Plants remove nutrients from the soil and air to enable them to grow and reproduce. Some nutrients are needed in larger quantities and are termed macronutrients. Those needed in smaller quantities, the secondary and micronutrients, are just as important for achieving healthy plant growth. Most commercial fertilizers provide macronutrients: nitrogen (N), phosphorus (P), and potassium (K). Secondary and micronutrients may be supplied along with macronutrients or are manufactured in special formulations for plant use.
Nitrogen (N) is essential for plant growth and photosynthesis. Without N, plants could not produce amino acids which are needed to form proteins, resulting in stunted growth.
N is difficult to manage in crop production systems because N is easily leached from soils or can be immobilized by soil microbes, volatilize back to the air, or lost via denitrification in water-saturated soils. Symptoms of N deficiency include slow, stunted growth, pale yellow-green coloration, and premature dying of older leaves (due to N mobility in plants). N is not routinely tested by soil testing laboratories for making crop recommendations because of these losses. Instead, N recommendations are based on your experience and the crop’s yield potential.
While soil tests provide some information about plant N needs, tissue testing is the better option for deciding if and how much more N is required to meet yield goals. Most private testing laboratories can provide plant tissue N levels quickly to aid in nitrogen application decisions. Labs can test N from leaves, whole petioles, and petiole sap. Consult the testing laboratory for detailed collection instructions.
Phosphorus (P) is needed by plants for nucleic acids (DNA/RNA) and in energy storage and transfer (ATP). Root formation, early plant growth, crop maturity, and seed production are all stimulated by P. Symptoms of P deficiency include stunted growth, purple coloration to leaves, delayed maturity and poor fruit or seed development.
Crops respond to P when soil tests indicate that levels are very low or low. When tests indicate adequate or high P, crops may respond to P fertilization if the fertilizer is placed near the plant or when soils are cold. Phosphorus may be banded near the seed as a starter fertilizer regardless of soil P levels. Soils that have received regular manure applications often have very high P levels, so knowing the past history of a field is very important in making fertilizer recommendations. Phosphorus is strongly adsorbed to soil particles and very little is lost via leaching.
Potassium (K) is essential for the translocation of sugars and starch formation. It is important for plant water use regulation. Potassium encourages root growth, increases disease resistance, improves fruit quality, and boosts winter hardiness. Symptoms of K deficiency include browning on the leaf margins, weak stalks or stems, small fruits, and slow growth.
Crops respond to K when soil tests indicates that levels are very low or low. Where levels are adequate or high, crops may respond to K when drought-stressed. Most often, K fertilizer should be broadcast rather than banded or side-dressed unless K levels are low. Most vegetables require larger amounts of K than P during a growing season. Some very coarse sandy soils have low K reserves and may require frequent applications to maintain K at an optimum levels.
Secondary and Micronutrient Management
Calcium (Ca), magnesium (Mg), and sulfur (S) are often called the secondary elements. Calcium levels in Utah soils are quite high but may not be readily available to plants. Calcium is a component of plant cell walls and membranes and does not move around in the plant. Calcium is transported around the plant with water so when crops are drought-stressed, young tissue may not receive enough Ca. Symptoms of Ca deficiency include “tip burn” of young leaves in lettuce or cabbage, blossom end rot of tomato, pepper or melons, terminal bud death, premature blossom drop in bean or tomato, or very dark foliage.
Soil Mg levels can be quite high but can still be deficient in vegetable soils. Magnesium is part of the chlorophyll molecule and is needed in photosynthesis. It is very mobile in the plant so deficiency shows up in older leaves. Symptoms of Mg deficiency include:
• Interveinal chlorosis of older leaves
• Leaf curling
• Leaf margin yellowing
Sulfur is an important nutrient for plants. It is an essential component in several amino acids and thus needed for protein synthesis. Symptoms of S deficiency are:
• Yellowish colored leaves
• Small spindly plants
• Slow growth
Sulfur deficiencies can occur when irrigation water is very pure or when high-analysis, low-S fertilizers are used regularly. Onions and plants in the cabbage family (cole crops) have high S requirements.
The micronutrients include boron, chlorine, copper, iron, manganese, molybdenum, and zinc. Boron (B) is needed for meristem growth and acts as a binding agent between cell walls. Deficiencies are most common in the young growing points as B is not mobile around the plant. Boron may be deficient in intensively managed vegetable crop soils. Deficiencies are likely to occur in bulb and root crops, cole crops, and tomatoes. Over application of B can be toxic to plant growth, so DO NOT exceed recommendations levels.
Chlorine (Cl) deficiencies are quite rare. Chlorine is required for photosynthetic reaction in plants and deficiency symptoms are:
• excessive root branching
• leaf bronzing
Copper (Cu) deficiencies are also rarely observed in Utah. Copper is needed for enzyme activation and plays a role in vitamin A production. Plants deficient in Cu are:
• have chlorotic shoot tips
• pale green in color
Iron (Fe) deficiency is a common problem particularly when plants are over-irrigated. Iron is required for chlorophyll formation, photosynthesis and nitrogen fixation. Soils with very high pH or aeration problems often are Fe deficient. Symptoms of Fe deficiency are:
• interveinal chlorosis
• terminal tip dieback
• general leaf discoloration
Manganese (Mn) deficiencies are not that common. Manganese is needed for enzyme activity and works with Fe in chlorophyll formation. Excess Mn may induce Fe deficiency with similar deficiency symptoms.
Molybdenum (Mo) deficiency are quite rare. Plants need Mo to transform nitrate-N into amino acids and N-fixing bacteria cannot use atmospheric N unless it is present. Deficiency symptoms are:
• stunted growth
• cupping of leaves
• yellowing of leaves
Zinc (Zn) is occasionally deficient in Utah soils. Zinc helps regulate enzymes and other growth regulating processes. When plants are Zn deficient they may have:
• a rosette growth form
• fewer flower buds
• mottled leaves
If you suspect a deficiency, it is important to have the affected plants tested.
Plants commonly obtain nutrients from the soil through their roots. Plants can also absorb a limited amount of some nutrients through leaves. If the soil has been properly managed, soils can supply all the nutrients a crop needs to grow and produce high yields. If a nutrient becomes deficient or unavailable during the development of the crop, foliar nutrient applications may then be beneficial. Foliar feeding is not recommended for the macronutrients but is commonly used to correct micronutrient deficiencies. Nutrient concentration, application methods, and plant type all influence the effectiveness of foliar feeding. Consult your county Extension educator for more information on nutrient applications to plants.
How to Understand Fertilizer Labels