FROST HEAVE OF ESTABLISHED ALFALFA PLANTS (MEDICAGO SATIVA) AT ITHACA, NEW YORK (SOIL FREEZING)

Frost upheaval of alfalfa plants occurs in agricultural regions where soil freezing takes place and snow cover is limited. Displacements are commonly ascribed to 'jacking' by freeze-thaw cycles in the fall and spring. However, comparison of the heaving pressures generated by these conditions with those produced during mid-winter reveals that such a model is untenable in light of the forces required to uproot established alfalfa plants. This study was conducted to clarify the mechanism of plant upheaval during winter soil freezing. Additional objectives included description of soil heaving in the field and quantification of root morphological and disease parameters in relation to plant heave. A tagging technique was developed for direct measurement of crown uplift relative to the adjacent soil surface. Both soil and plant elevations were determined using a surveyor's level in conjunction with stable benchmarks. Measurements proved to be accurate within (+OR-)1.6 mm. Two hundred individual heaving records of 2- and 3-year old alfalfa plants were collected over two winters at Ithaca, New York. These showed mean soil frost heave cycles of 22.6 mm and 38.8 mm amplitude in 1982/83 and 1983/84, respectively. Soil movements were positively correlated with depth of frost penetration and changes in the soil water/ice content profile. Five types of plant response to soil heaving were observed over both winters. Predicted uprooting forces at the peak of the 1983/84 heave cycle ranged from 424 to 1,885 N/plant. Mean final uplift of the 2-year old plants above the soil surface was 6.8 mm, compared to only 2.1 mm for the 3-year old plants. Varietal differences in root system morphology did not affect heaving resistance during the mild winter of 1982/83. In contrast, the extreme conditions of 1983/84 produced several significant relationships between root parameters and soil movements. Maximal plant heave over the winter, the single most important predictor of final plant uplift, was positively correlated with crown rot score and distance to first lateral root. Uplift occurred primarily in mid-winter and not during alternate freezing and thawing in the fall or spring. This result suggests a model for established plant upheaval in which uplift is associated with deep soil freezing rather than 'jacking' by minor freeze-thaw cycles.