Season, Temperature, Soil pH, and Mg Fertilizer Effects on Herbage Ca and P Levels and Ratios of Grasses and Legumes1
Authors: Gross C. F. , Jung G. A. 
Published: July 2010

Low Ca and P levels and wide Ca:P ratios of forages have been implicated in several animal diseases of economic importance. However, information on seasonal, temperature, and soil amendment effects on Ca and P levels and ratios of forages of different species and cultivars is meager. A greenhouse study was conducted to determine seasonal, temperature, soil pH, and Mg fertilizer effects on herbage Ca and P levels and Ca:P ratios in one or more cultivars of tall fescue (Festuca arundinacea Schreb.), timothy (Phleum pratense L.), reed canarygrass (Phalaris arundinucea L.), smooth bromegrass (Bromus inermis Leyss.), orchardgrass (Dactylis glomerata L.), redtop (Agrostis gigantea Roth.), perennial ryegrass (Lolium perenne L.), Kentucky bluegrass (Poa patensis L.), alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), ladino and white clover (T. repens L.), alsike clover (T. hybridum L.), birdsfoot trefoil (Lotus corniculatus L.), and crownvetch (Coronilla varia L.).

Plants were grown in 4‐liter pots containing Hagerstown silt loam (Typic Hapludalf; fine, mixed, mesic) under natural daylength and near‐ambient air temperatures. Minimum temperature was not permitted to drop below 1 C. Initial soil pH levels were 6.0 and 7.0 in combination with 0 and 672 kg Mg/ha (as MgSO4). During a 13‐month growing period, grasses received 1,300 kg N/ha (as NH4NO3), and grasses and legumes were provided 600 kg K/ha (as K2BO4). Plants were clipped to 5 cm, generally at 2 to 3‐week intervals. During semidormancy, plants were harvested at longer intervals but not analyzed.

Grasses and legumes, except white clover, had higher Ca and lower P levels (P < 0.01) in late autumn than in May under similar temperature regimes. During periods of alternating cool and intermediate temperatures, Ca levels of grasses and legumes were higher (P < 0.01) with the intermediate temperatures. As the average air temperature increased from 17 to 26 C, Ca levels of grasses and legumes and P levels of legumes increased or decreased depending on the species, but P levels of grasses decreased. Differences in Ca levels of grasses and legumes that were related to increased Ca saturation percent of the soil CEC, varied markedly in magnitude depending on the species and cultivar. Increased Mg saturation of the soil CEC above 13 and 22% lowered Ca levels and Ca:P ratios of grassa and legumes, respectively.

Redtop was the best and Kentucky bluegrass the poorest Ca accumulator among grasses. Penngift crownvetch was the best Ca accumulator and Vernal alfalfa and birdsfoot trefoil were poor accumulators among legumes. Grass cultivars did not differ consistently in P accumulation. Vernal alfalfa was the best and Regal white clover was the poorest P accumulator. Average Ca:P ratios of grasses were generally low, and legumes ranged from 2.5:l for Vernal alfalfa to >4.01 for the white clovers and Penngift crownvetch. With high Ca saturation percent of the soil CEC, many Ca:P ratios in autumn were above 4.01 for grasses and above 8.0:l for legumes.