biosystem

Fertilizers• Nutrients become available through

organic matter decomposition, chemical weathering of minerals, airborne additions, and fertilizers.• Nutrient analysis of the soil

determines the potential of the soil for supplying N,P,K, Ca, Mg, S, plus micronutrients to plants during the growing season.• Fertilizer ratio is the relative

proportion of primary nutrients (N-P2O5-K2O) in a fertilizer grade,

Nitrogen• NH4

+ and NO3-

forms taken up by plants• Loss of N can occur:

1) leaching of NO3- , 2) volatilization of NH4+ to NH3 (high pH soils), • 3) immobilization

by plant or microbe uptake, • 4) Denitrification

THUS it becomes important to give N - CREDITS for previous management activities. •legumes, manure or other organic additions with low C:N ratios are adding N to the soil•Thus this organic N needs to be counted into the total N available for crop growth •And thus reduce the need for fertilizer N.

Green Manure

Cow Manure

Phosphorus•Soil test for P 0-5 ppm = very

low• 6-10 ppm = LOW•11-15 ppm = medium

•16-20 ppm = high•> =21 ppm = very high•No reason to have application

of P if soil test > 21 ppm

P deficient tomato

If you apply a 14 lb sack of 22-3-14 how many pounds of N, P2O5, and K2O did you apply?

•N : 14 x .22 = 3.08•P2O5 : 14x .03 = 0.42

•K2O : 14 x .14 = 1.96

•To convert from the elemental analysis to the chemical analysis for phosphorous and potassium fertilizers, use this formula: • % P2O5 x .44 = %P

• % K2O x .83 = %K

Converting P2O5 & K2O (oxide from) into P and K (elemental form)

•% P2O5 x .44 = %P & %P x 2.29=%P2O5

•%K2Ox.83=%K %K x 1.2=%K2O

• For Example: A 30 pound sack of fertilizer (25 - 5 - 5) , is applied to your garden. • The amount of N applied=.25 x 30=7.5 lbs N • amount of P205 applied=.05 x 30=1.5 lbs & P=.44 x 1.5=0.66 lbs

P • amount of K20 applied=.05 x 30=1.5 lbs & K=.83 x 1.5=1.245

lbs.K

Soil Cation Exchange Capacity (CEC)•CEC is an inherent soil characteristic and is difficult

to alter significantly. • It influences the soil's ability to hold onto essential

nutrients and provides a buffer against soil acidification.•The higher the CEC, the more cations which can be

supplied. This is called the soil's buffer capacity.

Soil Cation Exchange Capacity

• In most soils, 99% of soil cations can be found attached to micelles (clay particles & organic matter) and 1% can be found in solution. • Cations in the soil (mainly Ca++, Mg++,

K+ and Na+) maintain an equilibrium between adsorption to the negative sites and solution in the soil water. • This equilibrium produces exchanges --

when one cation detaches from a site (leaving it free), another cation attaches to it. • Therefore the negatively charged sites

are called cation exchange sites. • The total number of sites is the Cation

Exchange Capacity or CEC

Cation Exchange Capacity

1)the number of cation adsorption sites per unit weight of soil or

2) the sum total of exchangeable cations that a soil can adsorb.

* CEC is expressed in milliequivalents (meq) per 100 g of oven dry soil.

Equivalent weight = molecular or atomic wt (g)

valence or charges per formula

Milliequivalent (MEQ)

1 meq wt. of CEC has 6.02 x 10 20 adsorption sites MEQ of Common Cations Element Na+ K+ Ca++ Mg++ Valence 1 1 2 2 Eq. Wt 23/1=23 39/1=39 40/2=20 24/2 = 12 MEQ wt .023 .039 .02 .012

Sample calculation for equivalent weight for lime or CaCO3

CaCO3 - formula wt. = 40 + 12 + 48 = 100

charges involved = 2

eqwt. = 50

meq = .05 grams

Or one meq of Lime = .05grams

Calculation of CEC with % clay and % OM

Assume Avg CEC for 2% OM (200 meq/100g) Assume Avg CEC for 10% clay (50 meq/100g)

CEC = (2% OM x 200) + (10% Clay x 50) soil with 2% OM and 10% Clay 200 x .02 + 50 x .1 = 4 + 5 = 9 meq/100 g

Predicting CEC

1) sum of cations : remove all cations and total the amount

2) NH4+ saturation: soil is saturated with NH4+ - the NH4+ is replaced by Ca++ and the NH4+ removed is measured.

