Proceedings

The use of fertiliser enhancers to improve the nutrient potential of an organo-mineral fertliser

Pawlett, M.1, Deeks, L.K.1, Noble, A.2 and Sakrabani, R.1, 1Cranfield University, UK, 2Demeter Technology, UK

(free)

Total nitrogen (N) and phosphorus (P) content in biosolids varies between 3–5 and 5–7 % (w/w) respectively. The addition of urea to biosolids produces organo-mineral fertilisers (OMF), which are more nutritionally consistent. The availability of the nutrients in OMF to crops can limit its agronomic value. Dicarboxylic polymers have been designed to improve the availability of N and P to crops. However, the effects of these polymers on soil nutrient cycling following OMF application are unknown. Two laboratory incubation experiments were established to evaluate the ability of two additives, one targeting P and the other targeting N cycling, to enhance the fertiliser value of an OMF. Nutrient effects of the additives with the OMF were compared to an untreated control. Available N and P were determined periodically. The results showed that the additives did not enhance the available P content of OMF as the inherent P is locked up and not bioavailable. The additives only enhances P available when water soluble P is present in the first place. In addition the additives did not enhance N availability. This short experiment showed that dicarboxylic polymer additives did not enhance the nutrient value of the OMF.

Keywords: Phosphorus, nitrogen, availability, additives, biosolids, DGT, Olsen’s phosphorus, organomineral fertilisers,

 

Introduction Vast quantities of nitrogen and phosphorus fertilisers are used to meet crop demands However up take of nutrients by crops is lower than the amount of added fertiliser due to low efficiency which results in wasted resource. Any excess nitrogen and phosphorus fertilisers results in potential environmental implications such as eutrophication, groundwater pollution, and biodiversity loss (Harrison and Webb 2001). Misselbrook et al. (2000) estimated that 90% of all ammonia emissions in UK are from agriculture. Various products are becoming available commercially which aim to improve fertiliser efficiency by increasing plant available nutrients. Efficient use of such products may benefit yields, economics, and reduce the environmental impact of fertilisers.

Geochemical factors, such as pH and soil ions (precipitation reaction), influence P solubility in soils and hence availability to crops. Sorption (fixation) of P reduces the quantity to the crop. If conditions are favourable (pH, soil moisture, organic matter) residual P provides a reserve for plant P uptake. However residual P may also be lost by leaching (Barrow, 1980) or surface erosion. Sorption is also influenced by Oxyhydroxides of Fe and Al, particularly in highly weathered or acidic soils (Borggaard 1983; Wood et al. 1984) but also in calcareous soils (Carreira et al. 1997). Alkaline soils commonly contain carbonates (Ca, Mg) that can adsorb P causing precipitation as calcium or magnesium phosphates. The phosphate ion is more strongly absorbed than the ion it replaces (Barrow 1999). We utilised a dicarboxylic polymer, which we shall refer to as Product A, to adsorb divalent and trivalent metal cations, thereby reducing the sorption of P and increasing plant availability.

Urea applied to the soil surface is prone to nitrogen losses through both ammonia volatilisation and nitrate leaching after nitrification. Urease enzymes catalyse the rapid hydrolysis of urea to ammonium and bicarbonate ions (Rachhpal Singh and Nye 1986). Ammonia formed by the disassociation of ammonium will volatilise to ammonia gas. Nitrification is the biological oxidation of ammonium (NH4 +) with oxygen by ammonia oxidising bacteria and archaea to nitrites (NO2 -) and subsequently nitrates (NO3 -), which are vulnerable to loss through leaching. Current management practices designed to reduce ammonia volatilisation do not completely eliminate N loss (Freney et al. 1992). A maleic-ilaconic (carboxylic) co-polymer, which we shall refer to as Product B, was used to increase the efficiency of nitrogen fertilisers by inhibiting urease activity (to reduce ammonia volatilisation) and nitrification (reduce nitrate leaching). Product B aims to maintain optimum urea and ammonium ratio to maximise crop yield potential.

Two separate incubation experiments were derived to evaluate the ability of Product A and Product B to improve the performance of an organo-mineral fertiliser (OMF) by increasing available phosphorus and nitrogen respectively within the soil. Detailed characterisation of OMF used in this experiment which is derived from nutrient-enriched biosolids granules is explained by Antille et al (2013). Product A has been demonstrated to have the potential to improve crop yields and reduce the environmental footprint of phosphorus fertilisers (Sanders et al. 2012). Product A was applied at the manufacturers recommended concentration (0.224% w/w) and twice the recommended application (0.5% w/w) to determine whether increasing the quantity on the fertiliser alters available P. Soil Phosphorus was monitored over 196 days (Olsen’s method) and also assessed at the end of the incubation period using Diffusive Gradients in Thin-films (DGT-P) (Zhang et al., 1998) and a resin capsule method (UNIBEST, USA) at 196 days. The rationale is that DGT-P and resin capsule methods more accurately determine bioavailable phosphorus compared to Olsen’s method. The hypothesis for the first experiment involving Product A were: i) Product A reduces soil P sorption and hence increases the availability of OMF derived phosphorus within the soil, ii) increasing the quantity of Product A (% on the fertiliser) improves performance, iii) the method of P analysis selected has implications for the results. In the second experiment, ammonium (NH4-N) and nitrate (NO3-N) concentrations in the soil were observed following application of OMF with Product B incorporated. The rate of NH4-N accumulation will provide an indirect indicator of urease activity, and hence the potential loss of N through volatilisation. The rate of nitrate accumulation acted as an indicator of nitrification, and hence the potential loss of N through leaching. The hypotheses were: i) Product B will reduce urease activity, and hence reduce the quantity of ammonium that accumulates in the soil; ii) the inhibition of nitrification will result in less nitrate accumulation.

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