Too Much of a Good Thing? Garden Fertilization Tips

Author: Emily Morrow, WVU Extension Agent – Jefferson County 
A productive garden soil is one that receives a lot of care and attention, including efforts to build up the soil’s health and organic matter over time. A common misconception with fertilizer is if a little is good, then more will be better. 

When seeking advice on fertilization, you might come across common tips, such as putting 19-19-19 fertilizer on the garden at the end of every season. In theory, it seems to make sense – you’re putting on the same amount of each element each year. However, not all crops, soils or fertilizers are created equal. Proper fertilization is all about the right time, the right place and the right amount.

The Damage of Too Much Fertilizer

Excess fertilization leads to excess salt buildup in soils, resulting in reduced water uptake by roots. Plants may appear wilted, leaves may be browned or scorched, and growth can be stunted. Even manure, when too much is applied, can lead to nutrient leaching and excessive growth that inhibits proper crop development. If using manure, it should be aged for at least six months or composted properly. 

Understanding Fertilizer Numbers

While there are many important elements that contribute to soil health, the three that play the largest role in plant health are nitrogen, phosphorus and potassium. These three are often referred to, in shorthand, as NPK. 

When choosing a fertilizer, the three numbers on the bag indicate what percentage of that fertilizer is a source of nitrogen, phosphorus and potassium, in that order. A 50-pound bag of 19-19-19 fertilizer contains 19% N, 19% P2O5 and 19% K2O. A fertilizer that contains all three nutrients is known as a complete fertilizer, even if those nutrients are not present in the same amounts. A fertilizer that contains only one or two of the key elements is known as an incomplete fertilizer. In some situations, an incomplete fertilizer is the best choice to bring your nutrients up to an optimal rate. This is why a soil test is a vital first step to determining if you soil is in need of fertilization and how much is the proper amount.

Getting the Right Rate

1. First, soil test the area. Soil testing is free through WVU and provides recommendations on how to fertilize your soil, based on the crop you are going to grow. Visit our article on soil testing for more information on how to take a soil sample or contact your local WVU Extension office.
2. When the results are returned, the middle of the first page will list if your soil’s levels of phosphorus, potassium, calcium and magnesium were measured to be low, medium, optimum or in excess. The bottom of the first page provides fertilization recommendations (Figure 1).
3. A soil should only be fertilized if that nutrient is below optimum levels. 
4. If your soil test report shows phosphorus is below optimum, using 19-19-19 as the nitrogen source is an option for a few years until soil test phosphorus is in the optimum range.
5. If your soil test report shows phosphorus as in or above the optimum range, 19-19-19 is not the best option.
6. When your soil’s phosphorus is at or above optimum and soil test potassium is low, 0-0-60 (potash) is a good choice to build potassium.
7. When both soil test phosphorus and soil test potassium are excessive, which often occurs in garden soils, urea 46-0-0 is a good choice for nitrogen.

Your local WVU Extension agent can help you determine the correct type of fertilizer and the correct rate to supply the nutrients needed. Look at page two of your soil test report to contact your county agent.  

Figure 1. An example of lab test results on a soil test report.