The existence of soluble organic forms of N in rain and drainage waters has been known for many years, but these have not been generally regarded as significant pools of N in agricultural soils. We review the size and function of both soluble organic N extracted from soils (SON) and dissolved organic N present in soil solution and drainage waters (DON) in arable agricultural soils. SON is of the same order of magnitude as mineral N and of equal size in many cases; 20-30 kg SON-N ha -1 is present in a wide range of arable agricultural soils from England. Its dynamics are affected by mineralisation, immobilisation, leaching and plant uptake in the same way as those of mineral N, but its pool size is more constant than that of mineral N. DON can be sampled from soil solution using suction cups and collected in drainage waters. Significant amounts of DON are leached, but this comprises only about one-tenth of the SON extracted from the same soil. Leached DON may take with it nutrients, chelated or complexed metals and pesticides. SON/DON is clearly an important pool in N transformations and plant uptake, but there are still many gaps in our understanding.

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.

Stockdale, E. A., Shepherd, M. A., Fortune, S., Cuttle, S. P. (2002). Soil fertility in organic farming systems - fundamentally different? ? Soil Use and Management, Soil fertility in organically managed soils, 18, (Suppl.), 301-308 Sponsorship: DEFRASoil fertility is de?ned as the ability of a soil to provide the conditions required for plant growth. It is a result of the physical, chemical and biological processes that act together to provide nutrients, water, aeration and stability to the plant, as well as freedom from any substances that may inhibit growth. Within this de?nition, it is useful to distinguish between those components of fertility which change relatively slowly, perhaps over the course of a rotation, or in some cases, decades, and the more immediate contribution from materials such as fertilizers and manures. The term `inherent fertility' is used to describe these more stable characteristics, while recognising that they are, to a large extent, products of soil management. We conclude that, although nutrient management in organically managed soils is fundamentally different to soils managed conventionally, the underlying processes supporting soil fertility are not. The same nutrient cycling processes operate in organically farmed soils as those that are farmed conventionally although their relative importance and rates may differ. Nutrient pools in organically farmed soils are also essentially the same as in conventionally managed soils but, in the absence of regular fertilizer inputs, nutrient reserves in less-available pools will be of greater signi?cance.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.

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