Between 1989 and 1999, 33 experiments tested the effects of potassium (K) fertilizer on the yield and quality of potatoes. The experiments were done on a range of soil types and used varieties and management conditions common to modern commercial production. The average yield in these experiments was 48 t/ha. Nearly half of the experiments were done on soils that had exchangeable K values < 120 mg/l (MAFF Indices 0–1) but use of K fertilizer resulted in statistically significant increases in fresh weight yield in only seven experiments. Generally, soil exchangeable K was a poor predictor of the probability of a yield response. Potassium fertilizer caused an increase in dry weight yield in only four experiments and these experiments were characterized by the absence of irrigation, soils with small amounts of exchangeable K and use of determinate varieties. Re-examination of published data supported the findings in the current work: potatoes are not particularly responsive to K fertilizer and the optimal K application rate is rarely > 170–210 kg K/ha. When applied at the optimal rate, the effects of K fertilizer on tuber dry matter concentration were nonsignificant. Exceeding the optimal K application rate caused occasional reductions in tuber dry matter concentrations particularly if potassium chloride (KCl) was used. In the two experiments where it was tested, application rate and form of K had no effect on crisp fry-colour. The effect of K fertilizer on tuber K concentration was measured in 21 experiments and on average each tonne of fresh weight yield was associated with 4·2 kg K. The range in values was large, 2·8–5·7 and related to soil exchangeable K.For fertilizer recommendations based solely on the probability of a significant yield response to K fertilizer it is suggested that no more than 210 kg K/ha be applied even on soils with < 120 mg exchangeable K/l. For fertilizer recommendations based on crop K removal, an uptake value of 4·8 kg K/t fresh weight (FW), as has been suggested, would be adequate, although errors in the estimation of yield may lead to over or under application of K. Since there was little evidence to support fertilizer policies that apply more K than is removed by the crop a fertilizer recommendation system based primarily on the probability of a yield response would be more than sufficient.

The effects of different rates of N fertilizer (0-180 kg N/ha) were tested on the growth, yield and processing quality of sugarbeet in 34 field experiments in England between 1986 and 1988. The experiments were performed using soil types, locations and management systems that were representative of the commercial beet crop in the UK. The responses obtained showed that current recommendations for N fertilizer use are broadly correct, but large differences occurred on some soil types, in some years, between the recommended amounts and the experimentally determined optima for yield. The divergence was largest when organic manures had been applied in the autumn before the beet crop. Calculations using a simple nitrate-leaching model showed that much of the N in the manures was likely to be leached, the extent of leaching being much less if the manure application was delayed until spring. In these circumstances, spring measurement of inorganic mineral N in the soil could improve fertilizer recommendations. In situations where higher than optimum rates of fertilizer N were used, the extra N had little effect on yield. Increasing the rate from 0 to 180 kg N/ha increased the amount of nitrate left in the soil at harvest by only 8 kg N/ha. The amount of inorganic N released into the soil from crop residues at harvest increased by 50 kg N/ha with N application rate, and the fate of this N has not been established.

Between 1989 and 1993, 17 experiments tested the effect of cover crop species, sowing date and destruction date on cover crop dry matter (DM) yield, N uptake and on soil mineral nitrogen (SMN) content. All the experiments were carried out in Suffolk, Norfolk, Lincolnshire and Yorkshire on sandy-loam textured soils after crops of cereals or oilseed rape had been harvested. The largest DM yields were obtained with early sowings and averaged 1·6 t/ha. Cover crop N uptake was less dependent upon sowing date and averaged 35 kg N/ha. The average reduction in SMN was from 46 to 32 kg N/ha. Differences between cover crop species were small when compared with season/site variations.Cereal cover crop DM yields were closely related to the thermal time accumulated from the first significant rainfall after sowing, whilst the yields of non-cereal cover crops were more affected by the moisture content of the soil at sowing. The amount of SMN in the soil at sowing had little or no effect on cover crop yield. The yields of cereal cover crops were much more predictable than those of non-cereal cover crops. Water usage by cover crops was estimated to be 20 mm/t DM and large cover crops delayed the onset of leaching and reduced the amount of water leached. However, even in dry autumns and winters, soils are likely to reach field capacity before the following beet crop is sown. Due to their large C[ratio ]N ratio (20[ratio ]1) little N would be mineralized after cover crop destruction. Cover crops comprising volunteer cereals and weeds often performed as well as the other cover crops and in most cases will be the most cost-effective cover crops.

Between 1995 and 1999, eight response experiments tested the effects of magnesium (Mg) fertilizers on the yield of potato crops grown in East Anglia, the Midlands, the West and Southwest of England. In addition, a further six experiments tested the effects of varying nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca) supply on the tuber concentrations and uptake of Mg by potato crops. The experiments were done on soils that contained varying amounts of exchangeable Mg and K but were still typical of soils used for potato production.In the eight response experiments, use of Mg fertilizer had no effect on total tuber fresh weight yield even though yields were often much larger than the national average yield. Increasing the N supply to the crop was often associated with an increase in the concentration of Mg in leaves and stems. This may have been due to N facilitating Mg uptake or a consequence of N delaying canopy senescence and, thus, delaying the translocation of Mg from haulm to tubers. Compared with the effects of N, varying the Mg and K supply to the crop had small and inconsistent effects on crop Mg uptake. Since the experiments also showed that Ca supply and soil K[ratio ]Mg ratio had no effect on crop yield and erratic effects on tissue Mg concentration, fertilizer recommendation systems based on ratios of nutrients in the soil cannot be endorsed. When these current experiments and older, published experiments are taken into account there is little justification for applying Mg fertilizer to soils with Mg Indices > 0 and on soils with Mg Index 0 an application of c. 50 kg Mg/ha would be sufficient.

S U M M A R YIncorporation of large amounts of straw (8-15 t/ha dry matter) into the soil had no effect on the incidence of soil pests and diseases or sugarbeet seedling population densities in experiments performed over three seasons (1984/85 to 1986/87) in Suffolk. Straw incorporation had no effect on sugar yield at the recommended rate of nitrogen fertilizer application, but the sugar yield and nitrogen uptake were reduced in one year by the incorporation of straw when the rate of applied nitrogen was low. It is probable that incorporating straw reduced the amount of nitrogen leached over the winter; however, the longer-term implications of straw incorporation remain to be assessed.