4.1. VEGETATION ASSESSMENT
The vegetation assessment was carried out on nine fields. At Birkett Hall, Springfield, Cowbridge, Chaplefield and Saunders were studied. For White House Farm, Front Field and Back Field were studied. For Reeds Farm, both areas of set-aside were studied (North Field and South Field), and for Woodham Lodge Farm, there was just one large extensive field to examine.
4.1.1. DATA COLLECTION
The vegetation assessment was carried out using a 0.25 m2 quadrat. A systematic method was used by placing the quadrat down every twenty paces along a prescribed transect. Sampling started on the border of a field following a transect through the centre of the field to the opposite side. Two to three transects were taken depending on field size.
For each quadrat, all monocotyledonous and dicotyledonous species were listed and the percentage cover of each species estimated. Also, for Elymus repens (couch), Phleum pratense (timothy), Avena fatua (common wild oat),Rumex obtusifolius (broad-leaved dock), Senecio jacobaea (ragwort), Cirsium arvense (creeping thistle) andCirsium vulgare (spear thistle) the number of individual plants present was recorded.
In total 455 quadrat cells were recorded for the nine fields, averaging 50 quadrats per field. Hubbard (1992) and Rose (1991) were used to identify species.
4.1.2. VEGETATION ANALYSIS
To analyse the quadrat data a series of indices were used. The quadrat data has been interpreted in a number ways to provide comparisons of the vegetation cover for the nine fields surveyed. The methods used by Poulton and Swash (1992) were followed to examine the vegetation characteristics, by determining species richness, a diversity index and a dominance index, as described below:
Species richness Total number of species recorded per field
Diversity index Average number of species per quadrat in field
Dominance index Number of species in field achieving dominance in at least one quadrat
In addition to the indices used by Poulton and Swash (1992), three more indices have been developed to analyse the data in a more specific manner:
Frequency The frequency of occurrence of each species recorded as a percentage of all quadrat cells for the field.
Abundance The mean number of plants present per 0.25 m2 for couch, broad-leaved dock, ragwort, creeping thistle and spear thistle.
Cover The mean percentage cover per 0.25 m2for broad-leaved dock, ragwort, creeping thistle, spear thistle, couch, slender foxtail, barren brome and false oat grass.
Under the Weeds Act 1959 the occupier of land is obliged to control problem weeds i.e. Cirsium arvense, Cirsium vulgare, Rumex crispus, Rumex obtusifolius and Senecio jacobaea (Willoughby, 1996). These weeds are controlled by herbicides on arable land, but during set-aside have the opportunity to spread. For analysis of cover,Cirsium arvense, C. Vulgare, Rumex obtusifolius and Senecio jacobaea are classed as problem weeds.Arrhenatherum elatius, Bromus sterilis, Elymus repens and Alopercurus myosuroids are classed as problem grasses. For the analysis of abundance, Elymus repens is examined in addition to the problem weeds.
Finally, a comparison of the species composition of the field margins and field cores for Front Field, Back Field, Spring Field and South Field was carried out.
4.2. SOIL SAMPLING
4.2.1. SOIL COLLECTION
Soil samples were collected from the top 7.5 cm using a 2 cm soil corer. Samples were taken along transects within the field following the vegetation assessment. At least 25 samples were taken for each field, following the ADAS guidelines for the sampling of soils under long leys (MAFF, 1979). The bulked sample was placed into a polythene snap-bag. The samples were then air-dried in aluminium trays and sieved through a 2 mm mesh. The less than 2 mm fraction was rebagged and stored until needed for laboratory analysis.
4.2.2. SOIL ANALYSIS
The soils were analysed for organic matter content (by loss on ignition), pH, Cl, NH4, NO3, NO2, PO4, SO4, Si, Ca, Mg, Na, K and particle size. These cover the major nutrients required by plants (Russell, 1961). Soil water extracts were analysed on the auto-analyser for Cl, NH4, NO3, NO2, PO4 and Si. SO4 was analysed using a Palintest Photometer. A digest was carried out to measure the total Ca, Mg, Na and K in the soil.
Ca, Mg, Na and K were measured on the Atomic Absorption Spectrophotometer (Varian Model 1475).
For a full laboratory methodology see Foster (1986).
Soluble and exchangeable ions
15 g soil and 150 ml of dionised water was placed into a stoppered polythene bottle and placed on a flask shaker for 15 minutes. The bottle was then transferred to a centrifuge for 15 minutes at 3000 rpm. The liquid was decanted and then filtered through glass fibre paper using Buchner apparatus, and bottled for analysis.
