Foliar or Soil Application: Which, and When

Soil application feeds the root system and supplies the crop over the whole season. Foliar application puts nutrients directly on the leaves, where they can be absorbed quickly. Each route has different strengths and limits, and most practical programmes use both. The question is not which route is better in general, but which one suits the nutrient, the crop stage and the problem in front of the grower.

Foliar feeding has a long history in horticulture, where it was often used to correct deficiencies that appeared during the season. Its use has grown in arable and plantation crops as growers have learned that it can complement the soil supply, particularly for micronutrients and at critical stages such as flowering and grain filling. The key is to use it for a clear purpose, not as a general tonic.

Strengths and limits of soil application

Soil application supplies large amounts of nutrient and builds a reserve in the soil, which the crop can draw on through the season. It is the only practical route for the main macronutrients, because the crop needs far more of them than a leaf can absorb in a single spray. It also allows the nutrient to be placed where the roots are, and it can be timed around the crop's uptake curve.

• Supplies a large amount of nutrient and builds a reserve in the soil for later use.
• Depends on root activity, soil moisture and soil chemistry, so uptake can be slow when conditions are cold, dry or waterlogged.
• Some nutrients become fixed in the soil and are not available to roots when applied this way, particularly phosphorus and micronutrients on high-pH or high-clay soils.
• Requires an application pass through the field, which costs fuel and labour, and may be difficult to time in wet seasons.

Strengths and limits of foliar application

Foliar application delivers small amounts of nutrient quickly, which makes it useful for correcting a shortage that has been diagnosed during the season. It can also bypass soil fixation, which is one reason it is used for micronutrients such as zinc, manganese and iron on alkaline soils. For some crops, a foliar spray at a critical stage can support flowering, fruit set or grain fill when the roots cannot supply enough.

The limits are equally clear. A leaf can take up only a small amount of nutrient at a time, so foliar sprays cannot replace the main supply of nitrogen, phosphorus or potassium. Concentration is limited by the risk of leaf scorch, and the leaf surface is waxy, so absorption depends on the product, the droplet size, the humidity and the time of day. A spray that is too concentrated or applied in strong sun can damage the leaf rather than feed it.

• Delivers small amounts of nutrient quickly, useful for correcting a deficiency during the season.
• Can bypass soil fixation, which helps with some micronutrients on alkaline or high-iron soils.
• Cannot supply large amounts of nutrient, so it works as a supplement to the soil programme rather than a replacement.
• Leaves can be scorched if concentration, timing or spray coverage is wrong.

Choosing the route

A useful rule is to use soil application for the main nutrient supply, and foliar sprays to correct a diagnosed shortage or to support a critical stage. The soil programme sets the base, and the foliar programme handles the exceptions. This keeps the total cost low while giving the crop a response when it needs one.

The decision also depends on the nutrient. Nitrogen and potassium are usually supplied to the soil, because the crop needs large amounts. Micronutrients such as zinc, boron and manganese are often better handled with foliar sprays, especially when soil tests show that the soil supply is locked up. Phosphorus is a case in between: foliar sprays are of limited use for the main supply, but they can help when the root system is weak early in the season.

Timing and conditions for a spray

Foliar sprays work best when the leaves are open and the air is moist, which usually means early morning or late afternoon. Avoid spraying in the heat of the day, when the droplets dry quickly and the leaf may be scorched. Avoid spraying just before rain, because the product will be washed off, and avoid spraying a stressed crop in drought, because the leaf may not be able to absorb it efficiently.

Spray volume and coverage matter. The aim is to wet the leaves evenly without run-off. Calibrate the sprayer, use the volume recommended on the label, and check the coverage on a sample of leaves. A spray that misses the lower leaves will give uneven results, and a spray that runs off the leaves wastes product and can cause soil contamination.

Planning a combined programme

A practical programme sets the main nutrient supply in the soil, then uses foliar sprays to support the crop at known sensitive stages. Plan the sprays around flowering, fruit set or periods of stress, rather than at fixed intervals. Record the product, concentration, weather and crop response for each spray, so the programme can be improved in the next season.

• Use a soil test and a tissue test to decide which nutrients need the soil supply and which need a foliar correction.
• Time foliar sprays to the crop stage and the weather, not to the calendar.
• Keep the total number of sprays low, because each pass adds cost and some leaf damage risk.
• Check the crop two weeks after each spray to see whether the response is what was expected.

Foliar sprays and crop protection products

Growers often want to combine a nutrient spray with a crop protection product, to save a pass through the field. This can work, but it is a risk. Some nutrients, particularly calcium and magnesium, react with pesticides and change their activity or cause precipitation. Check the label of each product for tank-mix permission, and run a small jar test before a full tank is mixed.

Adjuvants and surfactants change the wetting and spreading of the spray, and they can improve uptake of foliar nutrients. They also change the risk of leaf damage. Use only the adjuvant recommended on the label or by the supplier, and keep to the rate specified, because more adjuvant does not give better results.

Recording the response

Record each foliar spray with the product, the concentration, the date, the crop stage and the weather. Two weeks later, record the crop response, including any leaf damage. Over several seasons, the record shows which sprays gave a clear response and which did not, and it helps the grower decide where foliar feeding is worth the cost.

Common questions

Is foliar feeding a substitute for soil fertility? No. Foliar sprays correct short-term shortages or support critical stages, but they cannot supply the large amounts a crop takes up over the whole season. A healthy soil and a sound soil programme remain the foundation.

How many sprays are too many? There is no universal limit. Each spray should have a clear purpose and a label-approved concentration. Repeated sprays increase the risk of leaf damage and the cost, so each one should be justified by a diagnosis or a stage target.

Can I spray in the afternoon? Late afternoon can work in cool weather, because the leaves are still open and the spray dries more slowly. In hot weather, avoid the afternoon altogether, because the leaves close and the product can dry before it is absorbed.

Recommendations depend on local soil, climate and crop conditions. Follow the product label and local regulations, and consult a qualified agronomist for field decisions.