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Cretaceous limestone

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.


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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