Effects of phosphorus and organic nutrient sources on improvement of soil fertility and maize (Zea mays L.) production on selected soils in Kenya.

Abstract

Maize yield in Kenya is low primarily due to decline in soil fertility. Inorganic fertilizers are however, too expensive and cause environmental pollution. Organic manures are low cost therefore, have great potential in increasing yield. The quality, quantity and placement methods of these diverse materials is however, not lcnown. The supply of organic materials on farms is also insufficient, thus the need to develop other organic resources. Pymarc is a by-product of pyrethrum processing that could be used as a fertilizer. The use of organic manure as P source is limited by the low P content. Supplementation with inorganic P fertilizer is therefore required. A series of pot and field experiments were conducted in 2003 and 2004 to determine NPK release pattem from Pymarc and the optimal mode and rate of application and effect of combining Pymarc or Tithonia with TSP on maize yield and the net benefits. Pot experiment was conducted in 2003 in a greenhouse at Egerton using Phaeazems, Cambisols and Ferralsols. A factorial experiment in a CRD replicated thrice with rates of Pymarc equivalent to 0, 2, 4, 6 and 8 tormes ha"! was tested. Samples were taken at 0, 2, 4, 6, 8, 12 and 16 weeks for NPK analysis. Field experiments were conducted in 2003 and 2004 with Pymarc at Egerton (Phaeozems), Molo (Cambisols), Kosirai (Ferralsols) and Tithonia at Kosirai (Ferralsols). Pymarc at 0, 4 and 8 tonnes ha" and Tithonia at 0, 2.5 and 5 tonnes ha" were each tested for suitable mode of application as spot versus broadcast in a 2 x 3 factorial arrangement in a split plot design replicated thrice. Nitrogen released afler 112 days of incubation in pot experiment ranged from 142 mg N kg'1 in Ferralsols to 163 mg N kg'l in Cambisols and 170 mg N kg" in Phaeozems. Phosphorus mineralized from Phaeozems, 6.1 mg P kg“ and Cambisols, 7.2 mg P kg'1 was twice the amount of P mineralized from Ferralsols, 3.1 mg P kg'l. Potassium mineralized at 1 12 days of incubation was 101 mg K kg" in Phaeozems, 100 mg K kg'l in Cambisols and Ferralsols was 45 mg K kg". Increasing rate of Pymarc application led to increase in the amount of Nitrogen, Phosphorus and Potassium released. Cambisols had the highest potential mineralized NO of 182 m kg", 7.6 PO mg kg" and 101 KO mg kg" than Phaeozems ofNQ 163 mg kg", 6.4 P0 mg kg" and 99 K0 mg kg‘ and Ferralsols, NO of 14s mg kg", 3.6 PO mg kg“ and 49 KO mg kg“. Ferralsols had the highest NPK mineralization rate constant 0.12 KN (d'l) and 0.16 KP (d'1) while Phaeozems had highest Potassium rate constant of 0.12 KN (d'1). Nitrogen and Potassium released was consistently related to C: N, lignin, N, Pol: N and Pol+Lig: N ratio. Pymarc significantly increased root biomass, NPK concentration leaf, stover, grain yield, macrofauna fiesh biomass, diversity and abundance. Pymarc and Tithonia using the Kosirai experiment had a significantly higher macrofauna fi'esh biomass, diversity and abundance than Pymarc. Positive and significant correlations were observed between Nitrogen and Lignin, Plant Residue Quality Index (PRQI) and earthworms; Phosphorus and PRQI, Potassium and Lignin, PRQI, earthworms and centipedes, Lignin and PRQI and earthworms, polyphenols and termites, ants and centipedes. Negative correlations were also observed between Nitrogen and Ants; Lignin and Polyphenols and PRQI and Earthworms. Phosphorus applied as Pymarc at 4 tonnes ha" or Tithonia at 2.5 tonnes ha’1 in combination with TSP at 13 kg P ha'l had the highest relative grain yield. Tithonia integrated with TSP had the highest relative grain yield increase. The addition of either Pymarc in combination with TSP at Egerton, Molo and Kosirai resulted in added maize yield of 110%, 103% and 117%, respectively. At Kosirai, Tithonia in combination with TSP resulted in added maize yield of 123% compared to TSP, Pymarc or Tithonia alone. Maximum net benefits were obtained at ll kg ha" when Pymarc was applied at Egerton, Molo and Kosirai for both spot and broadcast placement methods, respectively. The NPK release pattem is influenced by soil characteristics, amount, and quality of litter. Thus the release of nutrients by Pymarc and Tithonia and their effect on maize production makes them suitable for improvement of soil fertility and maize production when applied at 4 t/ha and 2.5 t/ha or integrated with TSP at 13 kg/ha, respectively.