Using 4R’s for Nitrogen Management in Horticultural Crops

Nitrogen is a major input for crop production in Ontario. However, nitrogen use efficiency (NUE) is low, with only about 40-65% of the total applied fertilizer N recovered in crops. Therefore, effective N management provides opportunities for enhancing fertilizer nitrogen use efficiency and for reducing the environmental footprint of crop production, which minimizes fertilizer requirements and saves input costs. This is possible by using 4R nutrient stewardship.

The 4R’s are all about making key decisions on fertilizer products along with the timing, placement, and rates of application. And high fertilizer nitrogen prices are providing an incentive to manage N this spring with as much efficiently as possible.

Most chemical N fertilizers used in Ontario are ammonium-based that are usually applied from mid-April thru June. For the most effective use of N fertilizers, split application is suggested compared to one-time application which means application of N fertilizers two or more times to the crops to better time fertilizer N with crop demand.

For example, an in-furrow or band application of starter N at planting followed by side dressing of N fertilizer in June when seedlings are at V6-V7 stage of their growth is effective in corn. But in non-fertigated fresh market tomatoes, pre-plant application of nitrogen followed by side-dressing after the first fruits are set is recommended. You should avoid late side dressing to protect the roots and to minimize blossom-end rot.

On the other hand, the most efficient uptake of soil-applied nitrogen into the tree occurs during active spring growth, so it is recommended to apply nitrogen in the spring after active tree growth begins in tree fruits. Application of nitrogen in a 1- to 2-foot band under the drip line or broadcast throughout the orchard are common but it is important to note that we should increase the rate by 20%–30% for a broadcast application compared to side-banding. The fact we increase the broadcast rate tells us that broadcasting fertilizer is not as efficient. We can lose up to 50% of the N through ammonia (NH3) volatilization compared to banding or in-furrow application.

Fertigation is an efficient method of nutrient application in fruits and vegetables in which fertilizers are injected through an irrigation system. In this system, nutrients are applied daily or weekly compared to 1-2 applications in soil application of granular fertilizers. Therefore, synchronization of nutrient supply and crop demand enhances nutrient-use efficiency. It also reduces labor and ground water contamination.

Variable Rate Technology (VRT) is another tool that allows farmers to apply fertilizers, water, chemicals, and seed at different rate across a field. There are two types of VRT: map-based and sensor-based. Map-based VRT adjusts product application based on a pre-generated map of the field. However, sensor based VRT uses mounted sensors that measure soil properties or crop characteristics in real-time.

The adoption of Best Management Practices such as addition of organic manures or compost, crop rotation with legumes, cover cropping and green manuring can add 10-200 kg N/ha, depending on the amount applied, crops selected, soil types, and growing condition.

Also, the use of enhanced nitrogen fertilizers such as slow-release fertilizers, controlled release fertilizers and those containing nitrification or urease inhibitors also minimize N losses and improve crop nitrogen use efficiency in field and horticultural crops.

Soil and plant tissue testing might help you to determine how much nitrogen is required for your crops. Large soil test deficiency can be addressed at pre-plant while in-season demand can be met through fertilizers and foliar applications. Therefore, spending few dollars in testing soils every three years or plant tissues every year could return thousands from your farm. It can also save money on other nutrients and find pH problems.

Tejendra Chapagain

Soil Fertility Specialist, OMAFRA