Clover and grass foliar nutrient responses to phosphorus and molybdenum fertilisers, and herbicide application on a summer dry hill pasture

Abstract

Pastures on the Port Hills, Canterbury, are typically grass-dominant growing on low Olsen P soils and enduring summer-dry conditions. Subterranean (sub) clover is the preferred legume on steeper dryland hill slopes, but clovers struggle to compete with resident grasses for soil moisture and nutrients. Nutrient concentrations in sub clover and grass foliage were sampled each spring, from 2021 to 2023, in an experiment with factorial combinations of phosphorous (0 or 60 kg P/ha,-P or +P), sulphur (0 or 50 kg S/ha, -S or +S), and molybdenum (0 or 0.06 kg Mo/ha, -Mo or +Mo) fertilisers, and plus or minus autumn grass-supressing herbicide. In the unfertilised controls, sub clover had higher foliar concentrations of N, Ca, Mg, B, Cu and Zn than the grasses which had higher concentrations of P, K, S and Mo, which are the nutrients commonly applied in fertilisers to support clover growth. Applying P increased foliar P from 0.16 to 0.27% in sub clover and 0.21 to 0.34% in grass in 2023. Sub clover and grass foliar S concentrations were mainly affected by sampling year. The +Mo treatment only elevated sub clover Mo in 2022 from 0.05 to 0.11 mg/kg. However, each year +Mo increased sub clover foliar N% from 4.1 to 4.5% in 2021, 3.7 to 4.0% in 2022, and 4.3 to 4.8% in 2023. Grass foliar Mo was elevated by +Mo from ∼0.45 to 1.42 mg/kg in 2021 but then concentrations declined to 0.99 and 0.67 mg/kg in 2022 and 2023 respectively. Grass Mo values were, on average, 10 times higher than in the sub clover. Grass N% was increased where herbicide was applied, increasing from 2.6 to 3.0% in 2022 and 3.6 to 3.9% in 2023. In 2023, the +P+S+Mo fertiliser treatment increased N, P, S, Mg and B in fertilised sub clover compared with control. To maximise clover nutrition and growth on this low P site the regular addition of superphosphate, with molybdenum every four years, is recommended in conjunction with grass control in autumn.