Project
title / EFFECT OF FARM MANURE ADDITIONS ON SOIL QUALITY AND FERTILITY / DEFRA
project code / SP0501

Department for Environment, Food and Rural AffairsCSG 15

Research and Development

Final Project Report

(Not to be used for LINK projects)

Two hard copies of this form should be returned to:
Research Policy and International Division, Final Reports Unit
DEFRA, Area 301
Cromwell House, Dean Stanley Street, London, SW1P 3JH.
An electronic version should be e-mailed to
Project title / EFFECT OF FARM MANURE ADDITIONS ON SOIL QUALITY AND FERTILITY
DEFRA project code / SP0501
Contractor organisation and location / ADAS Gleadthorpe Research Centre
Meden Vale
Mansfield
Notts. NG20 9PF
Total DEFRA project costs / £ 180,489
Project start date / 01/04/1998 / Project end date / 31/03/2002
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CSG 15 (9/01)1

Project
title / EFFECT OF FARM MANURE ADDITIONS ON SOIL QUALITY AND FERTILITY / DEFRA
project code / SP0501

The objective of this project was to determine, on contrasting soil types, the medium-term effects of repeated farm manure applications on soil quality and fertility, focusing particularly on the turnover of soil organic carbon (SOC) and nitrogen (N), and effects on selected soil physical, chemical and biological properties. Many of the claimed improvements in soil quality and fertility following farm manure additions are based on anecdotal evidence. This project evaluated the effects of manure OC inputs on key soil properties, utilising four existing experimental sites with a history of repeated farm manure applications. At ADAS Bridgets (23% clay) and Harper Adams University College (12% clay), cattle farmyard manure (FYM) and slurry were applied annually from 1990 to 1994 and from 1998 to 2001. At ADAS Terrington (28% clay), pig FYM and slurry were applied annually from 1994 to 2001, and at ADAS Gleadthorpe (6 % clay), 6 rates of broiler litter (0-25 t/ha/yr) were applied annually from 1992 to 2001. Repeated applications of cattle FYM and slurry at Bridgets and Harper Adams supplied a total of c. 25 and 11 t/ha of OC over 7 years, respectively. Total OC loadings from pig FYM and slurry over an equivalent period at Terrington were 5 and 14 t/ha, respectively, while repeated annual applications of 5-25 t/ha broiler litter at Gleadthorpe supplied 13-65 t/ha OC over 9 years. The effects of these organic matter additions on key indicators of soil quality were measured in spring 2001. The main findings are summarised below:

Soil physical properties

  • At Gleadthorpe, repeated broiler litter applications (25 t/ha) over 9 years increased the topsoil available water capacity (AWC) by 23% and porosity by 6 %, and decreased topsoil bulk density by 5% (P<0.05). This increase was equivalent to 10 mm of additional plant available water in the top 30 cm of soil, which for unirrigated potatoes, would result in c. 2.5 t/ha extra potato yield (worth c. £200/ha at current prices). Also, the increased topsoil porosity and decreased bulk density will improve water infiltration rates into the topsoil and soil drainage. The repeated manure applications had no effect on AWC, porosity or bulk density at the other three sites.
  • Topsoil shear strength, structural stability and aggregate size distribution were not affected by the repeated manure applications.

Soil chemical properties

  • Topsoil total nitrogen (N) and extractable phosphorus (P), potassium (K) and magnesium (Mg) increased with broiler litter application rate at Gleadthorpe (P<0.05). At Terrington, pig FYM increased topsoil total N, and both FYM and slurry increased extractable P and K (P<0.05). Cattle FYM and slurry applications at Bridgets and Harper Adams had no effect on topsoil total N, but extractable P (Bridgets only), K and Mg were increased by the manure applications (P<0.05).
  • Total SOC was increased (15%) by the pig FYM applications at Terrington (P<0.05). However, total SOC was unaffected by the manure applications at Bridgets, Harper Adams and Gleadthorpe.
  • The light organic C fraction (LFOC) was increased by 0.8 g/kg (1.5 fold) by the broiler litter applications (P<0.05) at Gleadthorpe, despite no measurable effects on total SOC. This suggests that LFOC measurements may be a more sensitive indicator of changes in soil organic carbon than measurements of total SOC. However, LFOC was unaffected by the manure applications at Bridgets, Harper Adams and Terrington.

