pine_trees_turning_brown

Hope this helps!