Digest for total cation content
Approximately 0.6 g of sample was digested in a mixture of Perchloric acid (2 ml), Nitric acid (10 ml) and Sulphuric acid (2 ml). Once digested, the samples were diluted with H2O and re-heated. Once cooled, the solution was filtered using Buchner apparatuus and made up to 100 ml volume with H2O and 3 ml of Lanthanum chloride.
The pH was tested immediately after filtering using a Corning pH Meter (model 12).
Particle size analysis
Particle size analysis was carried out using a Malvern Laser. This was done as the particle size distribution of a soil can affect the competitive abilities of some plant species.
Low temperature loss on ignition
To determine the organic matter content of the soil, a sample was taken and oven-dried. Approximately 10 g soil was weighed into a crucible and placed into a muffle furnace for 12 hours at 450°C. The reduction in mass was recorded, and percentage organic matter determined using the equation:
Loss on ignition (%) = (MD – K) – (MF – K) ´ 100
MD – K
MD – oven dry soil
K – crucible mass
MF – crucible and soil mass after ignition
The auto-analyser was used to determine phosphates by colorimetry. Orthophosphate reacts with molybdate in acid solution to form phosphomolybdic acid. This is then reduced by ascorbic acid to an intensely coloured ‘molybdenum blue’ complex. The greater the level of phosphate in the soil water solution, the greater the intensity of the reaction. This is measured on an automated analyser at 660 hm.
Total Oxidised Nitrogen – Nitrates and Nitrites (NO3 and NO2)
The method for total oxidised nitrogen is based on the reduction of nitrate to nitrite by hydrazine-copper reagent and determination of the total nitrite content by the formation of an azo dye measured colorimetrically at 520 hm. To determine the nitrate concentration alone the determination is repeated with dionised water replacing the reducing reagent to give only the nitrite concentration. The nitrate concentration is the difference between the nitrite concentration and the total oxidised nitrogen concentration.
Chloride reacts with mercuric thiocyanate forming mercuric chloride which in turn reacts with ferric ions to give red ferric thiocyanate. This is measured using the auto- analyser at 480 hm.
Phenol and dichloroisocyanurate decompose in alkaline solution releasing hypochloric ions. Nitroprusside is added as a catalyst. The absorbance of the substituted indophenol is measured at 650 hm.
Silicates in solution react with molybdate to form a silicomolbdate complex. This complex is reduced to “molybdenum blue” which can be measured at 660 hm. Interference by phosphate is prevented by the addition of oxalic or tartaric acid.
Measured using a Palintest Photometer 5000. 10 ml sample used as instructed in the Palintest manual.
4.3. DATA ANALYSIS
To compare the chemical characteristics of set-aside with arable soils, boxplots have been produced using an SPSS Spreadsheet. To test the significance of the difference between set-aside and arable soils, a student T-test was carried out using an Excel spreadsheet. Full details of the boxplots and results for the T-test can be found in the appendix.
To test the specific hypotheses, a mean value for the soil data of each field was determined to compare with the vegetation data. Total N was determined using the equation:
Total N (mg kg-1) = NO3 (mg kg-1) * 0.2259 + NO2 (mg kg-1) * 0.3045 + NH4 (mg kg-1) * 0.7762
To test for relationships between soil data and vegetation data, the Student T-test was carried out.
A full COSHH assessment was carried out for the laboratory work. COSHH sheets can be found in the appendix.
The results obtained from this methodology will be used to test the following hypotheses. Hypotheses 1 – 8 are specific hypotheses which will be tested in Chapter 6. Hypotheses 9 and 10 are general speculative hypotheses which will be covered in Chapter 7.
1. High levels of N, P, and K result in a greater abundance of thistles, docks and ragworts.
2. Greater species richness is experienced on field margins.
3. Sowing a grass cover produces a lower species richness and species diversity after five years than natural regeneration.
4. Sowing a grass cover restricts the spread of weeds.
5. Greater species diversity and species richness is found in areas of lower nutrient supply.
6. Sowing clover results in higher nitrogen levels in the soil.
7. Heavily grazed fields have a higher nitrogen content than lightly grazed fields.
8. The annual cutting of set-aside promotes the spread of problem weeds.
9. Land under a 5 year non-rotational set-aside scheme is beneficial to the nutrient status of the soil, and therefore the future crop.
10. Land set-aside for five years could be detrimental to the environment with respect to increased nitrate leaching after the ploughing of the cover crop.