Nitrogen Fertiliser Placement and Crop Establishment
With the reduction in mechanical tillage, nitrogen fertiliser application has been pushed closer to sowing in both time and placement, and in some cases some nitrogen is applied at sowing. Continuation of this trend will necessitate close attention to fertiliser application particularly placement, rate and timing and a good understanding of the product differences and similarities, to maintain profitability and avoid crop establishment problems.
Application of nitrogen fertilisers at sowing can be achieved without sacrificing crop establishment by either minimising the concentration of product in contact with the germinating seed or placing the product away from the seed at sufficient distance that the product will not affect crop establishment. Application pre and post sowing provide the most flexibility.
Urea, as a compound, is not toxic to germinating seeds however the ammonia evolved from it, is toxic to seeds.
Fertilisers and Seed Germination
Toxicity to germinating seed and growing plants, emanating from fertiliser can arise from a number of ions (nitrite, chloride, ammonia, borate) and compounds associated with the fertiliser. Biuret, a “double” molecule of urea formed in small quantities during the manufacturing process can affect seedling development but has little effect on germination.
The relative "toxicity" of the ammonium fertilisers lies in the ammonia potential which is the result of the pH and the ammonium content, product solubility and the interaction with the salt effect (osmotic potential) of the fertiliser product. When combined with the high rates of application in most cropping systems, ammonia and ammonia forming fertilisers (urea and DAP in particular) create the greatest risk for toxicity among the fertilisers commonly used.
As seeds are living organisms (even in the dry state), being exposed to ammonia can reduce viability. Both ammonia vapours in the soil air and ammonia dissolved in the soil water cause damage from ammonium fertilisers. The damage can commence as soon as the seed - ammonia contact takes place and is intensified by the duration of exposure, the ammonia concentration, crop species tolerance and seed metabolic activity (stage of germination).
During seed germination there are three distinct phases - water uptake, cellular activation and root and shoot elongation. Crop establishment failure involving ammonia damage occurs during the cellular activation and to a lesser degree during root and shoot elongation, whereas the osmotic effect of fertiliser is more pronounced during the cellular activation phase.
The osmotic effect of fertiliser results from the speed with which the dissolved fertiliser solution moves outside the application band, thereby becoming more dilute as it moves away from the band. This is dependent on factors such as soil compaction, soil moisture percentage, permeability of the soil, organic matter content, nature of the soil colloids, the fertiliser salts involved and the salt concentration in the soil water at the time of fertiliser application.
Root and Shoot Elongation
In a situation where fertiliser is placed with or in close proximity to germinating seed, it is not only the effect on the germination process that determines the success or failure of the crop to establish, but it is also the effect on the rate and magnitude of extension of the roots and shoots. The presence of ammonia or high concentrations of fertiliser salts can produce a marked reduction in both root and shoot elongation in most crop species. If the rate of root extension is less than the rate of drying of the soil, poor growth follows. This can be made worse by disturbance of the soil below the seed in heavier soil types.
On encountering high ammonia concentrations, roots become stunted, are generally more branched near the seed, are thicker and may have a brown, scorched appearance at the tip. The reduction in root elongation and /or thickening under some circumstances, can also precipitate problems with uptake of some of the more immobile nutrients such as phosphorus, zinc and copper. Placement of high rates of ammonium fertilisers (particularly urea, DAP and BIG N) directly below the seed line of tap-rooted crops such as canola and cotton should be avoided or only considered after consultation with a suitable qualified person.
Ammonia in close proximity to a germinated seed may reduce the elongation of the shoot at reasonably low concentrations in some species. This occurs as a result of the depletion of energy in the seedling as ammonia interferes with energy transfer pathways. Combined with the effects of certain seed fungicides, sub optimal soil temperature and sowing too deep, it is a common for crops that have had excessive ammonium placed with the seed to germinate, but the shoot to fail to break the soil surface.
In contrast, the osmotic effect is normally seen as a delay in emergence and in more severe cases a failure of the seed to germinate associated with the inability of the seed to imbibe water.
Crop Species Sensitivity to Fertiliser
Crop plants have a great diversity in their tolerance to the range of environmental conditions in which they grow. For example, sorghum is generally more moisture stress tolerant than maize, barley is more salt tolerant than wheat. Diversity among crop species has also been shown in the area of nitrogen fertiliser tolerance when applied with the seed at sowing.
Recent research work funded by Incitec Fertilizers has shown that the tolerance of the crop species to ammonium fertilisers placed with the seed at sowing is related to the fertiliser product (ammonia potential and osmotic potential ), the application rate and to soil characteristics such as moisture content.
Generally, the range of tolerance between crop species indicated in Table 1 does not appear to be large enough to recommend an increase the rates from those already suggested in fertiliser handbooks and other similar publications. However, the data in the table below does indicate the crop species where extra caution may be required when ammonium fertilisers are applied near the seed e.g. cotton and canola.
With the advances in our of the interaction of fertiliser and seed establishment in recent years, and the improved technology in sowing implements, the fertiliser application rate and its interaction with the soil environment is still the prime determinant of crop establishment in most years. For individual sites and in individual years, modest modifications to application rates according to crop species may be advised given the extra information now available. The safest application method for high rates of high ammonium content fertilisers is to place them away from the seed by physical separation (combined N, P products) or by pre or post plant application (straight N products). For the lower ammonium content fertilisers e.g. MAP close adherence to the safe rate limits set for the crop species and the soil type is advised.