On-Farm Measurements of pH, Electrical Conductivity and Nitrate in Soil Extracts for Monitoring Coupling and Decoupling of Nutrient Cycles

Abstract

Concepts of interacting nutrient and proton cycles, “decoupling” of mineralization and uptake, and the steady state soil solution, developed mainly to assess impacts of acid rain on forests and their catchment waters, are very pertinent to biological agriculture. In particular, they illustrate how decoupling of soil-plant nitrogen cycling also decouples cycling of protons and basic cations (chiefly Ca, Mg, K), and can result in acidification of soil and in loss of cations as well as of nitrate. In biological agriculture, loss of cations may be more important than loss of N because there is no equivalent to biological nitrogen fixation to replace them. It is proposed that on-site measurements of pH, EC (electrical conductivity) and nitrate (using semiquantitative nitrate strips) in 1:1 water extracts of soil are convenient tools for on-site monitoring of coupling/decoupling phenomena. Values of pH, nitrate and EC of soil samples taken from a variety of soil types, crops and farming systems are reported and relationships between the variables are examined. Several practical questions examined in the course of the studies provide examples of how the measurements can be of value in biological husbandry. As predicted, there were strong linear relationships between nitrate concentrations and EC for samples from a given system or region. For many purposes, EC values give the same type of information as nitrate values, and are simpler and cheaper to obtain. Values of EC and nitrate were lowest under sod, intermediate in cultivated ground, and highest in cultivated ground to which manures or compost were added. In a laboratory experiment, growth of plants was found to reduce soil extract nitrate and EC to a greater extent than did incorporation of immobilizing residues. In eight comparisons of soils from potato or grain crops grown organically with those from crops grown with synthetic fertilizer on the same or nearby farms in eastern Canada and Maine (USA), EC values were consistently lower, and nitrate values the same or lower under organic management; there was a trend for pH to be higher under organic management. The techniques were used to monitor seasonal changes in the soil soluble nutrient pool and in lettuce tissue nitrate in an intensive organic vegetable production system on Vancouver Island, and to examine an intensive organic crop/livestock system in Colombia for possible sites of leakage of nutrients.