Archive for Wednesday, March 20, 2013

Garden Calendar: What to know about spring fertilizer applications

March 20, 2013


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How to collect a soil sample for testing:

• Identify an area such as a lawn, garden, flower bed, or around a specific tree(s).

• Take 10 or more slices of soil with a trowel, spade or knife from random locations within the given area. Slices should be 6-8 inches deep for gardens and flower beds, 3-4 inches deep for lawns, and 8-10 inches deep for trees and shrubs.

• Mix the slices together and remove rocks, mulch, plant roots, etc.

• Bring 2 cups of the soil to K-State Research and Extension-Douglas County, 2110 Harper St., Lawrence, between 8 a.m. and 5 p.m. Monday through Friday.

Upcoming classes:

• Gardening 101, 9 a.m.-noon April 6, pre-registration required, $10 or $15 if taken with Gardening 102 on Aug. 24, Dreher Family 4-H Building, 2110 Harper St., Lawrence

• Spectacular Shrubs, 10 a.m.-11 a.m. May 15, Dreher Family 4-H Building, 2110 Harper St., Lawrence

• Growing Growers, workshop series and apprenticeships available, see

Please call 843-7058 or visit for more information about upcoming classes.

Warmer weather should be around to stay soon, and my garden gloves and pruners are already waiting by the door for their chores.

These early days of spring are a good time to give many species of plants supplemental nutrients in the form of fertilizer, but how much and what kind really depend on what you are trying to grow and what your soil has already.

This probably sounds more complicated than picking up a bag of your favorite rose food, bloom booster, steer manure, tree spikes, etc. There seems to be a fertilizer for everything. Taking the time to get it right will save you money and help your plants grow better, though.

The first step to getting the right fertilizer is determining what nutrients are already available from the soil in your garden or other area where you are trying to grow plants.

Soil can be tested for pH and nutrient values through K-State Research and Extension. In Douglas County, we are fortunate to have a grant from the Douglas County Conservation District that will pay for up to 10 basic soil tests per resident per year. The regular price for the basic analysis is $8 per sample; other tests, such as organic matter content and micronutrient contents, are available for a small additional cost-recouping fee.

With the soil test results you will receive recommendations based on the needs of the plants you are trying to grow. Vegetables require more phosphorus and potassium than shade trees, for example.

A soil test will also provide you with recommendations on when to fertilize. Trees, shrubs, perennial flowers, and other similar ornamentals are the best fertilizer in early spring when new growth begins. Fruit trees are also best fertilized in spring. For vegetables, work fertilizer into the soil prior to planting, then add fertilizer as needed to specific crops. For irrigated lawns, wait until May and use a slow-release product. For nonirrigated lawns, skip the spring application and fertilize in September.

In the absence of a soil test, use a fertilizer that contains primarily nitrogen unless you know your soil is deficient in phosphorus and potassium. Soil in this area, especially soil with high clay content, often has adequate levels of these nutrients already, so adding more is unnecessary. In some states, fertilizers containing high percentages of phosphorus have even been banned from use unless a soil test has proven deficiency. Some researchers argue that the excess phosphorus pollutes waterways and groundwater.

Remember all those fertilizers for every different kind of plant? Ignore that part of the label and look for the three numbers in a row (such as 12-0-0 or 27-3-3). The first number indicates the percentage of nitrogen, the second number is the percentage of phosphorus, and the third number is the percentage of potassium.

Paying attention to the percentages of nutrient can also help you get the most bang for your buck — especially when you realize a fertilizer with 3 percent nitrogen is not a quarter of the price of a fertilizer that is 12 percent nitrogen.

The other good news about the fertilizer selection is that plants are still unable to tell the difference. Rose food might be just the thing for your tomato plants, or the lawn fertilizer could be the best thing for your apple tree. Determine how much nitrogen or other nutrients your plants need and make your fertilizer selections based on that information rather than a pretty label. The only caution here is if the fertilizer contains herbicide, which will also be listed on the label.

Finally, a word about trees: For fruit trees, fertilization may be necessary to sustain production. Overfertilization will also lead to a decrease in flower bud production, though, so have your soil tested to determine what your fruit trees need. Ornamental trees rarely need supplemental fertilization. If a tree is showing signs of stress, a soil test may determine if it has a nutrient deficiency and what to apply. If fertilizer is needed, granular fertilizer can be broadcast over the root zone with a formula based on either the size of the tree or the size of the root zone area.

— Jennifer Smith is the Horticulture Extension Agent for K-State Research and Extension in Douglas County. Contact her or an Extension Master Gardener at 843-7058 or


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