Soil biological properties

  • The size and activity of the soil microbial biomass was increased by the repeated broiler litter applications at Gleadthorpe; biomass N almost doubled and respiration rate increased 2-3 fold (P<0.05). Biomass N was also increased by the pig FYM (c. 30%) and slurry (c. 55%) applications at Terrington (P<0.05), and soil respiration rate by the cattle FYM (c. 50%) and slurry (c. 25%) applications at Bridgets (P<0.05). The manure applications had no effect (P>0.05) on microbial biomass C at any of the sites.
  • Potentially mineralisable N (PMN) was increased (P<0.05) by the broiler litter applications at Gleadthorpe (c.2 fold) and the cattle slurry additions at Bridgets (c. 35%), indicating that long-term soil N supply would be increased by manure additions.

Nitrogen mineralisation, crop yields and carbon returns

  • Spring soil mineral N supply was increased by the manure applications at all sites (P<0.05). The increases were greatest following the cattle or pig slurry applications compared with cattle or pig FYM, reflecting the greater proportion of readily available N (ammonium-N) in slurry.
  • Crop N offtake in the absence of fertiliser N was increased by the manure applications, demonstrating the need to account for manure available N supply when formulating fertiliser N policies. However, estimates of net N mineralisation through the construction of a simple N balance were variable, probably due to loss of N during the growing season.
  • Crop yields were largely unaffected by the manure applications, although there were indications at Bridgets and Gleadthorpe in 2001 of higher yields on the manure treatments (where allowance was made using MANNER for manure N supply) compared with the unmanured treatments receiving recommended rates of fertiliser N. The return of C in cereal straw, stubble and roots was on average 3 t/ha/yr where straw was incorporated (Bridgets only) and 0.5-1.5 t/ha/yr where straw was baled and removed.
  • Changes in topsoil total SOC, expressed as a percentage of the difference between manured and unmanured treatments over the duration of the experiments (%/yr), were related to total C inputs (P<0.05). On average, the addition of 1 t/ha/yr of organic C via animal manures and crop residues, increased SOC at a rate of 0.5%/yr and LFOC levels by 1.6%/yr.
  • The CENTURY model predicted SOC increases following the broiler litter applications at Gleadthorpe which were similar to those measured in the field. The model suggested that the repeated application of 25 t/ha broiler litter would continue to steadily increase total SOC, whereas an application of 10 t/ha broiler litter would only maintain SOC, albeit at a higher level than if no manure had been applied (where SOC was predicted to decline).

These results indicate that repeated farm manure applications have important and measurable beneficial effects on the physical, chemical and biological properties of arable topsoils, acting as a valuable soil conditioner and source of plant available nutrients. Indeed, the most sensitive indicators of soil quality and fertility changes were soil chemical (e.g. increases in plant available P, K and Mg supply and total N) and biological (e.g. increases in biomass N, PMN and respiration rates) properties. The only measureable effects on soil physical properties were on the loamy sand textured soil (at ADAS Gleadthorpe) which had received the highest OC loadings (upto 65t/ha). Changes in soil properties, particularly physical characteristics, only develop gradually and long timescales are needed to fully evaluate the contribution of farm manure applications to soil quality and fertility.

Scientific report (maximum 20 sides A4)
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CSG 15 (9/01)1

Project
title / EFFECT OF FARM MANURE ADDITIONS ON SOIL QUALITY AND FERTILITY / DEFRA
project code / SP0501

ROAME B: SCIENTIFIC OBJECTIVES AND PRIMARY MILESTONES ADDRESSED

Objective

  • To determine the medium-term effects of farm manure additions on soil quality and fertility, focusing particularly on the turnover of soil organic carbon and nitrogen.

More specifically, the objectives of the project were:

  • To evaluate the effects of repeated farm manure additions on key indicators of soil quality, viz; structural stability, strength, water holding capacity, microbial biomass C and N, and respiration.
  • To compare the amounts of C and N recycled in manure applications with measured soil C and total N levels.
  • To use the data generated to test current model predictions (eg. CENTURY) of C and N turnover in soils.
  • To determine the effects of manure additions on potentially mineralisable soil N and compare this with measured crop N uptakes.
  • To provide recommendations on the use of farm manures to maintain soil fertility and quality.

1. EXTENT TO WHICH OBJECTIVES HAVE BEEN MET

The work has shown that the addition of organic carbon to topsoils via repeated farm manure applications can raise soil organic matter (SOM) levels. This in turn, had measurable effects on selected soil physical, chemical and biological properties, namely: available water capacity (AWC), porosity, bulk density, biomass N, respiration rate and potentially mineralisable N (PMN). The data collected were used to test CENTURY model predictions of C and N turnover, which were in good agreement with the field measurements at ADAS Gleadthorpe. The results provide valuable information on the effects of farm manure addition on soil quality and fertility, and how manure additions contribute to the sustainable management of soils.

