Nitrogen affects a plant's leaf development. Pale green or yellow leaves indicate a nitrogen deficiency. Excessive growth and very dark-green leaves indicate an excess of nitrogen. Natural nitrogen supplements include blood meal, fish emulsion, herbivore manure and compost.
Plants in phosphorus-poor soil may have a purple cast to their leaves and will exhibit slow growth and poor production of blossoms and fruit. Excess phosphorus can tie up other nutrients in the soil, making them unavailable to the plants. Natural phosphorus sources include bone meal and rock phosphate.
Potassium deficiency signs are more subtle than signs of deficiency in either nitrogen or phosphorus. Weak, spindly plants that seem prone to insect damage and bear small, thin-skinned fruit are likely to be potassium deficient. It is easy to confuse potassium deficiency with a lack of water. Increased watering leaches potassium out of the soil, leading to a downward spiral. Natural sources for potassium include greensand, wood ash and granite dust.
If you're judging nutrient deficiency by plant appearance, it may be too late to do anything. Get a soil analysis before you start planting and you can give your plants what they need before they tell you what they're missing. Most university extension service systems offer low-cost soil analysis. Contact your local office for specific sampling instructions, but generally they will ask you to take samples from a few places in the lawn or garden, blend the samples into one and deliver it to the testing lab. Describe your proposed use for the soil being tested to get the most complete and most accurate recommendations. For example, you will not get the same recommendations for your lawn and a vegetable garden, and neither of those would match the recommendations for a flower bed.
While nitrogen, phosphorus and potassium represent the primary elements of plant growth, plants also need calcium, magnesium, sulfur, boron, copper, iron, manganese, molybdenum and zinc. Synthetic sources of NPK fertilizer contain few, if any, of these "micro-nutrients." However, they're generally available in adequate amounts in natural amendments such as compost and blended organic fertilizers. Natural sources also tend to be slow to release the components, which avoids the see-saw effect of going from too little to too much of an element.