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Plant Nutrition INTRO TO SOILS CH 12 Plant Nutrition Many soil factors affect plant growth Difficult/expensive to improve However . . . Supply of soil nutrients can be controlled . . . Plant Nutrition Essential Elements Nitrogen Promotes rapid growth and dark green color Needed for chlorophyll production Key component in protein Plant water efficiency Plant Nutrition Essential Elements However, too much Nitrogen . . . • Produces soft, weak, easily injured growth (lodging) • Prone to disease/insect injury • May slow maturity and ripening • Delays hardening-off • May impair flavor in some vegetables • Nitrates may accumulate Plant Nutrition Essential Elements Nitrogen is most important for crops grown for their vegetation, such as leafy vegetables, hay or turfgrass Nitrogen Cycle • Nitrogen moves around . . . • Atmosphere is ~ 78% N2 • Unfortunately, N2 (gas) is an unusable form for plants However . . . Nitrogen Cycle Bacteria can use this form; they may . . . • Use it to form protein for their own bodies, or . . . • Supply it to host plants • Eventually, both microbes and host plants do die; • This allows other microbes to . . . Nitrogen Cycle Mineralize the protein to ammonium ions (NH4+) • NH4+ may be . . . – taken up by plants – converted by bacteria (Nitrification) to NO3– lost to the atmosphere (Volatilization) as NH3 • See reaction p. 198 (NH4+ with Hydroxyl ions) Nitrogen Cycle • Study the Nitrogen Cycle (simplified) on p. 199 and Nitrogen Cycle handout • Additionally, lightning and fertilizer factories fix N Nitrogen Cycle Non-biological losses of nitrogen include: – Leaching of NO3– Ammonia Volatilization* *may occur in dry, alkaline, or recently limed soil Nitrogen Cycle Losses through production: • • • • Crop harvest Erosion Irrigation Liming Nitrogen Cycle Additions for production • Manuring • Growing legumes • Fertilizing Forms of Nitrogen in Soil 97-99% of soil nitrogen resides in organic matter • only a small percentage is mineralized to useable forms • average of 90 lbs/acre/yr . . . far short of typical crop needs • e.g. 150-bushel/acre corn crop contains ~ 190 lbs of nitrogen • See fig. 12-6, p. 201 Know this chart! Nitrogen Deficiency Symptoms: • Slow growth and stunting • Lack dark green color • Exhibit chlorosis • Lower leaves first affected • Grasses begin yellowing at blade tips • Extreme cases dry up – called “firing” Phosphorus • • • • • “Partners” with Nitrogen Part of genetic material ATP stores and transfers energy Spurs early and rapid root growth Helps plants use water more efficiently resist cold and disease and improve quality of grains and fruits Improves efficiency of Nitrogen uptake Phosphorus “Balances” Nitrogen • • • • • Hastens maturity Aids blooming and fruiting Important in crops flowers, fruits, seeds Promotes early and rapid root growth Major element in starter fertilizers Forms of Phosphorus in Soil • Provided by weathering of mineral . . . Apatite (calcium phosphate) • Anions are: H2PO4HPO4-2 (referred to as phosphate ions) Forms of Phosphorus in Soil • Much phosphorus is unavailable to plants • “fixed” in insoluble forms – Iron phosphates in strongly acid soils – Aluminum phosphates in moderately acid soils – Calcium phosphates in alkaline soils Maximum availability between 6.0 to 7.0 Phosphorus Deficiencies/Excesses • • • • P-deficient plants often have a purple tint Older, lower leaves affected first May exhibit darker than normal green color Delayed maturity and poor root systems • Excess P ties up nutrients such as iron (Fe) Potassium • a.k.a. – Potash • Elemental symbol “K” • Plants consume more K than any other nutrient except N • Cell walls and stems strength • Regulates stomates • Fruit development and ripening • Root and tuber crops • Potassium promotes tougher growth Potassium Improves: • • • • Stem strength; less prone to lodging Fight disease Increase winter hardiness Influences transpiration rate; conserves water supplies • Caution: excess/luxury consumption may inhibit uptake of calcium or magnesium Potassium in the Soil • Weathering releases Potassium ions (K+) • Stored on CEC and fixation in clays (fig. 12-11, p. 206) • Moves in soil: more than P, less than N • Most plant uptake occurs by diffusion Potassium Deficiencies • Less of a problem than other primary nutrients; some occurrence in sandy, leached soils • Excess N can inhibit uptake of K • Dry, cold, poorly aerated soil inhibits uptake; neutral pH is desirable for uptake • Deficiencies are seen as “marginal scorch” on edges of lower, older leaves