Agriculture Reference
In-Depth Information
Fig. 9.4
Implement for simultaneous sensing of pH and electrical conductivity of soils. For pH
sensing, a horizontally oriented sampling shoe (
insert top right
) is periodically lowered into the
soil and when lifted pushes a naturally moist soil sample against ion-selective electrodes. After
this, the electrodes are rinsed with water. Crop residues - if present - are swept aside by a row
cleaner. Calibrating of the electrodes is essential for precise sensing (From Veris Technologies,
Salina, USA, altered)
On the other hand, solving the problem with a substantially better resolution
while using the conventional method of analyzing in laboratories is economically
not possible. Because for the traditional laboratory method, the costs rise almost
proportional to the number of samples. Whereas with modern proximal or remote
sensing methods, the number of samples hardly affects the costs.
It is important to differentiate between
soil water pH
on the one hand and
soil
buffer pH
on the other hand. Ion-selective electrodes that sense hydrogen cations
indicate via their voltage output only the soil water pH, which reflects what the
plants experience. But the soil water pH explains nothing about the ion absorbing
capacity of the soil particles. This
ion absorbing capacity
results in reserve- or
exchange acidity that is indicated by the soil buffer pH. When conventional soil
laboratories analyze soil in order to present recommendations for liming, both soil
water pH as well as soil buffer pH are measured. The soil buffer pH is not an
a priori
existing feature of the soil, it is generated in the laboratory. Basically, soil buffer pH
is the resulting pH that develops after in the laboratory there was added a defined
amount of liming material. In the laboratory, the difference between the original soil
pH and the final pH that is created by the liming material, a buffering solution, is
recorded. This difference depends heavily on soil properties. It indicates how much
lime is needed to raise the soil pH to the level that is aimed at.
So for recording the
liming requirement
in a site-specific manner by on-the-go
sensing, the “buffering capacities” of the soils need to be taken care of in addition to
