The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 222

that K was released from non-exchangeable reserves or taken up from
below the sampling depth; this could also have occurred where efficiencies
were < 100%. The soils contain ample P and K after > 70 years of applica-
tion, and so the efficiency of use decreases with the amount applied. The
results do show, however, that FYM produced the most efficient use of P
and K, and a combination of FYM and fertilizers was generally the least
efficient. However, efficiency of use is only one way of judging the effective-
ness of manures and fertilizers. Combinations of FYM and mineral K
produced the largest yields and offtakes.
Plant availability and the utilization of P and K were controlled by the
nature and extent of the sorption surfaces in the soils, both mineral and
organic, and the capacity for fixation. Fertilizers were least effective in the
most strongly P- and K-fixing soil at Rothamsted, and most effective in the
sandy loam soil at Skierniewice, which held the nutrients in an available
form without fixing them. Also, where FYM increased the cation exchange
capacity (CEC) of soils by adding humic materials (Skierniewice; Blake
et al ., 2000a) or blocked K-fixing sites through adsorption onto clay
surfaces (Rothamsted; Goulding and Talibudeen, 1984), this improved K
utilization.
Deficiencies of other nutrients decreased the effectiveness of applied P
and K. FYM was generally more effective than fertilizers because these
nutrients were supplied in the manure. However, the release of N and
P from organic manures is strongly dependent on the mineralization
rate. This is obviously not likely to be a problem in East Africa, but at
Skierniewice the cold climate greatly reduced the rate of mineralization and
caused N and P deficiencies when only FYM was applied.
Conclusions
Researchers, farmers and advisors are faced with a dilemma. Organic
manures and crop residues should be recycled and used effectively to
maintain SOM levels, supply nutrients and make farming more sustainable.
In addition, C plays a central role in controlling the rate of N cycling. Soils
in which C supply matches N input cycle N tightly, keeping it within
the system; N-saturated soils deficient in C are more likely to lose N to
the environment. Carbon supply from manures and residues is therefore
beneficial. Manures also supply a broad range of nutrients, and they can be
more effective than fertilizers, especially in soils with a range of nutrient
deficiencies or which fix P or K or both. However, rates of mineralization of
N and P from SOM and manures are difficult to predict and even harder to
control. Rates of release are determined by environmental conditions and
are not necessarily well synchronized with crop uptake. This can lead to