Cretaceous limestone
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1. Introduction
Chalk developed 60 - 90 million years ago as geogenic sediment. Chalk consists of coccoliths and/or bryozoa, the uniform crystallite size of which comes to only a few µm.
This crystallite size given by nature cannot be achieved by the most complex grinding and screening processes carried out for other lime products which developed in other times of origin, such as Devonian, Jurassic.
For this reason the reactivity of the cretaceous limestones is much higher than that of other lime products.
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Coccoliths |
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2. Evaluation of the reactivity
In order to evaluate the reactivity relating to the actual appliance, the dissolution speed depending on time in acid is measured. According to the German rules and regulations for fertilizers (Düngemittelrecht), the minimum reactivity comes to > 30 % for carbonate products and > 10 % for dolomitic limestones. Additionally, the content of basic constituents is specified. Another quality criterion is the graining.
In practical operation, it turned out that these specifications do not sufficiently evaluate the practical value of lime. The comparison of different types of lime shows clear differences, which the practical farmer cannot clearly recognize in the present rules and regulations for fertilizers.
According to the German rules and regulations for fertilizers (Düngemittelrecht), the following three parameters are decisive:
- Basic constituents CaO + MgO > 30 %
- Grain size distribution 97 % < 3.00 mm 70 %< 1.00 mm
- Reactivity > 30 % at carbonates pH-value 2 > 10% at carbonates with > 25 % MgCO3
In practical operation this means that lime with high contents of basic constituents and a grain size distribution which conforms to standards at a reactivity of 30 % can be supposed to be sufficient, although this product is only available for longer periods of time for a utilization for plant and soil.
A solution is the neutralization value as expression for the temporal dissolution of lime, i. e. the increase of the pH-value in soil. The neutralization value includes the grain size dependence as complex quantity.
Table 1: Effective neutralization values of carbonate fertilizer limes
(Runge, P, Tagung Berichte Akad. Landwirtsch. Wiss. DDR Berlin (1988) 267, S. 315-320)
| Product |
Alkaline efficient ingredients % CaO |
Neutralization values % CaO |
Neutralisation values of alkaline efficient ingredients % CaO |
| Chalk |
50 - 55 |
40 - 50 |
80 - 90 |
| Dolomite |
57 |
28 |
47 |
| Devonian |
55 |
30 |
55 |
The table shows considerable differences between the neutralization values and the contents of basic constituents. Chalk reaches a neutralization value of up to 90 % while only 47 % is measured for Dolomite.
The naturally fine crystallite size of chalk of a few µm has a very favorable effect on the neutralization value, while considerably worse reactivities at other crystalline products from older ages are measured.
Considerations regarding the economic efficiency
Conversion factors:
CaO zu CaCO3 1:1,78
MgO zu MgCO3 1:2,09
Due to the conversion factor dolomitic fertilizer limes have the highest contents of basic constituents. The table relativizes this proposition.
Table 2:Products / Reactivities / Neutralization values
| Products / Chalk |
Alkaline efficient ingredients % CaO dry residue |
Reactivity to Sauerbeck /Rietz % |
Neutralization values acc. to Runge % CaO |
Neutralisation values of alkaline efficient ingredients % CaO |
| Chalk |
50 - 55 |
80 - 90 |
45 - 50 |
80 - 90 |
| Devonian |
55,0 |
30 |
30 |
55 |
| Dolomite |
57,0 |
25 - 40 |
28 |
47 |
The table shows that due to the high reactivity, > 80% of the analytically determined, basic constituents are available in the chalk products. Compared with that, these constituents are considerably lower in other lime products.
The high reactivity results in a quicker availability of chalk. Losses due to
- elutriation
- nutrient removal by the plants
- neutralizing effect when using acid fertilizers
are compensated faster. Limes with low reactivities are often not able to make available the compensation in the lime supply in this period of time or in the vegetation pauses considerable parts get lost as eluviation losses.
The farmer has to decide about the price at which he wants to purchase lime. Here it is helpful to take the availability / the neutralization value as basis.
Assuming 100 % as the value for chalk, because chalk has the highest neutralization values, the other products are to be evaluated as shown in the following table:
| Products / Chalk |
Neutralization values acc. to Runge % CaO |
Price relation |
| Chalk |
45 - 50 |
1,0 |
| Devonian |
30 |
0,63 |
| Dolomite |
28 |
0,59 |
When comparing the costs, a Devonian / Dolomite product would have to be offered with a price reduction of app. 40% at the production site of the Cretaceous limestone, in order to be able to offer the same quantity of neutralization value.
Additionally a 1.6fold to 1.7fold quantity has to be taken into account for the output
4. Magnesium supply
Despite the low reactivities dolomite products can play a certain part regarding the magnesium supply of soils. When supplying via dolimitic limestone, the magnesium demand can be a low-cost solution. Here the farmer has to take into account, however, that the availability of magnesium has to be fixed for longer periods of time. If magnesium which is quickly available is required, other magnesium compounds such as Kiserite and Optimag are to be applied. |
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