The effect of fertilizer nitrogen, phosphorus and potassium on the calcium, magnesium and phosphorus status of pasture cut for silage

Abstract

SUMMARYPhysiological problems in some Fife dairy herds were thought to be associated with mineral imbalance in grass cut for silage. Consequently trials at two sites were conducted to study the effects of three levels of nitrogen and phosphorus and two levels of potassium on yield and composition of pasture cut for silage.Nitrogen increased yield of herbage and reduced its clover content whereas phosphorus and potassium had little effect on yield or clover content. Herbage clover content and calcium concentration declined over the period of the experiment. Mean concentrations of calcium, magnesium and phosphorus were 5·8, 1·7 and 3·4 g/kg respectively. Concentrations of these minerals varied more with time of cutting and with season than with level of applied phosphorus and potassium.Nitrogen tended to reduce calcium concentration in mixed herbage and to increase magnesium concentration in the grass component; it did not affect herbage phosphorus concentrations, but narrowed the calcium-phosphorus ratio in the second cuts. Applied phosphorus increased herbage phosphorus concentration only at the site with the lower soil phosphorus status. Potassium tended to reduce magnesium concentration in clover and, at one site, in mixed herbage; it also tended to reduce herbage phosphorus concentrations in second cuts.Mineral uptake was increased by applied nitrogen. Mean annual uptake of calcium magnesium and phosphorus was 34, 10 and 20 kg/ha respectively.The effects of treatments on concentration and balance of these minerals are discussed in relation to ruminant requirements and maintenance of suitable levels of the minerals in the soil.It is concluded that normal applications of NPK fertilizer are not detrimental to satisfactory mineral balance in pasture.