Begin with a simple soil test. Look at pH and phosphorus levels in particular, but do not ignore other nutrients. If lime and phosphorus are required and you have a limited soil fertility budget, begin with lime. Work the phosphorus level up. Phosphorus is extremely important for maintaining legumes in the pasture.
Poultry litter is a good source of phosphorus as well as organic nitrogen.
If it is available in your area, it might be worth exploring. Use caution when pricing poultry litter. Price it on a 'cost of nutrient' basis. What might appear to be cheap fertilizer may be high if the concentration of nutrients is low. Evaluate your grazing strategy.