Fertilizing Your Garden

bag of fertilizerQuestion: I’ve had a vegetable garden for years. I’ve taken good care of the soil by adding different kinds of manure and compost for many years. The garden hasn’t produced very well the past couple of years so I had a soil test done as you suggested. I’m a little confused by the results. It looks like the only thing my garden needs is nitrogen. Is that right? What should I use? I’d prefer an organic fertilizer. D.M.

Answer:  Your soil test results showed excessive – off the chart – levels of phosphorus and potassium. (In fertilizers nitrogen (N) is the first number, phosphorus (P) the second, and potassium/potash (K) the third.) Soils in southeast Wisconsin are rarely deficient in P and K and repeated applications of manures and composts, which contain fairly high amounts of P and K, over many years increased the soil P and K. Excessive soil phosphorus can runoff into nearby streams, lakes and other waterways causing algae blooms and other problems. Very high levels of phosphorus can interfere with the plant’s ability to take up the vital nutrients iron and zinc. Don’t use manures, composts or any product high in P and K for three to five years. Over time the growing plants will use the excess P and K. Retest the soil in three to five years to determine the P and K levels. The results will tell you if you need to apply P and/or K.

You will need to apply nitrogen to your vegetable garden. Finding an organic source high in nitrogen, but low in phosphorus and potassium, is a bit challenging. Organic products can vary in nutrient levels but organic fertilizers will have the N-P-K percentages on the bag. Chose a product with the least amounts of P and K. Organic sources with high N, low P, and low K are feather meal 12-0-0, blood meal or dried blood 13-2-0, fish meal 9-3-0, fish emulsion 5-2-2, alfalfa meal 3-0.5-3, soybean meal 6-1-2, Milorganite 5-2-0.

A couple of nitrogen natural products are problematic for a vegetable garden. Chilean nitrate (sodium nitrate) 16-0-0 can be purchased alone or as a component of the product Pro-Booster 10-0-0, which is made from vegetable proteins, animal proteins, and Chilean nitrate. Although Chilean nitrate is natural, the USDA National Organic Program severely limits its use in organic production. Corn gluten meal 10-0-0 prevents seed germination so it should not be used in a vegetable garden as a nitrogen fertilizer.


Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from the UW-Madison. If you have a plant or gardening question, email Larson at barbara.larson@kenoshacounty.org or call her at 262-857-1945.