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Phosphorus, Potassium and Micronutrients. Importance of Phosphorous. Phosphorus is the 2 nd most often deficient nutrient Importance for plant nutrition: energy transfer — ATP genetic material — DNA protein synthesis — RNA. Importance of Phosphorous (cont.). P balances N
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Importance of Phosphorous • Phosphorus is the 2nd most often deficient nutrient • Importance for plant nutrition: energy transfer — ATP genetic material — DNA protein synthesis — RNA
Importance of Phosphorous (cont.) • P balances N • N delays maturity; P hastens maturity • N promotes vegetative growth; P promotes reproductive growth (flowers/fruiting). • P particularly important in crops in which we use flowers, fruits, or seeds • P spurs early and rapid root growth • P helps plants resist cold and disease
Solution P Plants take up P as: • Primary orthophosphate ion: H2PO4- (pH < 7.0) • Secondary orthophosphate ion: HPO42- (pH > 7.0) The form most common is a function of soil pH – both equally present at neutral pH
P Deficiency Symptoms • Purplish tint or a purplish margin on older leaves • Slow growth; stunted plants • Delayed maturity; poor fruit, grain, or seed development • Plant uptake of P is slow in cool soils (e.g. corn on cold days in April)
Center plot no P was applied. • Stunted growth and poor tillering are most visible early indications of P deficiency in wheat.
Phosphorous Cycle ADDITIONS: fertilizers/manure WITHDRAWALS: harvest/eroding sediments • Phosphorus is immobile in the soil (therefore, need to inject or incorporate into root zone) • There is no loss of phosphorus as gas or leaching
Phosphorous Cycle (cont.) • Soil P containing minerals release P in plant available forms VERY slowly • amount of P in ‘available’ form at any one time <1% • Phosphorus in fertilizers react with minerals in soil and changed to unavailable forms. • Remember plants absorb P as: 1. primary orthophosphate H2PO4- 2. secondary orthophosphate HPO42-
Eutrophication P erosion into streams accelerates growth of algae and water plants. Subsequent plant decomposition uses up oxygen in the water.
The Phosphorus Problem • Soil P level usually very low 2. P in rocks and minerals only very slowly available 3. When P is added in fertilizers it becomes “fixed” and unavailable for plant uptake. • < 30% of fertilizer-applied P taken up by crop during the year it is applied. • However, P level can be built up and eventually reach a ‘saturation’ point, at which point previous applied P will become available for plants. (Fig 13.17)
Figure 13.17 No off-site movement in drainage water until soil saturation (lb./acre)
The Phosphorus Problem (cont.) 4. Availability regulated by soil pH. • Maximum P availability occurs at pH of 5.5 to 7.2. (optimal 6.5) • pH > 7.2 then Ca and Mg react with P to form insoluble compounds • pH < 5.5 then Fe, Al, and Mn react to form insoluble compounds
Management for Phosphorus 1. Lime acid soils to pH near 6.5 where P most available 2. Increase organic matter content. -- OM is a source of P as it decomposes 3. Enhance mycorrhizae fungi symbiosis with crop roots. -- mycorrhizae significantly improve P acquisition. -- mycorrhizae enhanced by crop rotation, OM addition, and conservation tillage.
Management for Phosphorus (cont.) 4. ‘Banding’ vs ‘Broadcasting’ of P fertilizers • P is immobile in soil. If broadcast stays near the surface • Place P down in the root zone if possible • Banding also minimizes P contact with the soil and slows down P fixation Broadcast Banded
Common Phosphorus Fertilizers • Monoammonium phosphate (MAP) 11-52-0 • Diammmonium phosphate (DAP) 18-46-0 • Ammonium polyphosphate10-34-0 (liquid) • Triple superphoshate 0-45-0 • Ordinary superphosphate 0-20-0-12S • Phosphoric acid 0-52-0 • Superphosphoric acid 0-68-0
Crop absorption of mobile nutrients (Nitrogen) Plants absorb mobile nutrients from large volume of soil
Increasing plant population to obtain higher yield creates competition for N • Adding N fertilizer eliminates competition for N among plants • Allows normal plant development and increased yields. Competition among plants for N
Absorption is from just a thin cylinder of soil around each root. Crop absorption of immobile nutrients (Phosphorus)
No Competition Among Plants For Immobile Nutrients Even At High Yield Goals If supply of nutrient is adequate for 30 bushel/acre wheat yield, then supply will also be adequate for 60 bushel yield.
