Having picked up a book called "The Ideal Soil" i thought i had cracked this cation exchange thing , oh how little i knew . I asked Neil (see earlier photos) for an explanation of Small tustins numbers , this is what he said ,it makes the most comprehensive sense about the subject since i started looking ino it.
Hi Mark
Must say, I have been working with soils for over 25 years and have yet to find anything close to ideal.However, while the ideal soil may not exist in reality, components of an ideal soil can be identified and management processes put in place to optimise them.However, soil characteristics will change with climate,weather, cultivation, application of fertilisers,lime, manures,cover crops,specific crop and root exudates etc,making the soil a very dynamic medium with constantly changing target or optimum values of any given nutrient or component. Striving for the ideal value of any given parameter can be a fruitless task,but interpreting how parameters interact, and how to optimise soil or crop response to these interactions,can provide useful guidance to soil management strategies.One such guide is the concept of Cation Exchange Capacity(CEC).
CEC is a measure of the negative charge associated with clay and organic matter,and
the ability for this charge to hold,adsorb or react with materials with a positive charge.The more active organic matter and functional clay,the higher the CEC,the better the nutrient and water holding capacity of the soil,and the higher the yield potential or performance characteristics of the soil.CEC is expressed in terms of unit charge, typically as milli equivalents per 100 gms (meq/100g) or centimols of cationic charge per kilo (cmolc/kg).This unit measure can be related directly to the atomic mass and charge of any given cation. A charge of one meq/100g would hold 449 kg per ha of Calcium, or 269 kg per ha of Magnesium, or 876 kg per ha of Potassium, or 516 kg per ha of Sodium or 22.4 kg per ha of Hydrogen (the unit measure of
acidity).Similarly, CEC can be converted into mg per litre to match standard soil testing. However, the numbers are derived using different extraction protocols and can not be used under Cross Compliance for soil management purposes.Anion Exchange Capacity, or AEC, is the measure of positive charge associated with clay and organic matter, and is expressed in the same unit terms (meq/100g or cmola/kg). AEC is typically very low and the anionic
(negatively charged) nutrients such as nitrate, sulphate, chloride and borate tend to have high diffusion rates,moving at over 1 cm per day though
the soil profile.Typical ranges for CEC would be 1 to 4 meq for sands, 2 to 8 meq for silts, 6 to 20 meq for clay loams, 15 to 45 meq for clays, 35 to 75 meq for organic soils and upto 225 meq for peats.CEC is a useful means of determining the ability of the soil to hold nutrients and can be used to measure both chemical concentration gradients
and actual nutrient loading.
Concentration gradients are readily calculated as Base Cation Saturation
Ratios, or BCSR, by dividing the unit charge of a particular nutrient by the
total charge of the soil. Each cation has different behavioural characteristics at specific concentration gradients, dependent on soil textural class and clay chemistry.Although optimum Calcium uptake can occur at 20% BCSR, for most soils,
Calcium works best at 60% to 68% BCSR. However, as Calcium has a positive impact on the physical spacing of the clay colloid, target BCSR values for Calcium must be adjusted to match clay type and content, in some cases increasing to over 80% BCSR before sustainable, self-structuring clay aggregates can be formed. By contrast, Magnesium activity may be optimum at around 4% BCSR, but must balance Potassium and Calcium if sufficient uptake levels are to be achieved. Conversely, Magnesium has a very high water holding capacity and has a tendency to decrease clay particle spacing, increasing the plasticine limits of the soil. Increasing the Magnesium BCSR
on a sand soil would reduce drought stress and Potassium uptake, while decreasing soil structure and increasing water holding on a clay soil (ideal for black grass).
