PRESENTED BY AKHTAR MEHMOOD ROLL # DEPARTMENT OF BOTANY M.PHIL BOTANY FINAL SEMESTER

 Plant nutrient usually exist as ions i.e.  They carry an electrostatic charge.  The positively charged nutrients are known as Cations while  Negatively charged nutrients are called as Anions.  Example.  Cations. Ca 2+, Mg 2+,,K +,Na +, H +,Al 3+  Anions: NO 3 -,H 2 PO 4 -,HPO 4 2-,SO 4 2-,Cl - History  From 1920s to 1940s William Albrecht did a lot of experimenting with different ratios of nutrient cations.

 Exchangeable Cations  Cations bound to soil in varying degree  Strongly bound to silica or Soluble in soil solution.  Between these two extremes are the exchangeable cations, which are weakly bound to soil particles.  Soil particles carry net negative electrostatic charges as a result of processes of soil weathering, and organic decomposition.  These sites of negative charges are most predominant in the humus fraction of the soil, and on the edges of clay particles.  The bonding of these cations largely prevents their loss by leaching, but is not so strong that plants cannot extract them from the soil.

 The Cation exchange capacity of a soil is a measurement of its ability to bind or hold exchangeable cations. In other words, it is a measure of the number of negatively-charged binding sites in the soil.

Milli-equivalents (Meq.) of Selected Cations and Their Equivalent ppm Cation Atomic Weight Valence Milli- equivalents Equivalent ppmLbs/acre H+H Ca Mg K+K NH Al Zn Mn Fe Cu Na

ElementAtomic Weight ValencePpm to equal 1 milli equivalent Hydrogen1120 Potassium Magnesium Calcium202200

 To determine the CEC calculate the milliequivalents of H,K,Mg,Ca per 100 gm of Soil(meq/100 g of soil)by using formula: Formula  H,meq/100g soil=8(8.00-buffer pH)  K,meq/100g soil = lbs/acre extracted K/782  Mg,meq/100g soil= lbs/acre extracted Mg/240  Ca,meq/100g soil = lbs/acre extracted Ca/400  Na,meq/100g soil = lbs/acre extracted Na/460

Lab # Sampe l#Soil code Soil pH Buffer pH PKMgCaNa

 H,meq/100g soil=8( )=2.40  K,meq/100g soil = 221/782=0.28  Mg,meq/100g soil= 28/240= 0.12  Ca,meq/100g soil = 400/400=1.00  Na,meq/100g soil = 12/460=0.03  Total CEC=3.83 meq/100g soil

RatingCEC (me/100g)Comment Low5-12Low organic matter. Sandy soil Medium12-25Pumice soil, Lower fertility High25-40High fertility soil, High clay content. Very High40+Peat soils

 It refers to Elements that are basic or Alkaline in their reaction.e.g K,Mg, Ca & small amount of Na & Al.  Hydrogen is an element with a positive charge and acts like a cation however soils with significant saturation of hydrogen are acidic, or have a lower pH.  The measure is expressed as milligram equivalents per 100 grams of soil or shortened to “me”.

 Example  K=0.28meq/100g soil  Mg=0.12meq/100g soil  Ca=1.00meq/100g soil  Na=0.03meq/100g soil  CEC=3.83meq/100 g soil Total for bases=K+Mg+Ca+Na=1.43meq/100g soil Percent Base saturation= (1.43/3.83)(100%)=37%

 Exchangeable Cations can be divided into two groups.  Bases  Acids  Every CEC binding site must have a cation bound to it, to maintain electeroneutrality.  The soil pH Will be effected by whichever cations predominate on these exchange sites.  More base cations more alkaline soil  More acid Cations more acidic soil

 It is the fraction of the negative binding sites occupied by bases.  For example  A base saturation level of 75% means that three out of four sites are occupied by basic cations (remaining 25% by acidic cations).  Total base saturation is determined by following formula  Total base saturation= Ca+Mg+K+Na  CEC

 CEC also helps to characterise soils.E.g  Organic matter is the major source of Negative electrostatic sites there is a strong correlation between CEC values, and the amount of organic matter in the soil.

 CEC can give insight into soil quality and site characteristics.  Higher CEC likely indicates more clay, poor internal drainage, limited structure and soil compactation in high traffic areas.  Low CEC is indicative of sandy textured soils prone to drought that invariably needs more organic matter to improve water holding capacity, but have open grainy structure that resist compaction.

What we have learned  Clay and organic matter have negative charges that can hold and release positively charged nutrients.(The cations are adsorbed onto the surface of the clay of the clay or humus).That static charge keeps the nutrients from being washed away, and holds them so they are available to plant roots and soil microorganisms.

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