3) Estimation based on texture: Sand = 0-3 meq/100 g LS to SL = 3-10 Loam = 10 - 15 Clay Loam = 10-30 Clay = > 30 (depends on kind of clay)

USDA Textural USDA Textural TriangleTriangle

•A high CEC value (>25) is a

good indicator that a soil has a high clay and/organic matter content and can hold a lot of cations. •Soil with a low CEC value

(<5) is a good indication that a soil is sandy with little or no organic matter that cannot hold many cations.

http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm

Important concept of Base Saturation• Cations can be classified as either acidic (acid- forming) or basic. • The common acidic cations are hydrogen and aluminum• Common basic ones are calcium, magnesium, potassium and

sodium. • The proportion of acids and bases on the CEC is called the percent base saturation

• The concept of base saturation is important, because the relative proportion of acids and bases on the exchange

sites determines a soil's pH. • As the number of Ca++ and Mg++ions decreases and the number of H+ and Al+++ions increases, the pH drops. • Adding limestone replaces acidic hydrogen and aluminum cations with basic calcium and magnesium cations, which increases the base saturation and raises the pH.

Base Saturation vs pH

% Base Saturation - meq bases ÷ CEC x 100 % Hydrogen Saturation - meq H ÷ CEC x100 Example: Ap Soil Horizon Cations-- H+ Ca++ Mg++ K+ Na+ 9.4 14 3 0.5 0.1 CEC = 27 meq/100g (sum of cations) % base sat = 17.6 ÷ 27 x 100 = 65% % hydrogen sat = 9.4÷27 x100 = 35%

pH vs. Base Saturation-an approximate relationship

Buffering Capacity

The ability of soil to resist change in pH.The amount of H+ in the soil solution is

small compared with the “H+, Al 3+”

adsorbed on the soil colloids (reserve)

Neutralization (by the addition of bases) of the solution H+ (H+ is removed from the system) results in a rapid replacement of H+ from the exchangeable H+ on the soil colloid.

CaCO3 when added to soil will neutralize H+.

CaCO3 = Lime (dolomite = MgCO3 & CaCO3)

Why apply lime (CaCO3)? 1. helps nutrients become available

to plants (solubility vs. pH) 2. improves soil structure 3. provides nutrients for plant

growth -Ca & Mg 4. promotes growth of beneficial

microorganisms- they like pH=6.5 5. overcomes acidifying effects of

fertilizers 6. reduces metal toxicity to plants

(solubility vs. pH)

• Calcium and Mg - when soils are low in Ca, Mg, they have a pH problem and by adding lime or dolomite the pH and Ca, Mg problem is corrected.

Magnesium deficiency on corn

Use of Precision Agriculture: •Farmer’s fields have variable yields across the landscape. • Variations can be traced to management practices, soil properties and/or environmental characteristics. •Soil characteristics that affect yields include texture, pH, structure, moisture, organic matter, nutrient status and landscape position. •Environmental characteristics include weather, weeds, insects and disease. Source: http://extension.missouri.edu/explore/envqual/wq0450.htm

• The higher grain yields that appear

spatially related to the high pH area may be caused by favorable soil conditions related to pH. • Correlation between yield and a soil

parameter is not certain proof that pH is the cause of higher yields. • Past management of this portion of the

field may have been the more important factor resulting in higher yields because the area of high yield is substantially smaller than the area of high pH.• Aerial photograph, soil pH and 3-year

average grain yields for central Missouri farm

Field -- Soil pH -- Soybean Yield

• CEC and Soil Testing:• Because the CEC of a soil is relatively constant

unless large amounts of organic matter are added, it is not measured or reported with a routine soil test. • Ca : Mg Ratio and Soil Testing• Some soil testing labs will report ideal calcium to

magnesium ratios for plant growth. • However, most plants tolerate a very wide range

of soil calcium to magnesium ratios. • Adjusting the ratios of calcium and magnesium on

the exchange complex by adding gypsum (calcium sulfate) or Epsom salts (magnesium sulfate) has not been shown to significantly benefit plant growth. • Gypsum is primarily used as a soil amendment to

improve water penetration and increase the level of calcium in the soil.