Investment in precision farming technologies can be expensive and is not expected to be costeffective for every farm. Previous research and farm experience has shown that the amount of soil variability across a farm and within a field is of key importance for determining potential benefits from the adoption of precision farming. The research reported here evaluates the analysis of yield map sequences and electromagnetic induction (EMI) soil sensing as potentially cost-effective methods for identifying and mapping soildetermined ''management zones'' within fields. Both methods are shown to provide useful information for the provisional delineation of soil type boundaries and crop management zones, though soil examination in the field is still necessary to confirm specific soil characteristics.

SU MMARYThe efficiency of poultry litter nitrogen (N) utilization was studied in seven field experiments in eastern England during harvest years 1991 to 1994. Poultry litter was applied at different application rates in winter or spring, prior to sugar beet or potatoes. The mean manure N efficiency based on crop yields was 33 % (range 25-43 %) for sugar beet and 36 % (range 13-66 %) for potatoes. For potatoes, the manure N efficiency was greater from spring (mean 43 %) than from winter application timings (mean 30 %). The manure readily available N applied (i.e. ammonium-N+uric acid-N) and fertilizer N replacement values were well related (P<0 . 05) for both sugar beet and potatoes. Similarly, there was a good relationship (P<0 . 001) between the amounts of readily available N applied in the poultry litter dressings and measured elevations in spring soil mineral N supply. Where the poultry litter dressings supplied >600 kg/ha total N to sugar beet, root sugar concentrations were depressed (P<0 . 05) and amino-N concentrations increased (P<0 . 01). The soil mineral N supply following harvest of the sugar beet and potato crops was also increased where application rates supplied >600 kg/ha total N. Yield increases were also recorded in cereal crops grown the following season, but only where high rates of manure N (>600 kg/ha) had been applied. The current work has shown that the fertilizer N replacement value of poultry litter can be predicted based on the amounts of total and readily available N applied, providing guidance to farmers on appropriate reductions in inorganic fertilizer N applications to make allowance for poultry litter N supply.

Lime was applied in summer 1981 at rates up to 20 t ha-I prior to reseeding an unimproved peaty hill soil. A marked pH gradient with depth developed showing that 3 years after application lime had very little effect below a depth of 5 cm. Soil pH values for 0-7.5 cm samples were: nil lime -4.2; 1 t ha-' -pH 4.3; 2 t ha-' -pH 4.6; 4 t ha-' -pH 5.0; 6 t ha-' -pH 5.6. In the 0-2.5 cm layer pH values were much higher.In all years at least 80% of maximum yield was achieved from an initial application of 2 t ha-' lime. Botanical analysis showed that maximum persistence of sown species, perennial ryegrass and timothy, occurred from 2 t ha-' lime; 6 t ha-' lime was necessary for maximum persistence ofclover. Lime application had only small effects on the mineral composition of the herbage.

Replicated field trials were carried out at 23 sites in England and Wales between 1986 and 1988 examining the response of first-cut grass silage to different levels of early winter-or springapplied potash on dry matter (DM) yield and herbage cotnposition. Residual effects at the second cut were measured. In 1986/87 and 1987/88, the effect of additional potash at second cut was examined.Yield responses at first cut were measured at 13 sites {/'<005). Optimum potash rates (Kopt) ranged from 0 to over 160 kg ha"', the highest rate tested. The mean rate of response up to K-opt at responsive sites was 10-4 kg DM (kg KiO)" '. Herbage K^ increased with potash rate regardless of the requirement for maximum economic yield. Increasing potash rate reduced herbage Mg% by less than 0-020% which was less than the effect of soil Mg level. An increase of 50 mg 1"' in soil Mg (0-7-5 cm depth) increased herbage Mg by 0-034%. Early winter application of potash was generally as effective as spring application though it gave slightly lower herbage K and higher herbage Mg concentrations. Offtake of potash (K2O) at first cut was on average 12 kg ha"' more from spring than from early winter application.