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New organic fertilizers based on waste products are continually being introduced in agriculture. Their nitrogen (N) fertilizer value of their total N and mineral N content varies widely, creating a demand for standardized laboratory methods. This study evaluated some potential methods for estimating the N fertilizer value of different kinds of organic fertilizers. The methods were evaluated against the N fertilizer value obtained from a ryegrass pot experiment. Fifteen fertilizers were tested, including different kinds of manure, powders from meat, bone, blood and feathers, rapeseed cake, lucerne pellets, sewage sludge, biogas residue, vinasse and mussel compost. Mineral fertilizer equivalents (MFE) were calculated as the fraction of total N (MFE) or organic N (MFEorg) out of total N that has the same availability to plants as inorganic N. Mineral N content (% of total N added with organic residue) after 4 weeks of incubation of soil was correlated to MFE (r2 = 0.78), but was on average 17% lower. Warm water‐extractable N, amino acid N and crude fibre analysis all proved to be unsatisfactory as methods for estimating MFE or MFEorg. However, the carbon/nitrogen ratio accurately reflected short‐term plant‐available N through a negative linear relationship (r2 = 0.83) and would thus be a very useful method for estimating MFE, with MFE decreasing by 5% per unit increase in C/N ratio. The results also indicated that the analysis of near infrared reflectance (NIR) spectra can be an even quicker and cheaper method to estimate MFE of organic residues, but this issue requires further research.

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The phosphorus (P) fertilizer effect of a range of commonly available manure, waste treatment and by-product residues was tested in pot, field and incubation experiments. The effect of the residues on P offtake was compared with that of commercial mineral P (super phosphate) to calculate the mineral fertilizer equivalent (MFE). Possible relationships between MFE and P extractable from residues using different agents (ammonium lactate, citrate, water) were examined. Dry matter yield and P concentration were measured in ryegrass grown in pots amended with 14 different residues. The effect on the first cut (after 5 weeks) was significantly higher for residues with a low organic matter content, for example ash and biogas residues (MFE = 74-85%), than for many other products with higher organic matter content, for example meat meal (MFE = 44%), cattle slurry (MFE = 57%) and sewage sludge (MFE = 0-37%). However, the effect on two combined cuts (after 11 weeks) was more similar between residues (MFE = 40-60% for most residues). Ammonium lactate-extractable P (P-AL) in residues correlated better with MFE (r 2 = 0.48) than water-extractable or citrate-extractable P. Grain yield and P concentration were measured in a field experiment with spring wheat fertilized with four different residues. Pelleted meat meal had a similar effect on yield and P offtake as mineral fertilizer P, whereas two different sewage sludge and chicken manure had approximately 50% of the mineral fertilizer effect. The effect of residues on soil P-AL (the Swedish measure of easily available soil P) in the incubation experiment showed no correlation with MFE from the pot experiments.

The choice of manure application technique can affect both the spatial distribution of ammonium in soil and net nitrogen (N) mineralization, and thereby N availability to crops. In this study we compared net N mineralization and spatial ammonium distribution after different degrees of incorporation of solid chicken manure and cattle slurry into soil. Ammonium-specific fluorescing optodes were assembled with manure applied to soil in closed chambers and the spatial distribution of ammonium in different treatments was measured for 2 weeks. The results indicated that much ammonium from the manures was quickly adsorbed to clay particles. Consequently, the ammonium concentration in the soil solution was threefold higher in the sandy soil than in the clay soil studied. Ammonium was distributed over a larger soil volume from manure applied below the soil surface than from manure applied above. Because the optodes excluded ammonium adsorbed to soil particles, net N mineralization was instead studied in separate incubations using extraction with potassium chloride solution for determination of ammonium and nitrate. When manure was kept concentrated in lumps rather than being mixed with soil, nitrate levels were about five times smaller after 1 week and 5-10% more of the manure N occurred as mineral N after 2 weeks. There were no differences in net N mineralization between surface application and subsurface incorporation. In this study a new technique to visualize and measure ammonium patterns around manure in soil proved to be useful for evaluating ammonium distribution and adsorption, but net N mineralization required incubations.

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In uniformly managed fields, high yielding areas and areas with less plant-available soil nitrogen (Np) may have lower grain crude protein concentration (CP) due to smaller amounts of nitrogen available per kg grain yield. With site-specific fertilization, more nitrogen could be applied to such areas to achieve a better and more even CP within the field. To investigate the need for this and the possibility of estimating fertilizer demand from historical data, within-field variations in CP, correlations with yield and Np and their consistency in spatial distribution between years were investigated. Measurements were performed during 1998-2000 at 34 sites within a 15-ha field in southwestern Sweden. Winter wheat (Triticum aestivum) was grown in 1998 and 2000 and spring barley (Hordeum vulgare) in 1999. CP was dependent on both yield and Np, but was also affected by other factors. The spatial distributions of CP and grain yields within the field were not consistent between years, probably because fertilization relative to optimum varied differently between sites in different years. In 1998, CP and grain yield were negatively correlated and CP was larger at sites with more Np. CP had a positive correlation with both yield and Np in 1999. In 2000, CP was lower where yield was higher at most sites, but did not correlate significantly with either yield or Np. The positive correlation between yield and CP was connected with nitrogen fertilization below optimum, whereas the negative correlation was connected with nitrogen fertilization near or above optimum at most sites.