Crop establishment series
Reviewed December 2006
By M.G. Mason, Senior Research Officer, and R.J. Jarvis, Principal Research Officer, Soil Management, South Perth, and M.D.A. Bolland, Senior Research Officer, Soil Management, Bunbury
This Farmnote is one of a series on crop establishment practices which minimise soil disturbance and improve stubble retention. In partnership with primary producers, Agriculture Western Australia is focussed on improved and profitable farming which conserves or improves the quality of the State's soil resources.
The recent increase in the adoption of no-tillage crop establishment practices has brought with it some concerns about fertiliser applications. This Farmnote summarises existing data on the implications of concentrated bands of fertiliser for crop growth and weed control. Farmnote No. 71/96 describes fertiliser toxicity in no-tillage farming.
Soluble fertilisers placed in contact with crop seed can delay or reduce germination. In extreme conditions, even partially soluble fertiliser such as superphosphate can affect germination, but the very soluble nitrogen fertilisers have the greater effect. This reduction in establishment can occur in cereal crops but is more severe in crops such as canola.
The effect is usually due to a high concentration of salt from dissolved fertiliser around the seed. Toxicity from phosphate fertilisers appears to relate to the concentration of soluble phosphorus in the fertiliser.
Figure 1 shows that per unit of phosphorus (P), triple superphosphate (TSP) is potentially less toxic than superphosphate. For example, the TSP solution had a conductivity level about 1.5 times that of superphosphate, but it contains 2.2 times the phosphorus.
Figure 1. The potential toxicity of equal weights of four fertilisers as electrical conductivity (EC) over time as they dissolve into solution. EC is a measure of salt concentration.
The toxicity potential of superphosphate varies with granule size ranges. Diammonium phosphate (DAP) has the same amount of P as TSP, but the ammonium content of this form of fertiliser greatly increases its potential for toxicity. The addition of the soluble trace element compound, manganese sulphate, further increases the risk of toxicity.
Urea is more toxic still, due to a different mechanism. As urea breaks down, it produces ammonium carbonate which causes alkaline conditions around the fertiliser granules. This results in the production of ammonia gas which is highly toxic to germinating seeds.
Toxicity will be more likely with higher fertiliser rates.
With placement closer to the seed, contact between high concentration solutions and the seedling is more likely. Sowing machines such as air seeders, especially those with wider boots and points, often scatter seed and fertiliser, which lessens the close proximity of seed and fertiliser.
With no-till practices, seed and fertiliser are often placed close together, which increases the chance of germination being reduced by toxicity. Possibly counteracting this is less disturbance and a generally more compact, better moisture-holding soil near the seed, particularly if press wheels are used.
Greater toxicity has been seen where the seedbed has been left loose and dry, and in consequence holds less moisture to dilute the concentrated salt solution from the dissolving fertiliser.
If the spacings between rows are increased and the rate of fertiliser per hectare is maintained, the concentration of fertiliser with the seed, and hence the risk of toxicity, will be increased.
The toxic effects on germination can be avoided by banding fertilisers away from the seed or by topdressing (Table 1). Banding about 5 cm below or to the side of the seed rows is usually sufficient to avoid germination problems. Because N fertilisers can leach, banding N fertiliser above the seed rows was tried in trials, but it resulted in reduced establishment (Table 1) and reduced early vigour.
Table 1. The effect of placement of N fertiliser (urea) on the reduction of wheat plant establishment on light sand, East Hyden
|Placement of fertiliser||Reduction in plant establishment (%)|
|40 kg/ha N||80 kg/ha N|
|Topdressed across seed rows||Nil||Nil|
|Drilled in seed rows||48||77|
|Topdressed above each seed row||13||33|
|Deep banded below each row||Nil||Nil|
At least one case of reduced germination has occurred with banding urea and DAP below the seed at a high rate on wide spacings. This was thought to be due to poor band placement and/or ammonia gas percolating upwards from the concentrated band to the seed in fairly dry, light sandy soil.
Toxicity levels are highest when sowing into moist, light sands and when sowing in any soil type is followed by a warm dry period.
Toxic effects will be reduced or eliminated when good rains occur immediately after sowing. Rain dilutes the salt solution, dissolving ammonia and removing fertiliser from around the seed by leaching.
Toxicity from phosphate fertilisers is less on soils with a lower P status and with higher P fixing capacity, that is, a higher phosphorus retention index (PRI) or higher reactive iron content.
The rate of leaching of fertiliser depends on soil type and rainfall intensity and timing. Maximum leaching occurs in light sandy soils with high intensity rainfall.
While nitrogen is in the ammonium form, which is half the nitrogen in ammonium nitrate, it is not readily leached because it is held by exchange sites in the soil. Ammonium nitrogen is nitrified (oxidised) to nitrate nitrogen at varying rates depending on moisture, temperature and soil pH. In very acid soils, nitrification is very slow.
Nitrate nitrogen is readily leached by rain. Urea can be leached while it remains as urea, but is generally broken down into ammonium nitrogen within two to three days and is then available for nitrification.
A 'safe' level cannot be stated because of the above factors which influence the toxicity of fertilisers on germinating seed. For example, many P rate trials gave slower wheat emergence when only 50 kg/ha super was drilled with the seed compared with nil P. A certain level of fertiliser may greatly reduce germination, but the remaining plants may grow luxuriantly in a high fertiliser responsive soil and produce a higher yield than would have occurred had a lower, non-toxic fertiliser rate been used.
Keeping in mind the difficulty of predicting toxicity effects, 'safe' maximum rates of fertiliser to drill with the seed on standard row spacings, which will not normally reduce establishment, could be around the following:
- 350 kg/ha superphosphate for cereals
- 180 kg/ha superphosphate (9% P or its equivalent P rate as TSP, DSP for lupins and other pulses). The safe rate is lower for manganese superphosphate.
- 130 kg/ha superphosphate for canola
- 120 kg/ha 18:8 (18% N:8% P) for cereals, and 40 kg/ha or less for canola
- 120 kg/ha 18:20 for wheat; 40 kg/ha or less for canola
- 30 kg/ha urea for wheat; nil for canola
- 100 kg/ha ammonium nitrate and ammonium sulphate for wheat; nil for canola
- Nil muriate of potash for lupins and canola
If in doubt about toxicity, increasing the seeding rate will be worthwhile.
- Farmnote No. 71/96 'Banding fertilisers in no-tillage farming' (Agdex 540).