The effect of subsoiling on plant nutrition and pasture production

Abstract

An eighteen month field trial was conducted to determine the effect of soil loosening on plant nutrition and pasture production.

The field trial was conducted on an established (8 year old) mixed sward pasture. The soil type was a Templeton silt loam (yellow-grey earth) of low to medium fertility. Two layers were identified as likely to be restrictive to root growth (22-27 cm and 40-60 cm).

The statistical design was a randomised block consisting of thirty-six plots (8 m x 8 m) in four replicates. The three cultivation treatments of nil cultivation, aeration (to 27 cm) and subsoiling (to 47 cm) were carried out on the 10-Nov-90 and 10-0ct-90 respectively. On 22-Dec-90 three fertiliser treatments were applied: nil fertiliser, phosphate only (30 kg P ha⁻¹) and phosphate (as above) plus sulphate fertiliser (30 kg S ha⁻¹).

The pasture was harvested when plant height reached approximately 8-10 cm. Pasture production and macro nutrient concentration were determined for each pasture harvest. From 1-Aug-91 root length measurements were made at monthly intervals. Soil water content was measured at weekly intervals at six depths (20, 30, 35, 40, 50 and 70 cm) using Time-Domain-Reflectrometry. Soil bulk density and hydraulic conductivity were measured at two dates during the period of the trial.

Although root pruning by the loosening implements was suspected to have slightly reduced pasture production from 7-Nov-90 to 22-Apr-91, soil loosening led to a significant (P<0.10) increase in pasture production of approximately 1,300 kg DM ha⁻¹ over the second season (1991/1992) of the trial.

Loosening by cultivation of the compacted soil layers significantly (P<0.10) reduced bulk density (11 % reduction) and significantly (p<0.10) increased porosity. Through the creation of soil physical conditions which were more favourable for root growth, soil loosening allowed the earlier onset of spring root growth, and root growth rates were 0.06 mm cm⁻² d⁻¹ higher than in the nil cultivation treatment. Subsoil loosening created a more extensive pasture root system compared to the nil cultivation treatment. By 18-Jan-92 the subsoil loosened treatment had a significantly (P<0.10) larger root length (44% higher) at the 30-60 cm depth than the nil cultivation treatment.

The subsoiled treatment had a significantly (P<0.10) drier soil profile (0-70 cm) over the 1991/1992 season. This was considered to be due to higher soil drainage rates in late winter/early spring and a small increase in water use by the pasture.

Soil loosening was found to result in a significant (P<0.10) increase (5 kg S ha⁻¹) in sulphur uptake by the pasture plants. The Templeton silt loam of the trial site was found to be deficient in sulphur, with particularly low concentrations (<3 µg SO₄ ²⁻ g⁻¹) occurring in the top soil. The sulphate concentration was higher (10 µg SO₄ ²⁻ g⁻¹) in the lower soil depths (30-50 cm). The more extensive pasture root system at this depth in the loosened treatments therefore had greater access to this sulphur, and resulted in increased pasture uptake of sulphur and increased pasture production on the loosened treatments.

Pasture grown on the loosened treatments tended to have higher concentrations of several macronutrients (S, Ca, Mg and Na) particularly during spring and mid-summer. The concentration of these four macronutrients in this Templeton silt loam were also found to increase below a depth of approximately 30 cm. The greater root lengths below 30 cm in the loosened treatments allowed the pasture roots greater access and consequently higher uptakes of these nutrients.

In August 1991 a microplot trial was established in an attempt to establish accurately the nutrient uptake potential of roots present in the trial plots. Radioactive tracers (³²p and ³⁵S) were injected to three depths (25, 40, and 55 cm) in the soil profile. The 80 cm diameter microplots were harvested when pasture height reached approximately 8 cm. The pasture samples were then dissected, dried, ground and the tracer activity determined. The subsoiled treatment was found to have a higher cumulative ³⁵S percent recovery from the 55 cm injection depth, confirming the results of the main trial.