Mobile vs. Immobile Nutrients Mobile and immobile nutrients like fuel and oil • Fuel(the mobile nutrient); required in relation to the amount of work expected from the pickup • Similarly, N required in relation to the yield expected. • 30 bushel wheat = 60 lbs N/acre • 60 bushel wheat = 125 lbs N/acre • 100 bushel wheat = 240 lbs N/acre • Oil(the immobile nutrient); required more in relation to the level in the crankcase than by how much work is expected from the pickup. • Similarly, P requirement determined from soil test and does not depend on yield goal.
Mobility Concept of Nutrients in the Soil Soil nutrients that are immobile in the soil usually more effective if they are incorporated or injected below ground to where the roots can access. Soil nutrients that are mobile can be broadcast on soil surface and rain/irrigation will leach them into the soil
Importance of Potassium • Potassium (often called potash) is taken up by plants as the K+ ion. • K+ remains in ionic form in cells and plant tissue (not synthesized into compounds). • K+activates enzymes for the formation of protein, starch, cellulose, lignin, etc. (> 60 enzymes known to need K)
Importance of Potassium (cont.) • Potassium promotes “tough” growth (vs. N promotes soft growth). Plants well stocked with K have strong stems. • Promotes winter-hardiness.
The Potassium Cycle • Western Oklahoma soils are naturally very high in potassium… K fertilizers seldom needed. • OM not an important source of K+ . • No gaseous loss or microbiological process affecting K • K+ does leach (especially sandy soils)… but leaching less than NO3-
K Usually adequate K Usually deficient Potassium Deficiency Common in crops grown in weathered soils developed under high rainfall.
Deficiency Symptoms 1. Leaf tips and margins “burn” starting on old leaves. 2. Weak stalks, plants lodge easily. 3. Reduced drought tolerance 4. Legumes with small, white spots along leaflet margins.
Common Potassium Fertilizers Potassium chloride 0-0-60 • KCl • muriate of potash Potassium sulfate 0-0-50-17S • K2SO4 • Sulfate of potash
Micronutrients(Trace Elements) Iron (Fe) Chlorine (Cl) Manganese (Mn) Cobalt (Co) Boron (B) Molybdenum (Mo) Zinc (Zn) Nickel (Ni) Copper (Cu)
Concern for Micronutrients • Increased crop yields have resulted in greater removal of micronutrients from soils. • Modern, high-analysis fertilizers have little impurities. The older, less-pure fertilizers contained some micronutrients.
Concern for Micronutrients (cont.) • Increased knowledge of plant nutrition, and improved methods of analysis in the laboratory, are helping diagnose micronutrient deficiencies that might formerly have gone unnoticed. 4. Food grown on soils with low levels of micronutrients may provide insufficient humandietary levels, even though crop plants themselves show no signs of deficiency.
Micronutrients • Dominant role in plants is to activate enzymes. • Very minute amounts used; difference between sufficient and toxic level small • Apply only if soil and/or plant tissue tests indicate need.
Micronutrients Most Often Deficient In: • Sandy, leached soils • Soils with high pH -- Fe, Zn, Mn, Cu deficiency at high pH • Soils that have been intensively cropped and heavily fertilized with macronutrients only
Iron (Fe) • Important part of many enzymes (including formation of chlorophyll) • Taken up mainly as Fe2+ ion • Availability decreases at high pH (acid= available; alkaline=unavailable)
Iron (Fe) • Deficiency symptom: interveinal chlorosis on new leaves. • Deficiencies common in sorghum, soybeans, OWB’s Variety differences? • EX: WW Ironmaster, soybeans
