Nutrition - Monitoring nutrition can avoid problems later

It is important to determine nutrient levels in growing media during early spring to help identify fertiliser requirements for crops sold early in the season and avoid deficiency or toxicity problems prior to despatch.

Many nutritional issues can occur in the previous season - heavy rain, drought, pesticide applications and water quality. To achieve accurate results, send compost and/or tissue samples to a specialist laboratory. Results will only provide a snapshot of nutrient levels at the time of testing rather than the whole picture. Note any liquid feeding or top dressing carried on the crops when looking at results. Take into account plant species - some genera are more sensitive to fertilisers than others.

Make sure that when you have a delivery of composts containing controlled-release fertilisers (CRFs) the material is used as soon as possible or stored in a cool, dry place on site to avoid nutrient release.

There is always a need for topping up fertiliser levels in container stock in early spring, especially if crops have not sold or were potted late season on outdoor beds or if crops have been exposed to a mild, wet autumn/winter. Choosing the crops to take samples from depends on potting date, nutrient requirement, growing medium and plant size.

Know the quality of your water source. Mains water or mixtures of rain, borehole, mains and recycled water can have an influence on nutrient availability and release. Taking samples during times of high or low rainfall, seasonal changes and changes of water sources will provide useful information on any changes to nutrient availability and uptake. The quality of the compost sample will reflect the quality of the results.

Container-grown crops

Collect samples from a minimum of eight pots for a representative volume. Note down how the affected crop is being irrigated and the type of growing bed being used. Laboratories may have different methods to analyse nutrients so ask how the results were achieved. Some peat-free and peat-reduced samples require different analysis methods. For plants that have been potted on, do not take core samples from the original root zone because this will represent nutrient levels of the original pot, not the finished crop.

Top tips for compost sampling

- Avoid sampling from dry or waterlogged composts and from bed edges, crops growing under gutters and near doors.

- Do not take samples from the top or bottom of containers and avoid including too many roots.

- Where there are clearly different symptoms within a crop, take samples from "good" and "bad" areas.

- Avoid taking samples from the sunny side of the container.

Field-grown crops

Take samples from a uniform depth. If a field has different soil types or topography, divide it up into uniform sections and note the differences for better interpretation of results.

Take at least 20 cores from the field section to try and produce an average sample. Do not be afraid of taking too many cores along the row. Walk the field in a "W" format to ensure a good representative sample, excluding headlands. An "X" format is good enough for fields smaller than 0.5ha.

Top tips for soil sampling

- Avoid areas of dense weeds, tramlines, gateways, lime heaps, field edges, paths, ditches, roadways and parts of the field that have had bonfires to remove plants.

- Record the depth of sampling.

- Avoid slopes and other areas prone to run-off.

- Identify the area of the field and split up accordingly into 1ha sections.

- Avoid poorly drained areas and taking samples from very dry soils or from mole hills.

- Do not mix topsoil with subsoil in one sample.

- Use an auger. A spade is the next best method.

- Do not take samples after heavy rain or recent fertiliser applications.

- Keep the sample cool (refrigerated) until analysis.

Interpreting results

Always check compost, soil and tissue results with your FACTS-qualified adviser.

Fertiliser choice

Some are appropriate for container-grown plants and others for field-grown stock.

Soluble fertilisers

Most commonly used, usually with an irrigation regime. Best used when plants are actively growing. Nutrients are mainly absorbed by roots with the nutrient content adjusted to specific ratios of N (nitrogen): P (phosphate): K (potassium). Some formulations also contain trace elements and cater to different growth phases and growing situations.

The NPK ratio determines the fertiliser use. A 24/10/10 product would indicate a higher nitrogen content to promote vegetative growth. The easiest way to deliver this type of product is to apply a stock solution via dilutor at 1:100. Multiple applications may be required.

Use a conductivity meter to make regular checks on nutrient concentrations. Feeding this way has a low risk of crop damage because the solution is weak in concentration and is delivered in large volumes.

Some products contain acidifying agents for use in specific water (hard/soft) situations. It is a practical method of applying fertilisers, especially if used through automated irrigation sprinkler systems. This method may lead to nutrient loss, greater waste factor and potential environmental contamination.

Top dressing

An easy way of delivering nutrients, especially if crops are under overhead irrigation or grown outdoors, which helps products to wash in the growing medium and allow uptake by plants. Like soluble fertilisers, these have specific ratios of NPK but the range of blends can be limited.

Some products contain large levels of fast-release ammonium nitrate, which can damage roots or foliage, so spread the product over the whole compost surface without touching any stems or leaves.

Some products are designed to last five-to-six months but they normally supply nutrients for two-to-three months. Benefits include seeing where the product was applied. This method often requires only one application. Products must be watered in to avoid feeder root damage and being blown off pots. Where some products have a sticking agent, it can take time to break down.

Foliar fertilisers

Formulations are designed so that nutrients are absorbed by foliage. Products can be in granular or liquid form and contain slowand fast-release nutrients. Normally used as a quick-fix on crops showing deficiencies during the growing season. Useful when applied in tank mixtures with pesticides towards the end of the season. However, use on soft growth can scorch crops, especially in high temperatures and high light levels.

Push-in CRF tablets

Plugs are conical and made of CRF prills bonded together. A small hole can be made in the compost and the plug pushed into it. The tablet then starts to release nutrients with good moisture levels and temperatures above 12 degsC. Some of the newest products are made by using water-soluble adhesive so the prill falls apart when plants are watered.

Applying tablets is laborious and they are not easily inserted in root-bound pots. However, once inserted they are invisible to customers, remain in the compost and have a good environmental profile.

Fully updated by Dove Associates

Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned


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