Agriculture Reference
In-Depth Information
in a laboratory and providing soils with nutrients in such a way that a rather uniform
level of supply is obtained. It might require some efforts and costs to do this prepara-
tory first step in a site-specific way. The needs can be somewhat eased if a small aver-
age oversupply - at least for the major nutrients phosphate and potash - is accepted.
The second prerequisite - the removal by the harvest being the main factor of
changes in the nutrient level - implies that losses of nutrients by leaching, by runoff
of water or by erosion do not occur. For well managed fields, losses via runoff or by
erosion will be on the average rather small and will represent mainly exceptional
cases that take place in situations of adverse weather. But this does not hold for
losses by leaching
that result from natural rain or irrigation. In many agricultural
areas, these losses by leaching into the not-rooted subsoil and into the groundwater
horizon can be substantial as well as rather unpredictable. Whether the losses via
leaching still allow to rely on nutrient removal alone for a site-specific control of
fertilizing, depends on the ions that are involved. The main point is whether the
nutrients are absorbed by soil particles or bound within compounds that are not
soluble in water and whether hence their leaching downwards is prevented.
The
nitrate
and
sulphate ions
hardly are absorbed by soil particles, they are
easily dissolved in water and hence are disposed to leaching into the non-rooted
subsoil and into the groundwater. The leaching of these ions occurs especially on
sandy soils and somewhat less on soils with a higher clay content, however, soil
texture alone cannot eliminate the problem. Another factor that considerably can
reduce the leaching is nutrient uptake by growing crops in combination with pre-
cise fertilizing that is in line with the seasonal needs of plants. This is simply
because ions that are taken up by the roots of crops cannot leach any more. Yet this
fact can only help to diminish the leaching problem somewhat, it cannot do away
with this problem in geographical regions where during winter there is no nutrient
uptake. In short, for a site-specific control of nitrogen- and sulphur fertilizing, in
most areas it cannot be expected that the concept of nutrient sensing via removal
supplies reliable information.
The situation for fertilizing of
phosphate
is completely different. The phosphate
ions are very immobile within soils. Leaching of these ions does not occur. This is
mainly due to fixation in calcium-, aluminium- and iron phosphates that are not
soluble in water. Hence sensing the removal of phosphate via yield recording can be
regarded as a good choice for mapping the site-specific removal. In a subsequent
operation, the map can be used to control the spreading. The information about the
removal is obtained by simply applying a factor to the site-specific yield.
Potash
ions again present another situation. They do not leach anything as fast
as nitrate- or sulphate ions do. Yet they do not possess the immobility of phosphate
ions either. Potash that is not taken up by the crop can be held in the soil by clay
minerals or by organic matter. However, in order to prevent leaching in humid areas
and when no crops are growing, a clay content of 5 % or more is needed (Potash
Development Association
2006
). And for a site-specific concept, this minimum in
clay content is needed for about all cells within a field. So for sensing the potash
supply via removal by crops, restrictions in the soil texture must be considered.
For
calcium
and
magnesium
, depletion of reserves within soils due to removal
by crops seldom is a problem. Thus sensing the removal by crops hardly is
