By James Hoorman, Hoorman Soil Health Services
Plant nutrient availability is dependent on several factors including soil moisture, temperature, microbes, pH, chemical nutrient form, and plant root proximity. Plant roots require moist soil, adequate soil temperatures, and teaming microbes. The microbes generally make most nutrients available in a reduced form. Soils that are slightly anaerobic (lack oxygen) and slightly saturated result in reducing conditions for making nutrients plant available.
Farmers want to avoid the extremes to maximize plant growth and yield. For example, highly oxidized and dry soils (nutrients tied up) are just as bad as highly saturated compacted soils where nutrients may be available, but tend to leach or be lost with flowing water. Ideally, an inch rain is better than no rain or 3 to 5-inch rains. Roots cannot grow into saturated conditions so they need oscillating wet and dry cycles to absorb most nutrients efficiently.
Some soil nutrients are highly mobile while others are relatively immobile. Mobile nutrients move with the soil solution and are generally negatively charged ions (anions) or uncharged and tend to have high soil concentrations. Examples include nitrogen, sulfur, chlorine, molybdenum, and boron.
Immobile nutrients have little soil movement and generally have positive charges (cations) (except for phosphorus), tend to have low soil concentrations, are often bound to the soil cation exchange capacity (CEC), and precipitate as slightly soluble compounds. Examples include phosphorus, potassium, calcium, magnesium, manganese, iron, zinc, copper, and nickel. Soil pH plays a vital role in plant nutrient availability. For most nutrients, a slightly acid pH between 6.5 to 7 makes most nutrients more plant available except for calcium, magnesium, and molybdenum.
Nutrients can also be classified as how mobile they are in plant tissue. Mobile plant nutrients are easily remobilized from the older leaves to the new leaves under deficiency conditions. Deficiencies generally show up on the older leaves because the nutrients are easily mobilized to newer faster growing leaves. Examples include nitrogen, phosphorus, potassium, magnesium, chlorine, zinc, and molybdenum. Immobile plant nutrients are not easily mobilized. Deficiency symptoms are most pronounced on young plant leaves and parts that are newly grown. Examples include calcium, sulfur, manganese, iron, copper, and boron.
Calcium deficiency is difficult to see and shows up as blossom end rot in tomatoes. Calcium deficiency are a hidden source of reduced yield and is often caused by too much soil potassium (K), making calcium plant unavailable. Leaf tip necrosis is a sign of calcium deficiency. Calcium deficient leaves are stunted & mishappen. In corn, look for parallel lines (railroad tracks) near the mid rib. Calcium is critical for seed germination and early plant growth and activates 146 plant enzymes.
Iron (Fe) is essential for crop growth and plants take up Fe as the reduced ferrous (Fe²⁺) cation. Iron is a component of many enzymes associated with energy transfer, nitrogen reduction and fixation, and lignin formation. Manganese (Mn) activates several important metabolic reactions and plays a direct role in photosynthesis. Manganese accelerates germination and maturity while increasing the availability of phosphorus and calcium. Manganese deficiency in soybeans and corn is common and is often corrected by applying foliar fertilizer sprays.
Zinc, copper and molybdenum are important for activating many plant enzymes. Zinc (Zn) is taken up by plants as the divalent Zn²+ cation and is commonly associated with yield limitations. Required in small amounts, high yields are impossible without zinc because it activates over 300 plant enzymes. Copper (Cu) activates enzymes and catalyzes reactions in several plant-growth processes. Copper is closely linked to Vitamin A production and it helps ensure successful protein synthesis. Molybdenum (Mo) is a trace element required for the synthesis and activity of the enzyme nitrate reductase. Molybdenum is vital for the process of symbiotic nitrogen (N) fixation by Rhizobia bacteria in legume root modules. Molybdenum is often lacking in plant tissue and is another source of hidden plant hunger.
Boron (B) affects membrane stability, helps plants absorb calcium, and B supports the structural and functional integrity of plant cell membranes. Boron-deficiency symptoms first appear at the growing points. Nickel (Ni) is a component of the urease enzyme. Without the presence of Ni, urea conversion is impossible. It is required in very small amounts, with the critical level appearing to be about 1.1 ppm. The majority of plant available nutrients are processed by soil microbes, so improving soil health generally results in improved nutrient availability and nutrient efficiency. Since many deficiency symptoms look similar, visit hoormansoilhealth.com for a fact sheets with nutrient deficiency images.