Reevaluation of Fertilizer Application Practices – Vegetable Crops Hotline

Reevaluation of Fertilizer Application Practices

Fertilizer prices have increased dramatically this year. More than ever, we need to think carefully about whether we are using fertilizers wisely. This article will be one of a series to help vegetable growers reevaluate their fertilizer use strategies.

One of the lessons I learned in Plant Nutrition class in graduate school was the 4Rs of fertilizer management principles. It does not matter if you have heard about this before. Now is a good time to review the 4Rs to reevaluate the fertilizer application practices.

The Four Rs stand for Right Source, Right Rate, Right Timing, and Right Placement.

Right source considers the price of the fertilizers, ease of application; synthetic or organic source; efficiency of nutrient use, etc. Most fertilizer recommendations for field-grown vegetables list pounds of a specific nutrient per acre. Different fertilizers have different nutrient content; thus, supplying the same amount of one nutrient would require a different amount of fertilizers with different nutrient content, which leads to very different price ranges. Whether to use a liquid or a solid form of fertilizers depends on the application systems. If fertilizers are supplied through an irrigation system, the solubility of a solid fertilizer is an important consideration. A decent amount of nutrients can come from organic sources. Be sure to consider the soil organic matter and previous crops in determining the required nitrogen. A controlled-release nitrogen source that delivers small amounts of nutrients over a more extended period may work better than a soluble source that offers a large amount of nitrogen at one time.

Right rate starts from a soil test. A soil test helps determine the portion of the crop nutrient requirement that is already available from the soil. The rest needs to be supplied through synthetic fertilizers or organic source nutrients. Right rate depends on cropping systems. A single fertilizer application system may require a different amount of a nutrient compared to a frequent fertigation system. Right rate varied by crops, even varieties. Developing the right rate recommendation requires intensive research over multiple years and varies among cropping systems. Unfortunately, due to limited resources in this type of research, that information is not always available for various vegetables grown with different systems in every region.

Right timing takes into consideration the growth pattern of a crop. The nutrient requirement is typically low at initial plant growth and increases quickly as plants develop and set fruit. It decreases as fruit ripens. When frequent fertigation is used, right timing and right amount correspond to different plant growth stages. Without frequent fertigation, split fertilizer application is preferred over one-time fertilizer application to meet nutrient needs and limit fertilizer loss as much as possible.

Right placement refers to placing the nutrient where the plants have the best access, the root zone. Two general approaches to apply fertilizers are banding and broadcast. The idea for banding is to place fertilizers right where the plant roots are. Applying preplant fertilizers right at the beds or applying fertilizers through drip systems are approaches to band apply fertilizers. Typically, this approach maximizes fertilizers use efficiency. Broadcast is to apply fertilizers uniformly over the entire field. Whether banding or broadcast, the decision depends on the crops’ types, the developing stage, the management system, and the nutrient itself. If the crop grows an extensive root system, broadcast applying fertilizers, especially in the later season is efficient. Nutrient such as phosphorus is easily fixed in the soil and becomes unavailable. Place phosphorus fertilizers in a concentrated area close to the root zone can increase the chance of phosphorus remaining soluble and being available for root uptake. So banding works more efficiently for phosphorus fertilizers.

The 4Rs principles are general rules of applying fertilizers. We will discuss these rules in more detail in future articles under a specific crop scenario.

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