There are 4 type cones:
1- base area 1 cm2
2-base area 2 cm2
3-base area 3.33 cm2
4-base area 5 cm2.
According to ASAE standards, for soft soil they recommends base size 3.23 cm2 however the cone used is 30 degree top angle. Eijelkamp used 60 degree top angle.
I am not sure which brand I have used in years past. In hard soils, the smallest cone would likely be selected to push probe into surface to get reading. On soft soils, the larger or largest cone has more surface area and give best reading using the appropriate scale. One would adjust reading to report results per square cm. So the 5 cm2 probe reading would be divided by 5 to report per cm2. Definitely an organic muck would use the 5 cm2 and some may be so weak and difficult to walk on surface without breaking through or get reading. The 3.33 cm2 may be right for paddy field. The probe cone surface area should already be adjusted in its value, regarding cone angle question. You may have to do some cone surface geometry calculations to sompare cones and check. I think the cone uses its surface area of cone being pushed into soil, and not top diameter of cone. That would probably become evident when comparing cone values and their top diameter and cone surface dimensions being pushed into soil.
Pasturing: mechanical spread of liquid manure; cultivation, harvesting in arable farming. 5 cm2 , 60° top angle (Major > 0.6, Moderate 0.3 – 0.6, Minor < 0.3)
Sports fields: 1 cm2 , 60° top angle (Major > 1.4, Moderate 1.0 – 1.4, Minor < 1.0)
As a rule a cone with a 1 cm2 - base area and a 60º- top angle is applied in soil research fieldwork.
Source: Vereniging voor Landinrichting, 1992
Given this information, the angle and the type of soil you are planning to use it on, I'd go for the 5 cm2 option (60º angle).
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