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LAST LECTURE The nitrogen cycle and nitrogen in the soil THIS LECTURE Phosphorus and Potassium and their cycles ESSENTIAL ELEMENTS macronutrients Macronutrients (with content ranges in plant tissues) Carbon (C) 40-45% Potassium (K) 1-3% Hydrogen (H) 5-6% Calcium (Ca) 0.2-3% Oxygen (O) 45-50% Magnesium (Mg)0.1-1% Nitrogen (N) 0.5-5% Sulfur (S) 0.1-0.2 % SOIL PHOSPHORUS AND POTASSIUM Why is phosphorus so important? 1. Essential component of ATP (adenosine triphosphate) Molecular currency of intercellular energy transfer Used as energy source during photosynthesis and cellular respiration Consumed by many enzymes in metabolic reactions and during cell division. Phosphorus (P) 0.1-0.4% 2. Incorporated into nucleic acids 3. Phospholipid bilayer Cell membranes, composed of a phospholipid bilayer, control what goes into and out of a cell DNA and RNA Genetic instructions for the development and functioning of all living organisms * Notice the 5 N and 3 P in the ATP molecule Phospholipid Active transport across the cell membrane requires ATP Sugar-phosphate backbone Phospholipid bilayer 1 PRIMARY NUTRIENT (N-P-K) FUNCTIONS THE IMPORTANCE OF PHOSPHORUS Nitrogen (N). • Gives green colour to plant. • Induces vigorous, rapid growth in plants. • Increases protein and yield. • Aids and promotes seed and fruit development. • Plants cannot use N as a gas, it must be combined with other elements and in the correct forms. Phosphorus (P). • Important to germinating seedlings. • Contributes to early maturing crops. • Necessary for seed and fruit formation. • Essential for N fixation. • Stimulates root growth. P is involved in: P deficiency: Photosynthesis Nitrogen fixation Flowering Fruiting & fruit quality Maturation Root growth Tissue strength Stunting Thin stems Bluish-green leaves Delayed maturity Sparse flowering Poor seed quality • Similarly to nitrogen deficiency, the older leaves are often first affected Potassium (K). • Necessary for production and translocation of carbohydrates. • Produces plumper seeds. • Controls water intake and respiration. • Stiffens straw and stalks. • P deficiency is often difficult to diagnose, as visual changes are subtle The Problem in Soil Fertility THEPhosphorus PHOSPHORUS PROBLEM IN SOIL FERTILITY 1. The total P content of soils is low. 200-2000 kg/ha in uppermost 15 cm (topsoil) 2. Phosphorus compounds found in soils are often highly insoluble (it does not occur as a free element, but is bound in phosphates) 3. When soluble sources are added (fertilizers and manure) they often become fixed into insoluble compounds • 10-15% of P added is taken up by crop in year of application • Overfertilization for decades has led to saturation of the P-fixation capacity (large P reserves in N. American soils) • In contrast, P deficiency is a serious problem in sub-Saharan Africa (removal repeatedly has exceeded addition) http://www.efma.org/EPUB/easnet.dll/ExecReq/Page?eas:template_im=000BC2&eas:dat_im=000C23 N, P and K Fertilizer Use in USA IMPACT OF PHOSPHORUS ON ENVIRONMENTAL QUALITY 1. P deficiency: Land degradation Little P is lost in natural ecosystems as P cycles between living biomass and soils Once cleared for agriculture: (i) Soil erosion loss (ii) Biomass removal • P-supplying capacity decreases, even if total P is sufficient • Nodulation is affected by P-deficiency, thereby promoting N-deficiency Most problematic in most highly weathered soils • Warm, moist environments of the tropics • Oxisols & Andisols • Low availability of P when in association with Fe & Al • Lots of P needs to be applied to Andisols (Fig 14.20) Figure 14.1 2 Combined P & N deficiency limits biomass and promotes further erosion WATER QUALITY DEGRADATION DUE TO EXCESS P (& N) Point sources Sewage outflows (phosphates in soaps) Industries Non-point sources Runoff water Eroded sediment from soils in affected watershed “Too much of a good thing” 3 ECONOMIC AND POTENTIALLY ECONOMIC PHOSPHATE DEPOSITS OF THE WORLD THE PHOSPHORUS CYCLE Phosphorus in the soil solution • Very low