2. BACKGROUND

The sustainability of UK agricultural production is dependent on the long-term maintenance of soil quality and fertility. The Royal Commission on Environmental Pollution report ‘Sustainable Use of Soils (RCEP, 1996) and, more recently, the Draft Soil Protection Strategy for England and Wales (DETR, 2001) both highlight the need to maintain and improve the quality of UK soils. Many authors have sought to define and assess soil quality and fertility, and factors that may affect these properties (Doran & Parkin, 1994; LSQRC, 1996; SSSA, 1996). A number of soil properties have been suggested as indicators (Doran & Parkin, 1994), but perhaps the most fundamental property of a soil is its organic matter content (Sikora & Stott, 1996).

Soil organic matter (SOM) is one of the most important components of a soil, influencing a wide range of physical (e.g. soil structure and water holding capacity), chemical (e.g. cation exchange capacity and nutrient supply) and biological (e.g. nutrient turnover and microbial activity) properties (Carter, 2001; Johnston, 1986). Consequently, it has a central role in the maintenance of soil fertility, often having a beneficial effect on crop yields that cannot be mimicked by inorganic fertilisers alone (Johnston, 1986). In fact, many authors assert that a change in SOM status will lead to an improvement or deterioration in the fertility of agricultural soils per se, although few can provide quantitative evidence of which soil property or function is being affected (Johnston, 1986; Loveland et al., 2001). Arable cropping is generally associated with declining SOM levels due to oxidation following cultivation (Alison, 1973), with substantial decreases in SOM status observed following the ploughing out of grassland (Johnson & Prince, 1994). Between 1980 and 1994, soil organic carbon levels in arable and ley-arable soils in England and Wales decreased by an average of 0.58% (Webb et al., 2001). This not only has implications for the capacity of soils to supply water and nutrients, but their ability to resist erosion.

Approximately 9 million tonnes of OM (c. 5.2 million tonnes of organic carbon-OC) are returned to soils via farm manure additions annually in the UK (Chambers and Smith, 1998). The application of farm manures to agricultural land has been shown to increase SOM levels (Hountin et al., 1997; Persson & Kirchmann, 1994; Van Meirvenne et al., 1996) and to potentially improve soil quality and fertility (Haynes & Naidu, 1998). However, many of the claimed improvements in soil quality and fertility following farm manure additions are based on anecdotal evidence. Our hypothesis was that manure OC inputs would result in changes in ‘key’ soil physical, chemical and biological properties, which we would seek to measure and quantify for the contrasting manure types.

The objective of this project was to determine, on contrasting soil types, the medium-term effects of farm manure additions on soil quality and fertility, focusing particularly on the turnover of soil organic carbon and nitrogen, and effects on selected soil physical, chemical and biological properties. The soil properties measured were those recommended by Doran and Parkin (1994), LSQRC (1996) and SSSA (1996) as ‘key’ indicators of soil quality and fertility.

3. METHODOLOGY

3.1 Experimental sites

The study was undertaken at four experimental sites, on contrasting soil types, with a history of repeated farm manure applications (Table 1).

Table 1. Sites and treatments

Site / ADAS Bridgets / Harper Adams / ADAS Terrington / ADAS Gleadthorpe
Topsoil texture / Silty clay loam
(over chalk) / Sandy loam / Silty clay loam / Loamy sand
Soil series / Andover / Wick / Agney / Cuckney
% Clay / 23 / 12 / 28 / 6
% SOC* / 3.30 / 1.52 / 1.39 / 1.10
Manure type / Cattle FYM & slurry / Cattle FYM & slurry / Pig FYM & slurry / Broiler litter
Date established / 1990 / 1990 / 1994 / 1992
No. of manure additions (to 2000) / 7 / 7 / 7 / 9

*Typical arable topsoil OC level 2.8% (Webb et al., 2001), with c. 96% of arable/ley grassland topsoils containing 6% OC (Loveland et al., 2001)

