Recently I purchased a soil testing kit. In it one can test pH, nitrogen, phosphate and potassium. However, the accompanying manual only tells what pH is needed for specific plants. It does not tell you what the content of the others should be in the soil. Only that it can range from Very High to Very Low. How can I know what the nitrogen, potassium and phosphate levels should be for my crops?
Nutrients in the Soil
Certified GKH Gardening Expert
It doesn't say because it can vary greatly depending on age and variety of plants you are growing. For that style of soil test kit, you are looking for a rating of medium (or equivalent) to low-high in any of these nutrients.
Rule of thumb is that for flowering and fruiting and young plants, you want a slightly higher phosphorous as this helps with flowering and root development. For foliage plants, a somewhat higher nitrogen is good as nitrogen grow leaves. For established plants, a balanced makeup is needed.