K for thought: e-ifc No. 12, June 2007

K for thought

Balancing outputs with adequate, responsible and sustainable inputs

Accumulating potash balance per hectare on total area of major arable crops in England and Wales, excluding inputs from manures

Sustainable agriculture implies the long term maintenance of soil fertility. The figure on the right illustrates how this can change in relation to potash by considering the accumulating average potash balance in K₂O per hectare over the past thirty years as calculated by annual differences between inorganic fertilizer inputs and crop offtakes for the major arable crops in Britain: cereals, oilseeds, sugar beet and potatoes. Manures are not taken into account but this has little bearing on the calculation because the supply of K to the soil in this form is relatively low in arable farming.

Over the period between 1973 and 1992 arable farmers in England and Wales applied a little more potash fertilizer each year than was removed in harvested crops, so that by 1992 on average these arable soils had 65 kg K₂O more than they had in 1973. However since then, over the last 15 years, the soil reserves have been mined, drawing from the "soil potassium bank" approximately 200 kg/ha of potash, without "paying back" through fertilization for all that was removed by harvest over that period.

Such balance calculations are very useful in assessing long-term consequences of changing fertilization practices. Farmers should not wait for the inevitable loss of fertility resultingfrom potash depletion but rather increase potash fertilization to maintain balance.

Adapted from data published by PDA, November 2006.

Fertilizer broadcasting in Parana, Brazil: key to replenishing nutrients in the soils where the fertility status is fragile. Photo by A. Naumov.