Fertilizer Quality Is More Important Than Package Size

05-08-2000
As we move into the spring growing season home gardeners look to fertilizers to enhance both the lawns, gardens and landscape plantings. A general understanding of what is in a fertilizer package may help gardeners select the right material to meet their individual growing needs.

Before you proceed too far into lawn and garden fertilization remember that the best approach to any fertilizer program is to have a soil test made on the area to be treated. Soil testing information is available at all cooperative extension office in each county, at many larger garden centers and hardware outlets dealing with lawn and garden supplies.

Admittedly a soil test may recommend the application of lime and fertilizer to the tested area. However it is important to realize that the same soil test can recommend when lime and fertilizer is not needed or might even interfere with optimum plant growth.

When it comes to meeting the fertilizer needs of the lawn or garden you should purchase on a quality basis, rather than on bag size or price per bag. Value depends on the total amount of nutrients or plant food in the bag and the source of the nitrogen carrying portion in a turf grade material.

Law requires that the total amount of plant nutrients be shown on the bag. If the fertilizer contains slow release nitrogen materials, the per cent water insoluble (WIN) must be on the bag.

A complete fertilizer is generally recommended for most lawn and garden crops. A complete fertilizer is one that contains nitrogen, phosphate and potash. The percentage of each of these nutrients appears on the fertilizer bag. A 10-5-5 grade fertilizer would contain 10% nitrogen, 5% available phosphate and 5% water soluble potash. The nutrients are always listed in the order of nitrogen, phosphate and potash.

To determine product value, an 80 pound bag of 10-5-5 would contain 8 pounds of nitrogen, 4 pounds of phosphate and 4 pounds of potash for a total 16 pounds of plant nutrients. The remainder of the package is filler and carrier for the nutrients.

Plant response to the fertilizer is determined by the ratio between the nitrogen, phosphate and potash. Ratio of a fertilizer grade may be obtained by taking the lowest number in the grade and dividing it into each of the three figure in the grade. For example, in a 10-5-5 grade fertilizer, 5 is the lowest of the three numbers and when divided into each of the three numbers, will give a 2:1:1 ratio. Grades of 14-7-7, 16-8-8 or 20-10-10 would also have a 2:1:1 ratio. A fertilizer grade of 12-4-8 would have a 3:1:2 ratio.

Fertilizer ratios are an important considerations when selecting materials. Leafy crops like the lawn, lettuce, spinach or cabbage would benefit from higher applications of nitrogen or a 2:1:1 or higher nitrogen ratio. Most landscape plants and fruit producing crops in the garden do not need higher nitrogen levels and would be best served with a 1:2:1 or lower nitrogen ratio as might be found in a 5-10-5 grade of fertilizer.

Fertilizer grade affects the pocketbook as well as plant growth. Essentially the higher the percentage of nitrogen, phosphate and potash in a product the less will be needed to meet crop needs. Shopping for fertilizer should be like any other product. Calculate how many pounds of actual nutrients you are getting for the dollars invested. A large package of fertilizer with fewer nutrients may be more expensive in the long run. Make your purchases on quality and not quantity and your budget and plants will probably be happier.

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**Dr. Robert Nuss is a horticulturist at Penn State. He coordinates all extension horticulture programs. He has bachelor’s, master’s, and doctoral degrees in ornamental horticulture and has been on the Penn State faculty since 1966.