concentrations (0.001 to 1 mg/L) • Roots absorb phosphate ions, HPO42- (alkaline soils) and H2PO4- (acid soils) Uptake by Roots • • • • • • Slow diffusion of phosphate ions to root surfaces Mychorrizal hyphae extend outward several cm from root surface P can then be incorporated into plant tissues (Fig 14.9) Soil P replenished by plant residues, leaf litter, and animal waste Soil microorganisms can temporarily incorporate P into their cells Some soil P gets tied up in organic matter (storage & future release) Available P seldom exceeds 0.01% of total soil phosphorus Forms of Soil Phosphorus PHOSPHORUS REMOVAL FROM PONDS & SLOUGHS Organic phosphorus Calcium-bound phosphorus (alkaline soils) Iron-bound phosphorus (acid soils) Aluminium-bound phosphorus (acid soils) A small amount of phosphorus is removed via uptake through the plants’ roots. • • • • More phosphorus settles to the bottom of ponds and binds to the clay soil. Low solubility – not readily available for plant uptake P is slowly released from each of these types of compounds Leaching loss is low, but can play a role in eutrophication Unlike N, P is not generally lost in a gaseous form Gains and Losses • Losses from plant removal, erosion of P-containing soil particles and dissolved P in surface runoff water • Gains from atmospheric dust are very limited, but a balance is established in most natural ecosystems Leaching of P after saturation of fixed pool 4 POTASSIUM Soil Solution K: • Immediately available to plants. • The nutrient 3rd most-likely to limit productivity • Present in soils as K+ ion (not in structures of organic compounds) • Soil cation exchange and mineral weathering dominate its exchange and availability (as opposed to microbiological processes) • Causes no off-site environmental problems • Amount varies with fertiliser application, weathering and cropping history • Generally not enough to meet the requirements of the cropÆ fertilizer needed Exchangeable K: • K+ retained by negatively charged exchange sites on OM and clay minerals. Readily available: moves backwards and forwards at ok CEC Slowly exchangeable K: • Igneous rocks are a good source – alkaline soils keep it. • Activates certain enzymes. • Regulates stomatal opening • Helps achieve a balance between negatively and positively charged ions within plant cells. • Regulates turgor pressure, which helps protect plant cells from disease invasion. • Promotes winter-hardiness and drought-tolerance Potassium deficiency • Potassium adsorption to wedge sites on micaceous (2:1) clays. Lattice K: • K found within the lattice structure of clay minerals and is released slowly into exchangeable forms as these minerals weather. Leaching: • Amount of K added in fertilisers or manures exceeds the CEC of the soil, potassium can be lost by leaching. Æ higher risk on coarse sands. WORLD POTASSIUM CYCLE • Leaves yellow at tip (chlorosis) and then die (necrosis) ÆThe leaves appear burnt at the edges and may tear, leaving a ragged edge • White, necrotic spots may appear near leaf edges • Oldest leaves are most affected The Potassium Cycle • High concentrations in micas and feldspars Æ K between 2:1 crystal layers becomes available • Returned to soil through leaching from leaves and from plant residue decomposition • Some loss by eroded soil particles and leaching • Replenishment required in most agroecosystems (1/5 of plant K is typically removed in product). Excess in plants can cause a dietary imbalance in ruminants FACTORS AFFECTING K FIXATION Almost entirely inorganic 1) Nature of soils colloids 2) Wetting and drying 3) Freezing and thawing 4) Presence of excess lime 5 FERTILIZER AND MANURE K READING FOR THURSDAY The principal K sources are manures or muriate and sulphate salts. In animal manures, K not biologically fixed to other compounds, unlike N and P, and thus is readily available to plants. Chapter 15: Calcium Magnesium and Trace Nutrients (aka Micronutrients) Common fertilisers use the muriate (chloride) and sulphate salts of P. Manure and fertiliser potassium contribute to potassium in soil solution. 6