It seems I constantly get asked these two questions. “When should I fertilize my yard and how much do I need to use?” There’s no cookie cutter response to that one.
In general, fertilizer should only be applied during or right before plants are in active growth. This is especially true for nitrogen as it is easily lost by the soil. When plants don’t absorb nitrogen right away, part of what remains in the soil is broken down. The rest winds up in groundwater or our waterways.
This dormancy rule does not apply to adding lime to soil. Lime takes time to react, so applying in fall is often a good idea. I try to refrain from liming and adding nitrogen fertilizer at the same time anyway. Ground limestone and hydrated lime can cause denitrification, releasing nitrogen gas into the air, wasting money.
So why do we need lime anyway? Lime raises soil pH and makes plant nutrients more available. In short, fertilizers are utilized more efficiently and plant growth is better. However, if some is good, too much is not better.
This is especially true around most evergreen shrubbery, which should be maintained in moderately acidic conditions. Some plants such as azaleas, rhododendrons or blueberries should grow in strongly acidic soils, so they rarely if ever require additional lime.
Nitrogen is the most discussed nutrient when fertilizing lawns. Nitrogen deficiency results in stunted yellowish growth. Turf rarely requires additional phosphate except during establishment where it is important for young roots. Potassium is another story.
Potassium (potash) contributes to drought tolerance, cold hardiness, and disease resistance. It’s held tighter and not lost by soils as much as is nitrogen. That’s good. Fall is a great time to add potassium, while fall nitrogen fertilization is only effective if turf is in active growth.
Fertilizing lawns in summer can be risky. If rainfall is limiting, fertilizer can do more harm than good. Most of our fertilizers are applied as a mixture of salts. These can cause plants to lose moisture. We call that fertilizer burn.
Shrubbery generally should not be fertilized with nitrogen in the fall. Fertilizing after plants go dormant simply means it will be lost. A worse problem results when plants are still growing. Winterkill is a major possibility with fall nitrogen fertilization.
All of these problems are magnified if we use high analysis fertilizers. In theory, there’s no difference in applying 50 pounds of 10-20-20 compared to 100 pounds of 5-10-10. We can calculate the proper amount to use, but applying it uniformly is another story. Overlapping too much could be problematic using concentrated fertilizers.
Some folks think they should always add nitrogen to plants with yellowish color. This is not always true. The best thing to do is perform a soil test. It won’t reveal the level of nitrogen in the soil but nitrogen might not be the problem.
Sometimes yellow color is the result of low iron availability, a phenomenon called iron chlorosis. Wet soils, excessively dry soils and low light intensity can exacerbate this. It can be prevalent in lawns, shrubbery, flowers and vegetables. Adding iron or lowering pH slightly can change foliage color in as little as a couple days.
Ted Manzer teaches agriculture at Northeastern High School (firstname.lastname@example.org).