PDA Leaflets
Potash and biosolids
The
Potash Development Association reports in its leaflet No. 20 that where
biosolids (sewage sludge) are used, there is frequently a need to apply
a straight potash fertilizer to balance the N and P in the biosolids and
that serious soil fertility and crop yield problems will arise if this
is omitted. The leaflet gives an indication of typical nutrient contents,
the legislative regulations as they are established in the UK, and advice
on best fertilizer practice. The leaflet is available through PDA, by
fax: +44 1994 427 443 or E-mail: info@pda.org.uk.
NK top dressing
An explanation of why top dressing of K, together with N, can sometimes
be good practice, is given in Potash Development Leaflet No. 22. It is
important to match nutrient supply to the time of high demand by the crop,
around flowering, 
by
supplementing soil K with extra K where K supply is short, for example
on light textured soils with poor buffering qualities. During rapid growth,
uptake by crops can be as high as 10 kg/ha/day. Potash offtake is often
underestimated. For example, a typical 8.5 t/ha crop of wheat, with the
straw baled, removes 100 kg of potash. Light textured, shallow soils are
at risk from K leaching. Under these conditions potash is best applied
"little and often" and single, large dressings should be avoided.
K helps in N metabolism and therefore improves N fertilizer use efficiency.
In this context, IPI published, in 1993, Research Topics No. 13 on "The
Potassium-Nitrogen Partnership" by P.A. Gething, available at
the IPI bookshop for USD10 through ipi@ipipotash.org
or www.ipipotash.org.
For the great majority of cereal crops, potash removal is considerably
greater than that of phosphate. The PDA leaflet suggests that potash top
dressing offers a convenient way of maintaining the correct replacement
balance. The leaflet is available through PDA, by fax: +44 1994 427 443
or E-mail: info@pda.org.uk.
Potash for organic growers
"... organic production and conventional farming have many common objectives and are working with the same basic resources. It is most regrettable that promotion and advocacy of the different systems emphasises differences and suggests conflicts...". This sets the tone of PDA leaflet No. 23, (Potash for organic growers) in which it is pointed out that, for P and K, the aim of both organic and conventional farmers is the same.
Both work within natural systems to maintain and/or increase long-term
soil fertility and both use, as far as possible, renewable resources.
Conventional farmers must also take into account nutrients from organic
sources when deciding fertilizer use. 
Differences
between the two systems are more apparent for nitrogen where organic systems
rely on soil and biological nitrogen fixation and conventional systems
include mineral N in their nutrient management. Both systems take care
to reduce nutrient losses for both financial and environmental reasons.
Concerning potassium, PDA rightfully points to the fact that K, irrespective of whether the source is mineral or organic, behaves in soils in exactly the same way. When K is released into the soil solution, either leached out of decomposing organic manure, recycled plant residues or dissolved from potash fertilizers, soil solution K is in equilibrium with exchange sites on clay minerals and soil organic matter. The exchangeable K or readily available K is itself in equilibrium with the non-exchangeable fraction and ultimately with the native/matrix fraction. Increasing the K concentration in soil solution will lead to absorption processes to retain the surplus K; lowering the solution K leads to desorption processes in soils to replenish K taken up by the plant roots.
In this context, soil analysis provides the best practical guide to the adequacy of nutrients for plant growth and the need for nutrient supplementation, says PDA, and concludes, "... all systems of production should maintain an adequate supply of available potash to the plant. Nutrient management must balance inputs with outputs and losses...". Low soil potash will bring penalties. If K is limited, response to N will be reduced, biological nitrogen fixation will be low and crop health, vigour and resistance to stress, pests and diseases will suffer. These factors are of great importance in the philosophy of organic farming where natural resistance through balanced fertilization is an integral part of overall husbandry in the absence of agro-chemical protection.
Less understandable is the fact that muriate of potash, KCl is grouped under "prohibited materials". It is more or less a natural product, but is prohibited for its high Cl content. This is undesirable for some plant species sensitive to chloride. It has also been postulated that excess Cl may be deleterious to micro-organisms and earthworms, but no evidence has been found to support that belief. The aversion to chloride may derive from confusion with chlorine gas, hyperchlorite or other forms, which do not occur in soils and plants.
Leaflet No. 23 is available through PDA, by fax: +44 1994 427 443 or E-mail: info@pda.org.uk.