Plants get a majority of their nutrients from the soil that they grow in. The three main nutrients that plants get from soil are nitrogen, phosphorus and potassium. The best way to tell if your soil has a nutrient imbalance is to observe your plants closely for signs and symptoms. Imbalances of different nutrients are revealed by different symptoms, so it is important to know how to recognize them.
Watch for older leaves that turn yellow, while younger leaves appear green and healthy, along with general plant weakness, which indicates a nitrogen deficiency in the soil, according to the University of Nevada "Recognizing Plant Nutrient Deficiencies" fact sheet. Nitrogen promotes lush foliage and better productivity. Correct nitrogen deficiencies by amending the soil. Good sources of nitrogen are soybean meal, fish meal, blood meal and bat guano.
Recognize purplish or bronze-colored leaves as a phosphorus deficiency in the soil, as noted in "Recognizing Plant Nutrient Deficiencies." Phosphorus promotes rapid growth, strong root systems and big blooms. Add bone meal, colloidial phosphorous or rock phosphate to the soil to increase phosphorous content.
Be aware of potassium deficiency, indicated by leaves with yellow, translucent spots and browning edges. Potassium builds protein, aides in photosynthesis, and builds disease resistance. Soil also provides secondary nutrients and minerals that plants need. Sources of potassium include ground kelp, greensand or granite dust.