Long-term effect of superphosphate on accumulation of soil phosphorus and exchangeable cations on a grazed, irrigated pasture site

Abstract

The annual herbage dry matter yield, herbage P concentrations and quantities of P, K, Mg and Ca cycled by grazing sheep were calculated for a 37 year-old grazed pasture supplied with superphosphate at rates of 0, 188 and 376 kg ha -~ annually. The amount and distribution of inorganic and organic P and exchangeable K, Mg and Ca in the soil below the three grazed treatments was also measured and compared with that below a wilderness area which had not been used for agriculture.Increasing rates of superphosphate increased herbage dry matter yield, herbage P concentrations and thus the amounts of P ingested and, in turn, excreted by the grazing sheep. Annual quantities of K, Mg and Ca cycled back to the pasture in the form of excreta were also increased. The increased cycling of K by animals induced by increasing superphosphate applications resulted in greater losses of K and consequently concentrations of exchangeable and non-exchangeable 'fixed' K in soil decreased in the order wilderness > control > 188 > 376. Differences were evident to a depth of 20 cm. Some losses of Mg during cycling also occurred and concentrations of exchangeable Mg followed the order: wilderness > control > 188 = 376. Concentrations of exchangeable Ca increased with pasture development due to additions of Ca in lime and superphosphate. Concentrations of total soil organic P (0-4 cm) increased in the order: wilderness < control < 188 < 376 but for inorganic P and total P the control had a lower content than wilderness indicating losses of P during cycling in the grazed control treatment. Phosphorus fractionation suggested that with increasing superphosphate rates inorganic P primarily accumulated in a form adsorbed to AI hydrous oxides and as calcium phosphate compounds whilst organic P accumulated in both labile forms and forms associated with humic compounds. In the fertilised sites both inorganic and organic P accumulated in the soil profile to a depth of 20 cm.

The effects of sheep faeces on soil solution composition

“…Given that the concentration of P, Ca and Mg in the faeces changed little with time in the field as the weight decreased these data indicate that for these elements, leaching was not an important effect and that physical breakdown and/or transfer by invading organisms were more important processes. Haynes and Williams (1992) found between 47-65% of the P, 70-74% of the K, 24-34% of the Mg and 9-13% of the Ca in sheep faeces from animals on pasture were water soluble. Dickinson and Craig (1990) showed that covering cattle faecal pats to protect them from the direct effects of rainfall substantially reduced their rate of decomposition, but the faeces continued to decay through the action of soil organisms, stimulated by water ingress from surrounding soil.…”

Effects of grassland management practices and environmental conditions on nutrient concentrations in overland flow

“…The most obvious difference between the overland flow sites was the soil P level as it rose in the same order as the DRP concentrations in overland flow. However, the higher the soil P levels, the greater the P concentrations in herbage are reported to be, and the P contents in grass in turn can have a bearing on P excreted by grazing animals (Haynes and Williams, 1992). It can be said though that the differences in DRP concentrations of overland flow from the three sites were highly likely to be due to soil P status and/or to factors related to the soil P levels (e.g.…”

“…The amount of P in cattle faeces varies in accordance with the P levels in pasture herbage, and these, in turn, are dependent on soil P levels (Haynes and Williams, 1992). An accurate assessment of the P deposited in cattle manure at Warren 1 and Cowlands is therefore not possible.…”

Rooting depth and root depth distribution ofTrifolium repens×T. unifloruminterspecific hybrids

“…Trifolium uniflorum occurs naturally in dry environments (Tela Botanica, 2015), and hybrids with white clover have been found to have increased drought resistance compared with white clover cultivars (Nichols et al, 2014b(Nichols et al, , 2015a. Shallow rooting is also advantageous for phosphorus (P) acquisition (Ge et al, 2000;Lynch and Brown, 2001) as most soil P is concentrated near the soil surface (Haynes and Williams, 1992). The root distribution of T. uniflorum and the T. repens  T. uniflorum hybrids may therefore also reflect the soil fertility status in T. uniflorum's natural environment.…”