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Visual soil evaluation methods can provide a quick and easy, semi-quantitative approach to assessing the overall soil structural condition of a block of soil in three dimensions. To express this amount of information through other measures of soil physical condition (e.g. penetration resistance, bulk density or shear strength) requires a number of measurements at various depths and can be costly and time consuming. There is therefore a need to develop simple field methods to assess and monitor soil quality. In a survey of grassland soil compaction in England and Wales, soil visual evaluation methods were used alongside more widely accepted physical measurements of soil compaction (e.g. bulk density – BD and penetration resistance). Soil structural condition was investigated in 300 fields located on 150 farms, with one ‘mainly grazed’ field and one ‘mainly cut’ field selected on each farm. The visual soil evaluation methods were the visual soil assessment (VSA) method from New Zealand and the Peerlkamp (soil structure – ‘St’) method. Based on the Landcare VSA ranking score, 8% of the grassland fields were in poor condition (95% confidence interval = ±3), 54% (±6) in moderate condition and 38% (±6) were in good condition. Based on the Peerlkamp ‘St’ score, 12% (±4) of fields were in poor condition (‘St’ score 7.0). Notably, the soil visual evaluations using the VSA ranking score and ‘St’ score were well related ( P r 2 = 66%). At 30 field sites selected for more detailed investigation, there was an inverse relationship between ‘St’ scores and mid topsoil BD ( P r 2 = 25%), indicating that the measurement of BD in the middle of the topsoil provided an indication of soil structural condition, as determined by visual soil evaluation. Also, for the 300 grassland fields, there was a positive relationship ( P r 2 = 26%) and VSA score ( r 2 = 19%). The visual evaluation scores increased with increasing penetration resistance, indicating that better soil structure (as assessed visually) was associated with greater penetration resistance. This was contrary to the expectation that soils with better structure would be less dense than poorly structured soils and therefore would have lower penetration resistance values. The use of multiple predictor models showed that the two most important factors ( P = 0.02) influencing the VSA ranking score were (in order of importance): (i) soil organic matter content (positive relationship); (ii) soil sand content (positive relationship).

Results from the UK were reviewed to quantify the impact on climate change mitigation of soil organic carbon (SOC) stocks as a result of (1) a change from conventional to less intensive tillage and (2) addition of organic materials including farm manures, digested biosolids, cereal straw, green manure and paper crumble. The average annual increase in SOC deriving from reduced tillage was 310 kg C ± 180 kg C ha−1 yr−1. Even this accumulation of C is unlikely to be achieved in the UK and northwest Europe because farmers practice rotational tillage. N2O emissions may increase under reduced tillage, counteracting increases in SOC. Addition of biosolids increased SOC (in kg C ha−1 yr−1 t−1 dry solids added) by on average 60 ± 20 (farm manures), 180 ± 24 (digested biosolids), 50 ± 15 (cereal straw), 60 ± 10 (green compost) and an estimated 60 (paper crumble). SOC accumulation declines in long-term experiments (>50 yr) with farm manure applications as a new equilibrium is approached. Biosolids are typically already applied to soil, so increases in SOC cannot be regarded as mitigation. Large increases in SOC were deduced for paper crumble (>6 t C ha−1 yr−1) but outweighed by N2O emissions deriving from additional fertiliser. Compost offers genuine potential for mitigation because application replaces disposal to landfill; it also decreases N2O emission.&nbsp

The incorporation of chopped wheat (Triticum aestivum L.) straw into soil by tine cultivation (non-soil inversion) or ploughing was compared with burning straw followed by tine cultivation at six sites in England over a period of 11 years. Three sites had clay soils and three silty clay loam soils. Effects of straw management on weed incidence, take-all (Gaeumannomyces graminis var. tritici) infection and grain yield of following wheat crops and occasional break crops were studied. Soil mineral nitrogen and organic matter contents were measured at the end of the study. Incorporating straw by tines rather than burning reduced mean yield at all but one site. The yield reduction from tine incorporation ranged from 5 to 8% on clay soils and 3–18% on silty clay loam soils. Ploughing straw into soil only had an occasional adverse effect on yield of following crops. Much of the yield penalty associated with tine incorporation of straw was attributed to weed competition by Bromus spp. Difficulties in preparing a good seedbed, resulting in variable plant emergence, was the other main cause of lower yields with tine incorporation and in situations where plough incorporation reduced yields compared to burning straw. Method of straw disposal had no consistent effect on take-all infection. The effects of straw incorporation on soil mineral nitrogen and organic matter contents were small and inconsistent. There was no consistent effect of straw management practice on yield response to additional autumn application of nitrogen fertiliser. These results demonstrate that on those soils where ploughing is preferred, it is a suitable option for disposing of straw. Where non-ploughing methods have traditionally been used after straw burning they can still be employed with success, but occasional ploughing or planting of suitable break crops may be required to control grass weeds.

SUMMARY Recent ADAS field experiments have evaluated the use of soil mineral nitrogen (SMN) measurements as an estimate of available soil nitrogen supply on organic/peaty soils, and following ploughed out grassland or organic manure applications. The contribution of soil nitrogen supply to winter wheat and sugar beet crops growing on organic/peaty soils of the Black Fens in East Anglia was studied between March and September 1988. Nitrogen mineralisation during spring / summer meant that late autumn / early spring SMN measurements were not a good measure of soil nitrogen supply. Estimates of net mineralisation potential are required to predict soil nitrogen supply. Field experiments in 1987 (23 sites) and 1988 (14 sites) studied the response of winter wheat to increasing additions of fertiliser nitrogen in potentially large nitrogen residue situations, following the ploughing out of grassland or organic manure applications. No response to fertiliser nitrogen was recorded where SMN levels were above 250-300 kg/ha N, measured in autumn. Below these amounts of SMN, responses to fertiliser nitrogen increased with decreasing SMN supply but were somewhat variable between sites with similar amounts. Extension of the information from these response experiments has enabled ADAS to offer a SMN advisory service, which has provided farmers with the confidence to make full allowance for soil nitrogen supply in large and variable nitrogen residue situations.

In the region of 90 million tonnes of farm manures, supplying 450,000 tonnes of nitrogen (N) and 119,000 tonnes of phosphorus (P) are applied to agricultural land in the UK each year. These applications are a valuable source of plant nutrients but they are also recognised as the largest source of controllable pollution in present day farming systems. Over 70% of nitrate entering water systems is estimated to originate from agricultural land and the transfer of particulate and soluble P from agricultural land has been estimated to contribute 40-50% of P loads in watercourses. Ammonia (NH3) emissions from UK agriculture contribute c.80% of total UK emissions with losses following the land spreading of farm manures responsible for c.96,000 tonnes of NH3 each year.