Soil Test Summary for Urban Lawns and Garden Soils

Published: January 1, 2010

Missouri homeowners use many different fertilization practices to manage their lawns and gardens. Most of the soil samples received by the MU Soil and Plant Testing labs from homeowners for lawn and gardens test very high in Phosphorus and Potassium. This happens as a result of applying fertilizers and organic nutrient amendments without testing the soil first, causing soils to be overloaded with certain nutrients resulting in imbalance in nutrient uptake and poor plant growth. Urban lawns and gardens have been identified as one of the major sources contributing to non-point source pollution due to over application of fertilizers. So, I decided to study the soil test summary for lawn and garden soils from urban areas to educate homeowners about the importance of soil testing and applying nutrients based on soil test recommendations. This will not only minimize environmental pollution but will save consumers money by the preventing over-application.

The total number of samples received by selected urban counties in Missouri is provided in Table 1. This summary includes soil test data collected from Greater St. Louis (St. Louis, St. Charles, Jefferson and Franklin Counties) and Greater Kansas City (Clay, Cass, Jackson, and Platte) areas, Boone, Greene and Cole Counties.

Soil test results for samples received by the MU Soil Testing Lab from each urban county had been grouped for pH, soil test P and soil test K into ‘very low,’ ‘low,’ ‘medium,’ ‘high’ and ‘very high’ as per following guidelines. The University of Missouri soil testing albs measure salt pH (pHs- 1:1 0.01 M CaCl2), soil P by Bray P 1 extraction and soil K by ammonium acetate extraction. The water pH (pHw) values are in general 0.5 units higher than the salt pH (pHs).

• Soil pHs : Very low less than 5.0; low 5.0 to 5.8; medium 5.8 – 6.5; high 6.5 – 7.5; very high greater than 7.5;
• Soil P1: Very low less than 20 lb/ac; low 20 – 40 lb/ac; medium 41 – 60 lb/ac; high 61-120 lb/ac; very high greater than 120 lb/ac;
• Soil K: Very low less than 100 lb/ac; low 101 – 199 lb/ac; medium 200 – 259 lb/ac; high 260- 300 lb/ac; very high greater than 300 lb/ac;

The soil pHs, soil P and soil K distribution for samples received from greater St. Louis and greater Kansas City areas are presented in Fig. 1 and 2. The total number of home lawn and garden soil samples received from St. Louis and surrounding counties (St. Louis, St. Charles, Jefferson and Franklin) is 1260 and the Kansas City and surrounding counties (Cass, Jackson, Clay and Platte) is 616. Soil pHs was high (6.5 – 7.5) or very high (greater than 7.5; pHs values greater than 6.5 is equivalent to water pH values of 7 or higher) in majority of the samples tested from greater St. Louis area and greater Kansas City areas, 62% and 50% respectively. Likewise, the majority of samples tested had high (60- 120 lbs of P/ac) or very high levels (greater than 120 lbs of P/ac) of soil P levels (greater St. Louis area 60% and greater Kansas City area 67%). Similar trends were observed in soil test K levels for samples received from greater St. Louis and greater Kansas areas, 65 % and 89% percent of samples respectively. So based on this summary for lawn and garden soil-tests, the majority of homeowners have built their soil test P and K levels high and wouldn’t need any additional fertilizer P and K for their lawns and gardens. These results indicate the importance of testing soils and applying only the recommended levels of fertilizer N, P and K to soils. Very often we receive questions from homeowners who have built up their soil test levels so high and caused imbalance of nutrients resulting in poor performance in their gardens and lawns. Applying combined N-P-K fertilizers that are available in the lawn and garden stores without testing their soils results in a waste of money, and excess fertilizer N and P getting washed off to lakes and rivers and causing eutrophication and environmental pollution. This soil test summary report emphasizes the need for soil testing and adopting proper nutrient management practices to have healthy and productive lawns and gardens without polluting the environment. Once you have over-built the soil test P and K levels in excess by over applying mixed fertilizers without soil testing, it may take years to deplete the excess soil test levels down to optimum levels by growing plants.

The optimum soil pHs for most of the plants is 5.8 to 6.5 which translates to water pH values of 6.3 to 7.0 (pHs + 0.5 =pHw). Some acid loving plants like azaleas, rhododendrons and blue berries like acidic pHs. So it is important to test the soil and follow the recommendations to bring soil pHs levels to the optimum rates by applying lime or elemental sulfur.

The soil test pHs, soil P and soil K distribution for other urban counties such as Greene, Boone and Cole counties are presented in Fig. 3. The same trends in soil test pHs, soil P and K distribution in greater St. Louis and greater Kansas City areas were observed in these counties. This summary report emphasizes the importance of soil testing for lawns and gardens and adopting soil test based recommendations to apply fertilizer and other soil amendments. Taking these proactive steps encourages the growth of healthier lawns and gardens without wasting money and polluting the environment by over-application.