There's more to soft shells than Calcium or the type of calcium.
By offering "shells" do you mean Oyster shells or egg shells?
Protein, Adequate levels of lysine and methionine is need to produce good egg shells. When pullets begin laying, there is an increase in protein, vitamin and mineral requirements per day due to deposition in the egg. If dietary protein is too low or the amino acid requirements are not met, poor egg production and hatchability will occur.
Edited by Chris09 - 2/20/16 at 2:22am
Calcium, The egg shell is composed primarily of calcium carbonate. The pullet's requirement for calcium is relatively low during the growing period, but when the first eggs are produced, the need is increased at least four times, with practically all of the increase being used for the production of eggshells. Inadequate calcium consumption will result in decreased egg production and lower egg shell quality.
Hens store calcium in medullary bone, a specialized bone capable of rapid calcium turnover. As calcium stores are depleted, bones become brittle. In severe cases, hens are unable to stand. The condition is known as caged-layer fatigue. Birds on the ground or on litter floors recycle calcium and phosphorus through consumption of feces, and do not have caged-layer fatigue.
Calcium can be supplied in the diet as either ground limestone or oyster shell. Particle size affects calcium availability. Usually the larger the particle size, the longer the particle will be retained in the upper digestive tract. This means that the larger particles of the calcium source are released more slowly, and this may be important for the continuity of shell formation, especially in the dark period when birds do not ordinarily eat.
Periodically, dolomitic limestone is offered to the feed industry. However, dolomitic limestone(which is used in the steel industry) should never be used in poultry diets. Dolomitic limestone contains at least 10% magnesium, and this complexes with calcium or competes with calcium for absorption sites in the intestines. The consequence of feeding dolomitic limestone is induced calcium deficiency.
Young birds should not be fed a high calcium layer diet because the calcium/phosphorus ratio will be unbalanced, resulting in increased morbidity or mortality.
Phosphorus, The nutritional role of phosphorus is closely related to that of calcium. Both are constituents of bone. The ratio of dietary calcium to phosphorus affects the absorption of both these elements; an excess of either one impedes absorption and can reduce egg production, shell quality and/ or hatchability.
In addition to its function in bone, phosphorus plays a primary role in carbohydrate metabolism, is active in fat metabolism, and helps to regulate the acid-base balance of the body
Vitamin D, Vitamin D is required for normal calcium absorption and utilization. If inadequate levels of vitamin D are fed, induced calcium deficiency quickly results and egg production decreases.
Feed grade vitamin D comes in two forms, D2 and D3. In most animals, both are equally potent. In birds, however, D3 is substantially more active than D2. In poultry diets, therefore, vitamin D must be supplied in the form of D3.
Magnesium, Magnesium is needed for healthy bones, proper nervous system functioning, and energy metabolism.
Fat, Dietary fat is a source of energy and of linoleic acid, an essential fatty acid. A deficiency of linoleic acid will adversely affect egg production. Dietary fats also serve as "carriers" of fat-soluble vitamins, and some fat is necessary for absorption of vitamins. In fact, impairment of the absorption of fat-soluble vitamins (A, D, E, and K) is the most serious consequence of a dietary deficiency of fat.