Light, water, CO2, Nitrogen, P, K . Most of the components, as they are increased, productivity is increased up to a point where further increase has either no effect or negative effects. However one component cant increase in isolation to increase productivity. If plants do not have any of the above, they will not be able to grow well, if they do not grow they cannot feed the food chains. So these set a limit to primary productivity as without them in the correct amounts for plant life the food chain becomes stunted as, herbivores cannot eat the plants and the secondary and tertiary consumers cannot eat the herbivores and then if that happens they can have a reduction in population, and it really just snowballs from there. So they interact through plants to set the limits to primary productivity as these tend to be the basis of most food chains.