Planning ahead for the next cotton season begins in winter. Nutrient management is a good place to start.
The primary nutrients we apply to our cotton fields are nitrogen (N), phosphorus (P) and potassium (K), and all are important to maximise yield. But crop use and the plant requirement for each nutrient differ for cotton and your management should be aligned to ensure high fertiliser recovery.
Our first action is to build your nutrition management plan – this includes finalising your application rates and what application method suits your operation.
To build your nutrition budget conduct soil tests across your fields to generate background analysis, consider your long-term cropping history, irrigation allocation and seasonal weather forecast.
The application rate of all nutrients will depend on several factors on your farm, including crop rotation, soil fertility and/ or soil constraints, yield expectation and water availability. At the very least it is recommended that application rates should account for the nutrients that will be removed from your expected crop. The below table consists of seed export rates of the key nutrients at various yields. For more information on this and a basic nutrition budget tool, download the Australian Cotton Production Manual.
Graph: Nutrient removal at various yields from Australian cotton systems (Rochester 2014).
Once you have finalised your fertiliser product and rate, we can organise the application method and placement. For this we need to understand the cotton major nutrients and what their role is:
The key element for all plant growth, N should be available to the crop throughout the growing season but primarily aimed at plant uptake through the flowering period. Matching N supply to crop N requirements requires close monitoring and management because N availability is affected by a range of physical, chemical and biological processes that occur in the soil. These processes are influenced by climatic conditions such as temperature and rainfall intensity. Irrigation deficits and incidence of waterlogging also affect the amount of nitrogen taken up by the plant, retained in the soil or lost to the environment. Therefore, the key to maximising the return from N inputs is in applying the right fertiliser, at the right rate, at the right time, in the right place.
The majority of fertiliser loss during the season occurs early when high amounts of irrigation water are applied to the N rich soil when plant use is particularly low. N losses occur through both gaseous emissions and lateral leaching from the field into the tail water. To counter this period of high fertiliser loss, and provide adequate N to the crop, growers should apply most of the crop fertiliser N during the growing season after the first or second irrigation.
(Pictured above: variable cotton growth due to poor pre-season N fertiliser application).
P is particularly important for early plant growth, as P deficiency causes reduced seedling vigour, poor plant establishment and root development, delayed fruiting and maturity. Plants deficient in P will appear stunted with a red/purplish colour.
P is highly immobile in the soil meaning that it predominantly stays where it is applied. This makes the application of P fertiliser challenging in cotton systems because cotton roots do not congregate in areas of high P concentration like fibrous root systems of cereals plants and are not particularly good at finding bands of P in the soil. The aim of fertiliser P application should be to treat the largest volume of soil possible and be available throughout the soil profile where plant roots are active. Treating a large area maximises the fertiliser interception in the soil by the plant roots. Generally, only about 20–30% of the P applied as fertiliser is used by the crop in the year of application, with the remaining P requirement coming from other sources of P in the soil, of which fertiliser application in previous years has contributed.
Adequate K nutrition has been linked to reducing the incidence or severity of plant diseases and improving yield and fibre quality. There are several forms of K found in the soil and while most soils have large amounts of K only a small proportion (less than 2 per cent) is available to plants.
Premature senescence is a potassium-related disorder that can occur in cotton regardless of the supply of K from the soil. Other nutrients, including phosphorus, are deficient in affected plants, although not to the same extent as K. The disorder is chiefly caused by the imbalance between a plants nutrient demand due to a high boll load, and the plants' inability to meet this demand. Premature senescence can be compounded by stresses such as waterlogging, cool, cloudy weather or soil compaction which interfere with the plant’s ability to take up K, reducing the plants' capability to meet crop demand especially during the period of peak demand between flowering and boll fill.
When deficiencies are experienced later in the season, as the developing boll load is a strong and competitive sink for available K, the youngest mature leaf at the top of the canopy is often the first to show symptoms.
Zinc (Zn) is essential in small amounts for enzymes and plant hormones. Deficiencies can be seen in the leaves as interveinal chlorosis, cupping and possible bronzing, stunting, and may affect yield, maturity and fibre quality. Zinc is best applied to the soil as a broadcast and worked in with cultivation. Zn can also be successfully applied to crops as a foliar spray, for example, Zinc sulphate, which is the most effective and inexpensive form of Zn to apply to the soils or the crop as a foliar spray but is very restricted in its compatibility for mixing with early season crop protection products.
Iron (Fe) is an essential nutrient required in very small amounts for chlorophyll synthesis and some enzymes. Plant symptoms include interveinal chlorosis of the young growth and yellowing of the leaves. Although plentiful in the soil, most of the iron in soils is unavailable to plants. Availability is greatly affected by high concentrations of cations particularly manganese. Applications of P and Zn fertiliser can also reduce iron uptake. Waterlogging can lead to deficiencies in alkaline soils. Deficiencies are generally short-lived when related to waterlogging events and can be managed via a foliar application for most cotton soils.
Other essential nutrients such as copper, boron, calcium, magnesium, sulphur, manganese and molybdenum all have very specific roles to play in meeting the nutritional needs of a cotton crop. They are required in very small amounts and deficiencies are very rare.
During field preparations we can start the process of pre-season fertiliser application. Placement will differ according to when we apply the fertiliser and which nutrient we are applying
- P & K – During the bed forming operations or prior to cultivation is a particularly good time to apply the non-soluble nutrients such as P and K based fertilisers. The immobile nutrients should be mixed thoroughly throughout the planting hill to ensure high root interaction during the growing season. It’s crucial for P and K to be dispersed throughout soil for greater plant availably and uptake, so blended fertiliser should be spread and incorporated through tillage, this provides a greater chance for interception by plants roots and greater recovery by the upcoming cotton crop.
- Zn - Liquid Zn fertiliser such as zinc sulfate can be sprayed onto the surface and worked into the planting hill through cultivation prior to planting.
- N - In situations where high rates of N are required it is best to split the application amount with low to moderate levels applied pre-season and the remainder applied in-crop. The pre-season application of N fertiliser is best applied during the cooler months (June to August) and should be drilled to depth and offset towards the irrigation furrow (if applicable) to compensate for lateral movement within the plant bed during irrigation. Pre-season application of N is aimed to ensure early-season plant growth is not limited. N is highly mobile, therefore placement should aim to minimise the risk of losses through heavy rainfall or applied irrigations. Preseason N should be applied to depth (~25-30cm), and if possible, offset towards the irrigation furrow (this improves N saturation of the plant hill and reduces any chance of fertiliser burn to plant roots).
Small levels of P and Zn can be applied with the seed through liquid injection. This application may aid early season growth in soils with poor fertility and soil constraints.
This is the primary period to apply the bulk of the crop’s N requirement. Ensure N rate aligns with yield expectation and the application is timed to provide sufficient N fruiting demand and boll development. The application should be aligned with an irrigation event to ensure the immediate transformation of fertiliser N to mineral N which is available for crop uptake. Side dressing urea or anhydrous ammonia will reduce potential gaseous emissions. Water run N based products can be used during this period, but high application variability can occur if your applicator is not calibrated correctly to the irrigation system.
Mid to late season
Application of fertiliser mid-to-late in the growing season does have low recovery rates and the benefit to the current is not high, but they may aid to counter crop deficiencies i.e. deficiencies cause by severe waterlogging. For this purpose, a wide range of foliar products are available to enhance plant response to various deficiencies.
If higher application rates are warranted, options include spreading urea, followed by irrigation or water run liquid urea provide the more efficient application method in a cotton system compared to water run anhydrous.