NPK is a term used for the major (also known as macronutrients) fertilizers your plants need to thrive. When buying fertilizers you will find numbers listed on the product something like, 4-6-2. This helps us feed the right diet for the right plant when we learn to read these numbers. "4" is the "N", total nitrogen. "6" is the "P", available phosphorous (also called phosphate). "2" is the "K", soluble potassium (potash). Nitrogen is needed to promote healthy stem and leaf development. All plants need nitrogen to grow healthy, but plants such as leafy crops need a larger supply of nitrogen to keep them growing healthy, so when looking for a fertilizer for leafy greens such as lettuce or kale, watch for the "N" to be higher in numbers such as a 12-0-0 that is found in blood meal. Some other sources of organic nitrogen are alfalfa meal, soybean meal, fish meal and feather meal. Phosphorus helps the plants set their roots down quickly and vigorously and promotes fruit and flower development. While tomatoes need nitrogen, to much can give you all vine and no fruit. Choose the right fertilizer, something that has a higher "P" to provide plenty of phoshorous. 4-6-2 blend will provide the right amount of phoshorous to keep the flower and fruiting process going. Bone Meal and Rock Phosphate is an excellent source of Phoshorous. Potassium also aids in the development of fruit, but it also helps with overall health of the plants and keeps disease at bay. If root crops are rather small, your soil may be lacking in potassium. One of the our favorite organic sources for potassium is is Greensand, with the added bonus of adding plenty of minerals and helps build soil structure. Langbeinite is another option for high potassium (0-0-22), and it provides magnesium and sulfer which is great for alkaline soils.
Local extension services will test your soil for a fee or you can buy simple DIY soil kits to help you determine what your soil needs, and overtime you will learn your plants and what they are telling you they need.