| dc.description.abstract | Smallhold farmers are faced with declining soil fertility arising from nutrient depletion through
continuous harvests and insufficient return through fertilizers or organic resources and therefore a
resulting decrease in crop yield. CheWi8!1 f ilisers directly offset nutrient shortages while
organic reso rces recycle nutrients and offset soil organic matter (SOM) losses. Soil nutrient
availability in low input farming systems which have low inherent soil fertility depends upon
mineralisation of these organic resources which include crop residues, animal manures and native
SOM.
An 18-year old long-term experiment located at the National Agricultural Research Laboratories
(NARL) of the Kenya Agricultural Research Institute (KARl) was used to study SOM dynamics
and its effects on soil nutrients. The experiment began in 1976 and is intended to determine the
appropriate methods for maintaining and improving soil fertility through the use of fertilisers,
farmyard manure and maize stover under a continuous crop rotation of maize and beans. A
selection was made of 8 out of 18 experimental treatments for detailed characterization of SOM
pools and other soil parameters. These treatments were: maize stover (R), farmyard manure (M =
10T/halyr), fertiliser (F = 120 kg N & P205 /halyr), combinations of each of them (FR, FM, MR,
FMR) and the complete control (CT). Soils were collected eleven months after the most recent
input applications and organic matter and plant nutrients analyzed. Organic matter fractions
were recovered by a procedure: of soil dispersion, wet sieving (53!-lm), decanting to remove sand
and density separation using Ludox, a colloidal silica suspension of densities 1.13 and 1.37
g/crn-'. Analysis of variance, regression and correlation were then performed on the data.
Manure application had the greatest effect on chemical soil properties, causing significant
increases (up to 50%) in the measured parameters. Total SOM varied only slightly among the
treatments (1.4-1.7% C and 0.14-0.20% N), but was significantly (P<O.OO1) affected by manure.
Particulate organic matter (POM) and its density separates varied between land managements to
a greater extent than total SOM. The medium (1.13-1.37 g/cm3) fraction showed the greatest
sensitivity to land management (P<O.OO 1). All the organic matter pools correlated strongly with
maize and bean crop yields except the heavy fraction (> 1.37 g/cm3) N which correlated poorly
with both maize(R2 = 0.13, P< 0.05) and bean (R2 = 0.10, P> 0.05) yield. Manure and maize
stover had a highly significant effect on soil microbial biomass (SMB) C and N (P< 0.001).
There was a significant correlation between net anaerobic mineralizable nitrogen (AMN) and soil
microbial biomass C (r= 0.91) and N (r= 0.93). Maize stover addition reduced AMN significantly
(P< 0.001) by 14% compared to the manure treated soils. When stover was applied in
combination with fertilizer however, AMN increased. Extractable Nand P and exchangeable
cations (calcium, magnesium and potassium) varied significantly among the different land
managements and they correlated well with total soil organic C and N (P<O.OOl). Soils recieving
only fertilizer showed significant (P< 0.01) reduction in pH while manure and stover treatments
showed significant (P<O.OOl) increases in soil pH. Effective cation exchange capacity increased
with the addition of both organic and inorganic fertilizers in the order M > F > R. Use of manure
offered the greatest return ratio to investments (4.4) and a high C sequestration efficiency (5.5%,
18 g/KSh input). In the Kikuyu Red Clay-Loam (Humic Nitisols) Highland areas of Kenya,
smallhold farmers who recycle nutrients through livestock may be managing their soil organic
matter resources in a more efficient manner than those who rely on fertilisers or crop residues
alone. | en |