For the example of Small Tustins, the values on the results sheet show;
element CEC as cmol/kg Base Cation Saturation Ratio Target
BCSR for this type of clay soil Nutrient load in kg/ha above or below
target
Calcium, Ca 23.40 meq, BCSR = 23.40/27.90 = 0.8387 or 83.8%
ideal for this soil 74 to 78% gives 1158 kg calcium excess
Magnesium, Mg 0.94 meq, BCSR = 0.94/27.90 = 0.0337 or 3.37% ideal
for this soil 5 to 7% gives 541 kg magnesium deficit
Potassium, K 0.45 meq, BCSR = 0.45/27.90 = 0.0161 or 1.6%
ideal for this soil 2.2 to 3.4% gives 154 kg potassium deficit
Total CEC 27.90
Interpretation is the key here. The Calcium excess is making a significantcontribution to soil structure, aeration, drainage, water infiltration ,
biological activity etc and it would be detrimental to reduce this to the target value. Although the Calcium excess is creating a concentration gradient against which Phosphorus, Magnesium and micro-nutrients will struggle, Calcium must be maintained at this level, or above, if soil structure is to improve to facilitate cultivation, grass weed control, Nitrogen assimilation and uptake etc. The down side is transient lock-up with Phosphorus, Manganese etc, all of which can be addressed using appropriate placement fertiliser, seed treatment, foliar feed, biolgicals
etc.Similarly, Magnesium base loading is low, but increasing the Magnesium level
to lowest BCSR target value would have major impact on soil structure, increasing plasticine limits and water holding which would restrict cultivation and favour grass weed seed development, while restricting Potassium uptake and Nitrogen Use Efficiency etc. making it far better, cheaper and agronomically more viable to foliar feed Magnesium to optimiseNitrogen and Potassium uptake.The BCSR values indicate chemical interactions. Target, or optimum levels, can be determined for optimum nutrient uptake and soil formation, but must be metered to suit specific soil type, cropping and environmental parameters.
As for Phosphorus - Potassium balance, one of the vital factors is crop nutrient demand. In the main, UK arable cropping focuses of autumn establishment, biasing P demand to the first 30 days from germination and leaving K to stem extension and beyond. This is not the case with most other parts of the world, where P and K demand are more closely timed. As a result, our crops need P at planting and K with first Nitrogen, while others can put total NPK in the seed bed and forget them for the rest of the growing season. In addition, most soils used to determine CEC and BCSR did not have the buffer capacity or strong alkaline nature of the typical
calcareous UK clay, and as a result are not subject to the massive P-fixing and antagonistic lock-up issues seen here, so P fertiliser strategies need to be modified to reflect this.Striving for ideal values may have agronomic benefits, but may also have cost implications that are prohibitive. For the example given, with 1158 kg Calcium excess, it would take 2400 kg Sulphur, in addition to the S required
by the growing crop or lost to the soil system by leaching etc, to effectively reduce the existing Calcium level to the ideal level. At £450 per tonne, this application would cost over £1000 per ha, which would not include the cost of loss of soil structure as the clay colloids begin to collapse, delays in drilling and crop establishment as soils take longer to dry and become friable, or the impact on grass weed seed germination, the activity of residual chemicals etc. As soon as this soil is subject to cultivation, the inputs of steel and diesel would trigger Calcium release from parent material, re-setting the Calcium BCSR at an excessive level. If
the farming system was reliant on direct drilling, cover cropping and surface mulching of crop residue, and if the reduction in Calcium from excessive to ideal had associated agronomic benefits, then the Sulphur route may have value. But under conventional management, with high input cost and volatile output prices that are often break-even at best, it may be far better to keep the Calcium level in excess, gain the benefit of improved soil structure, and work around the antagonistic nutrient interactions.Similar stories could be told for other cations. Building soil Magnesium to ideal would restrict Potassium uptake, lower Nitrogen Use Efficiency,
increase disease pressure, reduce soil structure and favour grass weeds.
Foliar feeding Magnesium will give improvements in Nitrogen use, protein building, disease suppression, green leaf area, without affecting soil structure. Building soil Potassium level would reduce Magnesium availability further, increase the tendency for the clay to shrink and crack, lift soil pH and reduce Zinc uptake. But timing an application of Potassium with Nitrogen at the onset of stem extension could lift grain yield potential in cereals by 400 to 600 kg/ha, provided the crop had access to sufficient Phosphorus and Manganese at germination.Its great to have targets. Even better to have the means to measure, monitor and manage those targets. But sometimes striving for ideals can be misleading, especially in the dynamics of the soil eco-system.
Hope this helps
N
This is what i asked
I have looked at the soil test we had done for small tustins and I can't fathom out the actual amount of calcium, magnesium , potash, sodium , that is available in the soil, I understand that if we take the exchange capacity which is 27.9 and multiply that by 400(?) it gives the actual amount required in the soil for calcium, 240 for mg,780 for K, and460 for NaThis book is in lbs acre is this the right calculation.
We then can take the amount in the soil and work out the amounts required the trouble is I can't fathom how to convert meq/100g into kg /ha and thus into the amount in the soil and then work out how much ca etc we need to apply.The NRM report , is, ca is 23.4 meq/100g,K of 0.45,mg of0.94,Na of 0.31
Does this make sense..?
The next question is the relationship between P&K ?
M
