Years ago I had a bunch of hens that all began laying soft shelled eggs when they should (theoretically) have been getting sufficient calcium. They were all on standard layer diets at the time, but I was also providing dolomitic limestone in a hopper. The other thing of relevance (when I found out what I'd done wrong) was that they'd been raised on a home-made grower diet that was high in meat meal.
After researching as much as I could, I learned quite a lot about soft shelled egg laying!
DIET AND MANAGEMENT
1. Okay, this is obvious: calcium deficiency. Increase availability of shell grit! However there are many other causes of calcium deficiency, so keep reading...
2. Calcium isn't being absorbed properly. That is, the diet may be sufficiently high in calcium to make eggshells, but for some reason the bird isn't able to take it on board. There are a lot of things to include here. For instance, heavy worm burden, coccidiosis, enteritis and other illnesses may affect the gut's ability to absorb what it needs to from food. A diet too low in animal-based protein may also compromise gut health, as vitamin B12 comes predominantly from animal sources, and is vital in establishing a healthy gut wall. Finally, algae in the water, even in small amounts, can inhibit absorption of nutrients including calcium. Never let water go even faintly green! Overall, if the calcium is there in the diet, but shells remain soft, you may need to look at whether your chickens have a healthy digestive tract.
3. Calcium-phosphorus ratio may be upset. The proper ratio is not far off 1:10 (calcium to phosphorus). Now here's where my meat meal mistake comes in... Too much phosphorus compared to calcium stops calcium being absorbed. Meat meal is often too high in phosphorus, even though its calcium content is high as well. The result can be soft shelled eggs! Bandsaw dust from the butcher may also cause problems with calcium-phosphorus ratio. The fix is obviously to remove any unbalanced feeds and increase plain calcium. Laying hens need about 3-4% calcium in their feed.
4. Too much calcium fed to growing pullets will reduce their ability to absorp calcium later in life (when they start to lay). The damage caused by high calcium too early can be permanent.
5. Vitamin D deficiency. Vitamin D is necessary for the absorption of calcium. Without sunlight or a D3 supplement (like cod liver oil), it doesn't matter how much calcium is in the diet, it won't be absorbed.
6. Dolomitic limestone is *not* an appropriate calcium source for layer hens. The reason is that dolomitic limestone is too high in magnesium, and this competes with calcium for absorption. Plain crushed limestone should be fine.
7. Mycotoxins (toxins released by moulds) can inhibit nutrient absorption and affect shell quality (as well as laying generally). Remove any mouldy feeds and preferably only store chicken feed in airtight containers, and for no longer than a month or so. Even feed that looks perfectly clean and healthy may harbour mould toxins at levels that can harm hens.
Soft shelled eggs are more likely to be diet related, but can also be a virus causing Egg Drop Syndrome (or EDS). As EDS has been eliminated from commercial breeders it's less prevalent these days, so most cases of soft shelled eggs will *not* be virally caused. Still, if you have adult hens that suddenly and persistently start laying soft shelled eggs (for up to 18 days or so) then it may be EDS. In that case you may have to moult them (putting them off lay by giving low protein feeds) to bring shell quality back and get them over the virus. Interestingly, according to web information it probably originated from infected vaccines, after which it became transmissible between hens!
Apart from EDS there's another disease (more common) called Infectious Bronchitis (IB) that can cause soft shelled egg laying, but IB involves significant respiratory symptoms, and while some eggs will be soft shelled, the majority are wrinkly and misshapen. Thus IB is fairly obvious.
I hope this is useful... Might as well dredge the brain before it starts to disintegrate!