Clover and grass foliar nutrients in response to fertilizer and herbicide applications on Port Hills of Canterbury

Pastures on the Port Hills, Canterbury, are typically grass-dominant growing on low Olsen P soils and enduring summer-dry conditions. 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 autumn grass-supressing herbicide (plus or minus, +H or -H) set up in May 2021. The grasses sampled represented the mix of species in the pasture. 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, the nutrients commonly applied as fertilisers to support clover growth. Applying P increased foliar P concentrations 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% by 0.3 to 0.5% units. +Mo elevated grass foliar 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 N% was higher, by ~0.4% units, where herbicide was applied in 2022 and 2023. In 2023, the +P+S+Mo fertiliser treatment increased N, P, S, Mg and B in 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.