At ADAS Bridgets and Harper Adams Agricultural College, cattle FYM and slurry were applied annually from 1991 to 1994, supplying c.300 kg/ha total N (c.200 kg/ha N for FYM at Harper Adams from 1992), with an additional 150 kg/ha inorganic fertiliser N applied during the growing season. Both sites were long-term grass leys from 1987 to 1997, whereafter an arable rotation (cereals) was introduced. The manure treatments were re-introduced in autumn 1998 and continued for 3 years, with plots which had received only inorganic fertiliser N for the duration of the experiment (at recommended rates) used as a control. From autumn 1998, manures were applied at a rate of c.250 kg/ha total N, with supplementary inorganic N applied at ‘optimum’ rates according to MANNER (Chambers et al., 1999) predictions of manure crop N availability (except in harvest year 1998/99 when no additional inorganic N was applied). There were 3 replicates of each treatment arranged in a randomised block design (plot size: 9 x 8 m at Bridgets, 6 x 10 m at Harper Adams).

At ADAS Gleadthorpe, 6 rates of broiler litter ranging from 0 to 25 t/ha were applied annually to a typical sandland crop rotation (cereals & sugar beet) from 1992 (Shepherd and Bhogal, 1998). Supplementary inorganic fertiliser N was applied at ‘optimum’ rates according to MANNER predictions from harvest 1999 (no inorganic fertiliser N was applied between 1992 and 1998). There were 3 replicates of each treatment arranged in a randomised block design (plot size: 5 x 15m).

At ADAS Terrington, pig FYM and slurry were applied annually to a typical siltland crop rotation (potatoes, cereals and sugar beet) from 1994 at a target N rate of c.175 kg/ha (210 kg/ha since 1998). Supplementary inorganic fertiliser N was applied according to MANNER predictions of crop N availability from the start of the experiment, with plots which had received only inorganic fertiliser N for the duration of the experiment (at recommended rates) used as a control. There were 5 replicates of each treatment arranged in a randomised block design (plot size: 300m2).

3.2 Site characterisation

Topsoil samples (0-15 cm) were taken from each plot in autumn 1998 prior to the application of manures in this phase of the work. Subsamples were analysed for pH, extractable phosphorus (P), potassium (K) and magnesium (Mg), laboratory density, organic carbon (wet chemistry) and total N, according to standard analytical techniques (Anon., 1986). An additional subsample was also analysed for light fraction organic carbon (free and protected) according to the procedure of Gregorich et al. (1997), where air-dried soil was shaken with sodium iodide (density: 1.8 g/cm3) and centrifuged to remove the ‘free’ light fraction (trapped between aggregates). The residual soil was then re-suspended in sodium iodide, sonified and centrifuged to remove the ‘protected’ light fraction (within aggregates). The C content of both fractions was then determined by standard wet chemistry analysis. Particle size distribution was determined on a bulked subsample from each block (Anon., 1986).

3.3 Manure nutrient and organic matter loadings

The dry matter (DM) and total N content of each manure was determined on triplicate samples prior to land spreading, in order to calculate target application rates. A further sample was taken from each plot at landspreading and analysed for DM, total N, ammonium-N (NH4-N), total P, K and Mg, and organic carbon (OC), according to standard analytical techniques (Anon., 1986). The broiler litter was also analysed for uric-acid N.

3.4 Indicators of soil quality

A number of soil physical, chemical and biological properties, as recommended by Doran and Parkin (1994), LSQRC (1996) and SSSA (1996), were measured at each site in spring 2001, prior to inorganic N fertiliser applications.

3.4.1 Soil physical properties

3.4.1.1 Soil moisture retention, available water capacity and bulk density

Composite topsoil samples (0-15 cm) were taken from each plot, air dried and sieved (<2 mm) prior to the determination of moisture retention on triplicate subsamples at 1, 2, 5 and 15 bars (permanent wilting point) on a porous pressure plate (Anon., 1982). Three undisturbed soil cores (0-5 cm depth) were taken from each plot for the determination of moisture retention at 0.05 bar (field capacity), using a sand tension table (Anon., 1982). These latter cores were also used to determine soil dry bulk density (Anon., 1982). The total available water capacity (AWC) and easily available water capacity (EAWC) were calculated from the moisture retention data:

AWC = v(0.05) - v(15bar)(1)

EAWC = v(0.05) - v(2bar)(2)

where v is the volumetric moisture content at a specified pressure step.

Total porosity and air capacity at field capacity were also calculated from the bulk density and moisture retention data, respectively:

Total porosity (Tp) = 1-(Bd/Pd)*100(3)

where Bd is the soil bulk density and Pd is the estimated soil particle density (2.65 g/cm3).

Air capacity = Tp-v(0.05)(4)