The response of pastures in Northern Ireland to N, P and K fertilizers and to animal slurries: II. Effects of mineral composition

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Abstract
SUMMARY: Herbage samples from 16 trials testing the effects of N, P and K fertilizers and slurry on grassland were analysed for % N, P, K, Ca, Mg and Na. The data were examined to see if a fertilizer policy designed for optimum yield should be modified after taking pasture mineral content into account.The correct fertilizer nitrogen policy appears to be to apply nitrogen for the desired yield and to disregard any effects on mineral content. The correct fertilizer policy for phosphorus and potassium is to steer a middle course between depleting soil reserves and accumulating undesirable excesses. The quantities of phosphorus and potassium which should be applied to cut grass to maintain the status quo in the soils are discussed. The amount of phosphorus and potassium added in slurry must be allowed for in this calculation. As the amount of slurry available depends upon the farming system, an effective fertilizer policy for phosphorus and potassium can only be devised by studying the farm as a whole.At the first cut, the slurries provided on average 115 kg NH4-N/ha, 114 kg K/ha and 86 kg P/ha. Slurry potassium was somewhat less than half as effective as fertilizer potassium in increasing % K in the herbage at the first cut. Furthermore, the herbage recovered only 0·8 kg of the 86 kg of slurry phosphorus. In the short term, slurry was only just over 10% as effective as fertilizer phosphorus, presumably because much phosphorus in it is organically bound.