ON-FARM MEASUREMENTS OF PH, ELECTRICAL-CONDUCTIVITY AND NITRATE IN SOIL EXTRACTS FOR MONITORING COUPLING AND DECOUPLING OF NUTRIENT CYCLES

Concepts of interacting nutrient and proton cycles, ''decoupling'' of mineralization and uptake, and the steady state soil solution, develop ed mainly to assess impacts of acid rain on forests and their catchmen t waters, are very pertinent to biological agriculture. In particular, they illustrate how decoupling of soil-plant nitrogen cycling also de couples 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 imp ortant than loss of N because there is no equivalent to biological nit rogen fixation to replace them. It is proposed that on-site measuremen ts of pH, EC (electrical conductivity) and nitrate (using semiquantita tive nitrate strips) in 1:1 water extracts of soil are convenient tool s for on-site monitoring of coupling/decoupling phenomena. Values of p H, nitrate and EC of soil samples taken from a variety of soil types, crops and farming systems are reported and relationships between the v ariables are examined. Several practical questions examined in the cou rse 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 ty pe of information as nitrate values, and are simpler and cheaper to ob tain. Values of EC and nitrate were lowest under sod, intermediate in cultivated ground, and highest in cultivated ground to which manures o r 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 so ils from potato or grain crops grown organically with those from crops grown with synthetic fertilizer on the same or nearby farms in easter n Canada and Maine (USA), EC values were consistently lower, and nitra te values the same or lower under organic management; there was a tren d for pH to be higher under organic management. The techniques were us ed to monitor seasonal changes in the soil soluble nutrient pool and i n lettuce tissue nitrate in an intensive organic vegetable production system on Vancouver Island, and to examine an intensive organic crop/l ivestock system in Colombia for possible sites of leakage of nutrients .