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Crop scouting for nutrient deficiencies

By Ken Panchuk, PAg, Provincial Specialist Soils, Regina

Wheat field showing nitrogen deficiency
A wheat field where the nitrogen metering
system stopped during seeding showing strips of
pale green colour typical of nitrogen deficiency.
Crop scouting is an important practice in crop production that begins with regular visits to each field, looking for clues that may indicate that the crop’s health is not optimal. Generally, nutrient deficiencies show up in patches within fields and very seldom whole fields. When patches that are performing poorly are identified and marked using GPS, then the task of determining the cause or causes can commence.

Today, crop scouting can be done by walking fields, using digital technology like drones with sensors and handheld optical sensors, or by using the services of a crop consultant and a precision agriculture service provider. Regular scouting is required because growing conditions change, crop pests may appear or patches of crop nutrient deficiency symptoms may become more pronounced at later growth stages.

Plant tissue analysis is a measurement of the concentration of nutrients in similar plant parts, at the time of sampling, giving a snapshot of the plant’s health. Plant tissue testing is not a substitute for spring or fall soil testing. It is used to monitor the progress of a growing crop, to diagnose problem areas in a field and to detect nutrient shortages, hopefully before symptoms appear. 

Tissue testing has its limitations. Nutrient content varies with the plant part selected, growth stage and even variety. For diagnostic purposes, a representative tissue sample should be collected from the problem area, as well as a sample from a nearby normal area of the same field for comparison.

For a more comprehensive diagnostic tool, use a comparative tissue plus soil test. Take representative tissue and soil samples from the affected area and representative tissue and soil samples from a healthy area nearby within the same field. The soil analysis will be used to help confirm the nutrient deficiency shown by the tissue sample. 

Nitrogen (N) deficiency symptoms will appear on the lower leaves first and will be pale green to various degrees of yellowing. Keep in mind that there are other causes of these symptoms, such as low soil moisture conditions and/or normal senescence (dieback) of older leaves.

Phosphorous (P) deficiency symptoms are more difficult to diagnose visually. P is a regular input, plus the P soil bank can buffer P needs, which makes P deficiencies rarer than other nutrient deficiencies. Deficiency symptoms in the field will generally consist of spindly plants in patches, purpling under severe deficiencies.

Potassium (K) deficiency can be seen as yellowing of the leaf margins. Near maturity, lodging may be a clue that those patches are low in K. Look in fields where straw is continually being harvested for livestock use and where the livestock manure is being applied to other fields. K deficiencies are common in the north-east region of Saskatchewan, as well as sandy soils throughout the province.

Sulphur (S) deficiencies generally occur in patches and most commonly affect canola crops. Yellowing of the newest leaves and/or cupping with reddening of the leaf margins are indicators of S deficiencies in canola.

Nitrogen (N) deficiencies can be corrected by dribble banding (or split nozzles) liquid N fertilizer (with or without Agrotain) at the appropriate rate. For P, K and S, it becomes more of a rescue treatment. There are some P and K liquid products for foliar application at low rates that will provide some yield rescue. P and K are best banded at seeding time. S can be broadcast as ammonium sulphate on whole fields or in the patches. Treatments applied prior to bolting stage will give the best yield recovery. Moisture is needed for all post-emergent nutrient applications to effectively move most of the nutrients into the soil for root access. Foliar uptake of macronutrients is generally limited for field crops.

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