Set-aside has produced some significant changes in the soil nutrient status. Within just five years, nitrate levels have been reduced substantially, phosphates have accumulated, as has the organic matter content of the soil. On both sown and naturally regenerated fields a grassland sward has developed, dominated by perennial species.
The set-aside has improved the wildlife habitats on the farms. However, the data collected does not show any strong evidence that the soil nutrient status effects the characteristics of the developed vegetation. Although there are trends, these are not statistically proven. It is more likely that other factors play an important part in the vegetation development on set-aside, such as competition, the soil seed bank and dispersal patterns.
The management regime certainly affects the spread of weeds, as seen on Woodham Lodge Farm where the fields cut two to three times a year aid the spread of Cirsium arvense. This was a surprising result, as the object of frequent cutting is to reduce the weed population of the field. Field margins are still more diverse than core areas, suggesting that the length of time for a grassland ecosystem to fully mature exceeds the duration of a non-rotational set-aside period.
The sowing of a grass cover only slightly reduced the species diversity and species richness of a field, which suggests that sowing a grass cover to prevent the spread of weeds will not necessarily reduce the ecological value of a field. At Birkett Hall, Lolium perenne was sown, and it was evident that the faunal species, especially on the field margins, was very diverse.
The sowing of a grass cover will not necessarily prevent the spread of Elymus repens as suggested by Talling and Godwin (1994). On Cowbridge at Birkett Hall, Elymus repens managed to reach quite high levels in the sward.
In the naturally regenerated fields, the species diversity and species richness may have been higher, but many of the species present are not considered to be desirable by conservationists. For example, on South Field, the most diverse field studied, Arrhenatherum elatius, Cirsium arvense, Convulvulus arvensis, Elymus repens and Senecio jacobaea all occurred in relatively high numbers. All of these are considered to be problem weeds, which can often restrict colonisation by more desirable species.
Although the conservationist encourages the development of a diverse grassland sward, farmers are not compensated for reduced yields that may occur as a result of the residue of problem weeds in the soil, and the associated diseases with them. If Elymus repens is not eradicated sufficiently it could cause problems to the future yields on Reeds Farm.
The high organic matter and ammonium levels on set-aside will provide a large source of nitrates once the sward is ploughed out at the end of the set-aside period. Nitrate leaching may increase dramatically once mineralisation increases in the more moist conditions in the autumn after set-aside. Although nitrate leaching is reduced during the set-aside period, the sudden increase caused by farmers throughout the country ploughing up grass leys may cause serious problems.