Berry, P. M., Sylvester-Bradley, R., Philipps, L., Hatch, D. J., Cuttle, S. P., Rayns, F. W., Gosling, P. (2002). Is the productivity of organic farms restricted by the supply of available nitrogen? Soil Use and Management, 18, (Suppl.1), 248-255. Special issue - Soil fertility in organically managed soils Sponsorship: DEFRAThis paper reviews information from the literature and case studies to investigate whether productivity in organic systems is restricted by the supply of available N during the major phases of crop growth. Organic systems have the potential to supply adequate amounts of available N to meet crop demand through the incorporation of leys, N rich cash crop residues and uncomposted manures. However, this is seldom achieved because leys are only incorporated once every few years and organically produced crop residues and manures tend to have low N contents and slow mineralization rates. N availability could be improved by delaying ley incorporation until spring, applying uncomposted manures at the start of spring growth, transferring some manure applications from the ley phase to arable crops, preventing cover crops from reaching a wide C:N ratio and better matching crop type with the dynamics of N availability.Peer reviewe
Abstract. Nitrogen (N), phosphorus (P) and potassium (K) budgets were calculated 1 for 9 organic farms in the UK. The farms were on sandy loam, silty clay loam and 2 silty loam over chalk with stockless farming systems and cattle, pig and poultry 3 enterprises with a significant proportion of arable cropping. A soil surface nutrient 4 budget was calculated for the target rotation on each farm using information about 5 field management and measurements of the soil, crops and manure. Losses of N 6 through leaching and volatilization were calculated independently using the NITCAT 7 and MANNER models.
A technique for measuring net rates of mineralization under field conditions is described. Soil cores were incubated in the field in sealed containers with acetylene to inhibit nitrification and thereby minimize losses of N through denitrification. Mineralization was estimated as the difference between the mineral N content after a 14-d incubation and that determined from soil samples taken at the start of incubation. Mineralization in the spring and summer in unfertilized plots in the field amounted to 90 and 70 kg N ha -t in S.E. England under grass and grass/clover swards, respectively, and 40 kg N ha -~ under a grass sward in S.W. England. Daily rates of mineralization ranged from 0.02 to 1.90 kg N ha-l, with peak values related to re-wetting of the soil after dry weather. Laboratory incubation of soil showed that neither the low concentration of acetylene (2% v/v) adopted for field incubation, nor the accumulation of mineral N during incubation was likely to affect the total measurement, but that frequent and regular soil sampling was necessary to minimize the effects of changes in soil water content. Estimates for mineralization over the whole growing season (180 d) were obtained for two years from extrapolation of the early season field measurements and were, on average, 50% higher than predictions based on a chemical extraction index of potentially mineralizable N.
Abstract. This paper compares nitrate leaching losses from organic farms, which depended on legumes for their nitrogen inputs (66 site years) with those from conventional farms using fertilizers under similar cropping and climatic conditions (188 site years). The conventional farms were within Nitrate Sensitive Areas in England, but sites following special practices associated with that scheme were excluded. Nitrate losses during the organic ley phase (including the winter of ploughing out) were similar (45 kg N ha–1) to those from conventional long‐term grass receiving fertilizer N inputs of less than 200 kg N ha–1 (44 kg N ha–1) and from the grass phase of conventional ley‐arable rotations (50 kg N ha–1). Losses from conventional grass receiving higher N inputs were greater than from organic or less intensive grass. Nitrate losses following arable crops averaged 47 and 58 kg N ha–1 for the organic and conventional systems respectively, with part of the difference being due to the greater proportion of non‐cereal break crops in the latter. Thus under similar cropping, losses from organic systems are similar to or slightly smaller than those from conventional farms following best practice.
Abstract. Nitrogen (N), phosphorus (P) and potassium (K) budgets were calculated 1 for 9 organic farms in the UK. The farms were on sandy loam, silty clay loam and 2 silty loam over chalk with stockless farming systems and cattle, pig and poultry 3 enterprises with a significant proportion of arable cropping. A soil surface nutrient 4 budget was calculated for the target rotation on each farm using information about 5 field management and measurements of the soil, crops and manure. Losses of N 6 through leaching and volatilization were calculated independently using the NITCAT 7 and MANNER models.
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