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Sunflower nutrition and timing of fertilizer applications

July 10, 2019

Plant nutrition is key in determining sunflower yield and oil quality, and there are several issues to keep in mind when planning and executing fertiliser strategies.

Plant nutrition

Along with carbon, hydrogen and oxygen, plants need mineral nutrients to grow.

Mineral nutrients are dissolved in soil water and are taken in by the plant through the roots, and there are 13 known elements which are essential for plant growth.

Mineral nutrients

Nutrients can be divided into two groups:

Macronutrients

Macronutrients can be further broken down into two more groups, known as primary and secondary nutrients. 

Primary nutrients are used by crops in large quantities, typically at the rate of kilograms per hectare, and are usually the first nutrients to be found limiting yield.

They include nitrogen (N), phosphorus (P), potassium (K).

Secondary nutrients are calcium (Ca), magnesium (Mg), and sulphur (S), and are usually found in enough quantity in the soil so an additional fertiliser is not always needed.

Micronutrients

Sometimes referred to as trace elements, these are required in smaller amounts than the macronutrients, typically at the rate of grams per hectare, and include iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), boron (B), molybdenum (Mo) and chlorine (Cl).

Liebig’s law

A deficiency of any one of these elements, macro or micros, can limit yield and the less efficient use of the other nutrients.

This is known as Liebig’s law of the minimum, a law that states plant growth is dictated not by total resources available, but by the scarcest resource, and is often pictured as the staves of a barrel.

The staves of the barrel represent individual nutrients; the lowest stave in the barrel represents the least available nutrient that will limit the amount of water (yield) the barrel will hold.

Once the least available nutrient is supplied, another nutrient will then become the least available.

In other words, if your soil is deficient in phosphorous it doesn’t matter how much nitrogen fertiliser you apply, the crop will only ever yield relative to the available phosphorous.

This is an oversimplification of the complex interaction that plants and crops have with soils and nutrient availability, but the basic principle is true.

Nutrient flows and soil fertility

As a crop is harvested and grain leaves the field, and ultimately the farm, it takes with it minerals from the soil.

Most major elements are returned to the soil in plant remains; stems, leaves, roots, etc., however material that leaves the farm will remove elements from the nutrient budget.

For example, a six-tonne crop of wheat will remove 25kg of phosphorous and 55kg of potassium per hectare, and, unless those nutrients are replaced, soil fertility and crop yield will fall.

Nutrients are replaced with fertiliser, legumes (peas, beans, clover) that fix atmospheric nitrogen, and if animals are part of the farming system, manure and slurry that is returned to the fields.

It’s this process of replacement and recycling that helps maintain soil fertility and support crop yields.

In addition to recycling, avoiding or reducing the loss of nutrient by minimising leaching is equally important.

This can be achieved by utilising cover crops to lock up nutrients in plants and leaving no overwintered fallow land, and, if there is livestock in the system, applying animal manures in the spring and summer to reduce rain washing nutrients off the field.

Sunflower nutrition

Accurate soil sampling and testing are critical to determine the availability of nutrients and to target fertiliser applications across the whole crop rotation.

Knowing how much phosphorous and potassium is available in the soil will allow you to apply the balance required by the crop, or, if nutrient levels are high, avoid applying fertiliser, reducing cost and protecting the environment.

Sunflower seed is sensitive to fertiliser so aim to limit the amount placed in close contact to the seed at planting.

Nitrogen

Sunflowers are a deep-rooting plant and access nutrients from depth in the soil profile and will usually yield well at quite low levels of soil nitrogen

High levels of nitrogen can lead to excessive vegetative growth, increased diseases, delaying maturity and reducing seed oil content and quality, including a reduction in protein and oleic acid.

Low levels of nitrogen will limit crop yield.

Research suggests that nitrogen applied just before floret initiation will influence the final number of seeds, while later applications from floret growth to anthesis will influence single seed weights.

Phosphorus and potassium

Phosphorus and potassium applied in fertiliser moves slowly through the soil, managing these nutrients for yield is based on maintaining levels in the soil for the needs of the rotation, rather than an individual crop.

Once the soil is deficient in phosphorus or potassium, a fresh application is unlikely to be available for uptake by roots in time to benefit the crop being grown.

To maintain soils at the correct Index it is usually enough to replace the amount of each nutrient expected to be removed from the field in the harvested crop.

Soils should be sampled every three to five years to check if phosphorus or potassium levels are being maintained, building up or running down, and an appropriate fertiliser strategy can be put in place.

Sunflowers use low levels of phosphorus, so applications of 40 to 60 kilograms of P2O5 per hectare is usually enough to replace off-take by the crop and maintain soil levels.

Sunflowers use high levels of potassium, but most are returned to the soil after harvest, so applications of 40 to 60 kilograms of K2O, will maintain soil levels.

Micronutrients

Sunflowers are sensitive to boron deficiency, particularly on calcareous and sandy soils, look out for poor seed-set and large areas of hollow seeds.

Based on the soil analysis data, Potassium, Boron, and plant activators and bio stimulators can be applied during Fungicide and/or Insecticide treatment to help to achieve better seed fertilization during the flowering time and get better yield at harvest.

Soil acidity

Soil pH is the measure of how acid or alkaline the soil is and affects the availability of nutrients; macronutrients are less available in soils with low pH (acid); micronutrients are less available in soils with high pH (alkaline).

Lime can be added to the soil to reduce acidity and supplies calcium and magnesium that plants can use.

Lime raises the pH to the desired level, in soils with a pH range 6.0 to 6.5, nutrients are more readily available to plants, and soil microbial populations increase, which is a good thing because soil microbes convert nitrogen and sulphur to forms that plants can use.

Lime also changes the physical structure of the soil, helping to improve water and air movement. 

Have your soil tested to determine how much lime and fertiliser your crop needs, look out for indicators of soil acidity such as acid tolerant weeds and failed crops, and consider buying a portable pH meter.

Plant nutrition, fertiliser and the environment

Agriculture is the third largest contributor to greenhouse gases (GHG) by sector:

  • Nitrous oxide, a GHG 300 times more potent than carbon dioxide, comes from the production and use of nitrogen fertiliser.
  • One of the main sources of water pollution are nutrients (phosphates and nitrates);
  • Eutrophication is when water becomes overly enriched with nutrients (nitrates and phosphates) causing excessive algae growth leading to the loss of oxygen.
  • Excessive amounts of nitrates and phosphates must be removed before the water can be used for drinking.

Agricultural land must be managed carefully to avoid losses of soil and nutrients:

  • Maintain the correct level of phosphate, avoid any unnecessary build-up.
  • Reduce soil erosion by minimum cultivations or direct drilling, and utilising cover crops.
  • Avoid surface applications of animal manures when soils are snow-covered, frozen, waterlogged, or steeply sloping.
  • Apply fertiliser in several small amounts rather than as a single large application.

Contact us for more information. We have a range of sunflower hybrids suitable for different requirements.