If you're going to try and manipulate soil fertility in regard to diseases, you need to consider the specific disease/s you're trying to avoid, they may not necessarily all fit a black and white generalization. As an example, using diseases I'm familiar with on turf, dollar spot is favoured by low nitrogen and brown patch is favoured by high nitrogen yet is not an obligate parasite (remember Marschner said high N favoured obligate parasites but had the opposite effect on facultative ones). There is research to show that rust likes high N on several other plants, however.
Re soil test kits, there's a more extensive article on the accuracy (or not) of several of these at:
http://horttech.ashspublicatio...
As for your question, it's difficult when they don't give the results in amounts, but I would be inclined to go for medium on all, bearing in mind that if they test for nitrogen it is probably only nitrate nitrogen (NO3). Plants take up nitrogen in the forms of ammonium (NH4) and nitrate and the soil test for nitrate would not include organic nitrogen which has to be converted to NH4 and then NO3 by soil microbes before the plants can use it. Thus there can, in effect, be N there in organic form for the future that isn't measured. Going too high on K can induce deficiencies of N, Ca and Mg. Other nutrients in excess can cause deficiencies of others, also.
The only information on phosphorous for daylilies that I can think of are the deficiency symptoms according to some tests described in the AHS's 2002 Handbook (large number of new leaves that were dark green in colour). Also Dr. Scott Elliott wrote an article about his experiments on daylily nutrient deficiency symptoms in the AHS Daylily Journal fairly recently. I have a feeling he wasn't able to induce a P deficiency but I can't find that Journal at the moment - will have another look later unless someone else can find it first. (Edit: found it, as I thought he wasn't able to induce P deficiency in daylilies).
