Nitrophosphate as an alternative phosphate fertiliser for acidic sandy soils

Abstract

It is a common practice to use large quantities of reduced nitrogen (N) such as urea, ammonia (NH3) and ammonium (NH4+) as pre-plant (before planting) fertilisers in the Free State, Mpumalanga and North West where farmers apply monoammonium phosphate (MAP(33)) together with urea before planting. However, MAP(33) a source of phosphorous (P), which is pre-applied resulted in soil acidification, the accumulation of NH4+ and loss of cations. Nitrophosphate was therefore suggested as an alternative phosphate fertiliser to remedy low P use efficiency, soil acidity and to replace lost calcium (Ca) and magnesium (Mg). A study was conducted with the aim to compare nitrophosphate with MAP(33) when used as pre-plant fertilisers in acidic sandy soils commonly found in commercial agriculture. To meet this aim, two greenhouse experiments were conducted at the research facilities of Omnia (Pty) Ltd in Sasolburg, South Africa in 2013. The biomass, residual soil nutrient status and nutrient uptake of potted wheat plants were compared when fertilised with nitrophosphate and MAP(33). The experiments consisted of a completely randomised design (CRD) with two P fertiliser sources (nitrophosphate and MAP(33)), applied at four different rates (0, 15, 30 and 45 kg P ha-1) replicated five times. Urea was added to all treatments, except the controls, to ensure that all treatments received the same amount (106 kg N ha-1) of nitrogen (N). Micronutrients were supplied by topdressing with HIDROSPOOR at a rate of 2 kg ha-1. For the experiments wheat was planted in an acidic sandy soil (pHKCl of 4.1) collected from a commercial farm in Bothaville with a low Bray-1 P (15 mg kg-1), S (13 mg kg-1), ammonium acetate extractable Ca (79 mg kg-1) content and 5% clay. Results from the study indicated that the pH of soils treated with MAP(33) was higher than the pH of soils treated with nitrophosphate. The leaves of MAP(33) fertilised wheat had a consistently higher chlorophyll content than the leaves of nitrophosphate fertilised wheat, except for the period 2-3 weeks after emergence (WAE). MAP(33) fertilised wheat had 108% (first trial) and 105% (second trial) more root growth and 96% (first trial) and 167% (second trial) more leaf growth compared to nitrophosphate fertilised wheat. MAP(33) also resulted in higher sulphur (S), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), copper (Cu), manganese (Mn), zinc (Zn) and iron (Fe) uptake. There was a strong correlation (R2 = 0.81) between soil pHKCl and root growth where the lower soil pH resulted in impaired root development which adversely affected wheat growth and nutrient uptake. The secondary nutrients associated with nitrophosphate therefore did not improve wheat growth. Results from the experiments indicated that the MAP(33) treatments resulted in lower concentrations of S, N, K, Cu, Mn, Zn, Fe and Mo in the dry leaf matter than nitrophosphate treatments. The results of this study suggests nitrophosphate is not the preferable fertiliser to substitute MAP(33) as a pre-plant fertiliser in soils with a low pH. However, under less acidic soil conditions (pH 6.5) nitrophosphate resulted in better growth than MAP(33).