Intercropping of cereals and grain legumes is a technology intended to enhance biodiversity in organic agroecosystems, increase and stabilize yields and make better use of plant growth resources. The experiments were designed to investigate the intercrops' yield advantage in different soil, climate and plant competition conditions in organic farming. Field trials were carried out during 2007-2010 at the Lithuanian Institute of Agriculture at three different soil sites: on a loamy Endocalcari-Epihypogleyic Cambisol ( CMg-p-w-can) (Dotnuva), on a clay loam Endocalcari-Endohypogleyic Cambisol ( CMg-n-w-can) (Joniskelis) and on a sandy loam Hapli-Albic Luvisol ( LVe-ha) (Perloja). Pea ( Pisum sativum L. (Partim)) and spring wheat ( Triticum aestivum L. emend. Fiori et Paol.), spring barley ( Hordeum vulgare L.), oat ( Avena sativa L.) and spring triticale (* Triticosecale Wittm.) were sown as intercrops (50:50 - a relative proportion of grain legume and spring cereal seeds) or as a sole crop. The result showed that weather and soil conditions during plant emerge determined the optimal productive density and created a basis for competition between the intercrop components. Spring cereals were dominant due to higher aggressivity index and competition rate compared with peas. Under various soils' conditions the aggressivity of cereals increased with increasing density of pea. In productive soils (Dotnuva, Joniskelis), the highest competitive and suppressive power in the intercrops was exhibited by oats, in low productivity soils (Perloja) - the dominant species varied. Under the conditions of various soils, the production of pea intercrop with oats or barley was directly affected by cereals, while pea intercropped with wheat or triticale by the two components of the intercrop. In terms of grain yield, intercrops ranked as follows: pea/oats > pea/wheat, pea/triticale > pea/barley. In loamy soil (Dotnuva), these regularities were less consistent, therefore the advantages of intercrops' yield over sole crops' yield were negligible when the relative yield total (RYT) was >1.0, except for the pea intercropped with barley. In Joniskelis and Perloja, the relative yield total showed greater and more stable yield advantages of intercrops, except pea intercropped with triticale. The efficiency of intercrops stood out even more vividly in the treatments managed under organic cropping system for a longer period of time.
