PART 2 – The Importance of Chemical Soil Testing
To determine soil fertility, soil acidity (pH), and organic matter percentages, it is absolutely essential to test your soil. A soil test will tell you how much, if any, organic matter, lime, sulfur, and or fertilizer your soil needs. Determining whether or not your soil needs certain nutrient fertilizers, organic matter, lime or sulfur applications requires proper soil sampling at the proper root depth prior to planting your plants. Take enough random soil samples down to the average root depth of your crop and sample the entire area uniformly. For grass, samples are usually taken at a 3” depth. For trees and shrubs, soil samples are usually taken at a 6-8” depth but with good drainage, roots may penetrate even deeper so it’s smart to see if you have roots in the soil cores. Do NOT take soil samples after you have applied fertilizer or your lab results will be skewed. Soil should be tested before applying fertilizers, lime, sulfur, etc.
Figure 1: Sample several locations for a more accurate analysis in your landscape and lawn.
Many state labs will recommend you to take 10-12 uniform soil core samples per 8,000 square feet of the area sampled. A minimum of one cup of dry soil is recommended by many labs but some recommend 2 cups of soil to be sent in. Remember to scrape off the top mulch, grass/thatch, and debris before you take the soil samples and drop them into a clean plastic bucket so it doesn’t contaminate the soil. For proper soil sampling, see “Collecting Soil Samples For Testing”, Bulletin # HO-71–W from the Purdue Extension Publications. Once you’ve collected the uniform soil samples and mixed them together, crushed them to the size of wheat grains and dried them (without heat), take the soil samples to your County Extension office, which can then send them to a professional soil testing lab. Be sure to write down the “primary” crop you’re wanting to grow. Once the lab tests the soil, the written soil test report will tell you what deficiencies and toxicities you have and what you need to apply and how much you should apply prior to planting., whether it’s long-lived trees, shrubs, lawns, perennial flowers or vegetable gardens. So when are fertilizer, lime, and organic matter applications the most beneficial? When you have severe deficiencies. When nutrient levels are deficient or excessive, the crop suffers. Be sure your soils lab specializes in your particular crops because some labs only specialize in agronomic crops and not ornamentals. The power of doing soil tests every 2-3 years is huge so you’re maximizing yield and beauty and not wasting time and money and not polluting our streams, rivers, lakes, and ground water from excessive applications.
A routine soil test will usually provide you with four critical pieces of information including: soil pH (acidity), percent organic matter, and an estimate of plant-available phosphorous and potassium. Acidic plants need lower pH which is why annual applications of lime can raise the pH so high, the plants may die from micronutrient deficiencies. Do NOT add lime unless a soil test recommends it for your main crop. If you also have magnesium deficiencies, dolomitic lime (containing magnesium) should be used. People that add lime every year typically have an excessively high soil pH which can reduce the availability of phosphorous, iron, manganese, zinc, and boron. I’ve seen acid requiring tree’s roots growing out into lawns that were limed way too often and have foliar micronutrient deficiencies. A leaf nutrient tissue test will confirm nutrient deficiencies and also help determine the need, formulation, and rate of fertilizer needed.
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- Figure 2: Nutrient deficiencies can occur within individual trees in the same planting area.
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- Figure 3: Chlorosis is an indicator of many deficiencies and tissue analysis may be necessary to get the true determination.
The amount of organic matter you have in your soil is very important because the more organic matter you have in your soil, the better the water holding capacity, drainage and tilth you have. If your organic matter is less than 2-3% by weight, adding more organic matter in the form of compost, peat moss, aged manure, mulch, organic fertilizers, etc. is highly recommended. If you have more than 8% organic matter, you don’t need to add anymore for a while. Phosphorous, (P) is essential as well and stimulates root development, rapid growth, and quality flowers. If your phosphorous levels are low, < 30-50 pounds/acre, adding phosphorous and working it down into the soil is highly recommended. Phosphorous doesn’t leach downward readily like nitrogen and potassium do, so incorporating it down into the root zone BEFORE you plant is highly encouraged. Core aerating the soil and then applying recommended phosphorous helps it get down to the roots. For many plants, the ideal range for phosphorous is 50 to 100 pounds per acre with the midpoint (75#/ac) being ideal. Potassium (K) is also essential to your plants and promotes winter hardiness, strong stems, disease resistance and lessens drought stress. If your potassium levels are low, <200 pounds per acre, adding potassium will be highly recommended. For a majority of plants, ideal potassium ranges from 200-400 pounds per acre with the midpoint (300#/ac) being ideal. For routine soil testing, nitrogen is not normally tested by soil labs due to its rapid loss and variability. Nitrogen can be applied periodically for many crops but excess, quick-release nitrogen can stimulate too much vegetative growth, delay fruit production of garden vegetables and flowers and it can also create highly lignified thatch problems in lawns. This is why slow-release, Spring fertilizers like sulfur coated urea, urea formaldehyde, or organic fertilizers like Milorganite are highly recommended because they can prevent the excess surge growth and release the nitrogen throughout the season. If you’re going to be fertilizing your lawn in the Spring, do NOT use quick release fertilizers but choose fertilizers that have a high percentage of water-insoluble, slow release fertilizer. Slow-release fertilizers cost more but don’t pollute our environment and release their nitrogen slowly throughout the season so you don’t have to apply nitrogen every 2-4 weeks.
Remember, use only the amount of fertilizer or compost that is recommended because more is NOT better. The only time fertilization is going to be the most helpful and economically profitable and aesthetically warranted, is when you know you have severe deficiencies for your particular crop. DON’T GUESS – SOIL TEST!!!!
For more information, check out these great Purdue Extension Publications:
“Collecting Soil Samples for Testing” (HO-71-W)
“Cover Crops in the Home Garden” (HO-324)
“Home Gardener’s Guide” (HO-32-W)
“Indiana Vegetable Planting Calendar” (HO-186-W)
“Fertilizing Woody Plants” (HO-140-W)
Our Nation’s Best Soils Website!
- nrcs.usda.gov
- http://websoilsurvey.nrcs.usda.gov/app/ Excellent Soils Info on Your Site!
To visit Part 1 of this series – https://www.purduelandscapereport.org/article/why-is-professional-soil-testing-so-essential-part-1/
