One of the first things you should do is have a professional soil test done. Your local extension office should be able to help you both prepare a good testing sample, direct you to where to have it tested and help you interpret the results. This initial test is intended to provide a baseline to determine soil pH, composition and mineral content, organic matter content and existing nutritional levels and make appropriate suggestions for amending.

In general, ideal soil conditions are seldom present and will need to be created by the home gardener. Most soils will benefit by incorporating quantities of organic matter, like compost or composted manures. Cow manure is an excellent soil amendment but needs to be well-aged or thoroughly composted before being used in the garden. The presence or development of weeds or grasses typically indicates the product has not been properly composted.

It's hard to describe exactly what ideal soil conditions are but if the soil is loose and friable, has varied texture (indicating the presence of organic matter), drains well yet retains moisture and has healthy populations of worms and other soil creatures, you are off to a good start.

It may help to visit the Soils and Compost forum to gain additional information. Some of the regulars there take the subject to extremes but there is still a great deal of good information to be gained from reading through many of the current posts and the archives.

How to compost.

If you have a 4 gallon or larger plastic container with a lid (that's MOST important) you can make liquid fertiliser. You put your cowpats into a burlap sack, tie up the neck with enough string to also make a loop or a tie - and fasten the ends to the handle (so you can haul it out later). Fill the container about three fourths full with water and put on the lid to keep out flies.

Leave it in a warm place at the far end of the garden for about a month. The water will go a darkish brown. Add enough of the brew to fresh water in a watering can to look like weak tea or coffee and use it to water your plants.

You can also recycle the baggie your potting mix came it. Put in your cowpats and dampen them a little (no lakes at the bottom). Then put the baggie into a cool and shady spot. Leave it for six to twelve months (composting can be quite a slow process...). You'll know it's done when there is no smell, the pats have turned into a fluffy dark substance, and any tiger worms (red with yellow bands) - or whichever real worms in your area love to hang out in damp cardboard and rotting vegies -have left for new food.

At that stage your compost can be added to potting media. Just make sure that there is grit added to ensure a good root spread. Crumble it through as if you were making breadcrumbs.

You can do the same in autumn only then you can do a layer of old leaves and a layer of cowpats, alternating. And a sprinkle of water to keep it moist but not dripping wet. You can leave it out over the winter.

If you have a no-see area of the garden you can use containers such as large cardboard cartons (the sort you might get fridges or washing machines shipped in). Open up the bottom so your compost items can go directly onto the soil. Pin down the bottom flaps so the container stays where you put it.

You can add big pieces of material but they take ages to break down and need sieving out unless you don't mind them on your beds. Build in layers and make your cowpats a fairly frequent though thin layer.

You can also dig a trench in the garden and do exactly the same there.

Another possible is to make a mound - cowpats, covered with soil, covered with cowpats, soil - always ending with soil. You can do that in late summer and leave it over the winter, maybe with a soil protection such as old sacking or plain old carpet (no rubbery backing). When your ground is ready in spring that heap can be useful for growing pumpkins, or zuccini, or cucumber - curcurbits. And rhubarb also loves big feeds like that.

If your pats are dry and stone free you could put them through the mulcher if you have one. Be sure to wear a nose mask to keep the dust out. Just add the grinding to your soil and mix through. Damp the soil before planting.

You need nitrogen, carbon, & water to make good compost.

Nitrogen:
manure
grass clippings
kitchen scraps
coffee grounds (Starbucks & local coffee/doughnut shops are good sources, but you have to pick it up every day, as they can't keep "garbage" inside the restaurant overnight)
teabags

carbon:
autumn leaves
shredded paper & cardboard

Cardboard is easier to tear up if you soak it first.

Pile it all up together, add water/old coffee/tea/juice/beer/urine until it's about as wet as a wrung-out sponge, & sit back & wait for it to decompose into rich, ozone-smelling compost.

Mix it with your native soil, plant your plants, & mulch well with undyed wood mulch.

Be sure to plant each plant in its best location:
tomatoes in full sun
hostas in shade
grapes against a fence or other structure they can climb
etc

Best luck & have fun!

And it sounds that they've been sitting out in strong sunlight for a while which is guaranteed to turn thin plastic brittle.

Cockroaches will head for anywhere there's shelter, food, and a touch of damp - darn it! I don't think it means there's human waste in your manure (unless you have good reason to suspect this).

Compost heaps, the slow ones, are always home to many sorts of insects and invertebrates. It's part of the way they work. You can make a 'hot' heap' which reaches high internal temperatures if you have plentiful supplies of raw materials. For example - lots of freshly chipped tree leaves and twigs, or vast amounts of grass clippings mixed with fallen leaves. Those big piles - at least 4'x4'x4' to start with - can become very hot in the middle and destroy seeds, bug eggs, and some of the fungi at least. Many gardeners can only dream about having such abundance...sigh.