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Testing Soil Fertility for Greater Profits

If you want high yields and peak performance from your crops, soil fertility needs to be taken into account. By testing soil fertility, you can see what your plants are getting and what they are not, allowing you to adjust for soil nutrient deficiencies.

Click the links below for more information.


1. Why test soil?
2. Testing Your Soil
3. Organic Matter Matters
4. Sand, Silt or Clay?
5. Special Considerations
6. Nutrient Definitions


Why test soil?

In most cases, essential nutrients for plant growth come from the soil and are delivered by the root system. However, most soils don't contain the right amounts of all of the nutrients needed. Different types of plants can have very different nutrient requirements. How can you make sure your plants have access to everything they need for maximum performance?

One approach is to fertilize with all possible nutrients on a regular basis. Taking this route, however, wastes fertilizer and dollars, pollutes ground and surface water with excess nutrients, and risks toxic reactions to your plants from too much of an otherwise good thing. Yet this is exactly what you are doing when you fertilize without testing your soil first. Fertilizing without testing is bad for plants and bad for profits.

The ultimate goal for any testing program is profitability. You want to know precisely what investment in fertilizer dollars will give you the best possible return in plant quality and/or yield.

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Testing Your Soil

An easy way to make sure that your fertilizer dollars are used wisely is to take a soil sample and send it to a laboratory for testing. Ask your neighbors or Cooperative Extension agent to recommend a good laboratory to use. Call the lab before sending your sample to make sure you follow proper procedures and include all of the information required for a complete and accurate test.

The lab will want to know what crop or ornamental plants are present or intended for the soil to be tested. In that way, the lab can "calibrate" your test results and fertilizer recommendations to your plants.

It's important that your lab considers both what's lacking in your soil and the needs of your specific plants in making a fertilizer recommendation.

Test your soil at least once a year. Different types of plants will "use up" more or less amounts of specific nutrients during the season. It's important to test each year to find out what changes have taken place in your soil's nutrient composition. Once you're familiar with the process, consider trying some on-site testing for fast, in-season results.

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Organic Matter Matters

Like pH, the organic matter content of your soil can influence whether or not nutrients are available to your plants. Organic matter holds nutrients in the soil by reducing leaching. It also preserves soil moisture, and lessens the potential for erosion and compaction. An ideal range for most soils is between 3 percent and 5 percent organic matter. Sandy soils will have less, and clay or silty soils may have more.

Organic matter is continually being used up, including its breakdown into nitrogen, sulfur and other elements taken up by plants. Vegetable and animal matter returned to the soil in the form of mulch, cover crops, crop residue or manure increases the soil's organic matter content. Organic matter should always be a high priority for testing and replenishment.

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Sand, Silt or Clay?

Soil type, or soil texture, is another important factor influencing soil fertility. The percentage of your soil that is sand, silt or clay determines your soil type and, to a great extent, how good your soil is at holding nutrients. Soils that are predominantly sand lose nutrients easily through leaching, while soils that are mostly clay are better at holding on to nutrients until they are needed.

One way to obtain an estimate of your soil type is to check soil survey maps for your county. Check your public library, Cooperative Extension office, and/or your local USDA-ASCS or Soil Conservation Service office.

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Special Considerations

Some types of plants have very specific nutritional needs. Optimum amounts, formulations and timing of fertilizer applications can vary among different plants and soil types. Be sure to check with your Cooperative Extension agent or private consultant to create a nutritional program that's best for you.

Fruits and vegetables - Analysis of leaves or petioles during the season, done on-site or through a laboratory, can be extremely valuable in "fine-tuning" fertilization programs. Check with your soil test lab, Cooperative Extension agent or private consultant on the advisability of foliage testing for your crop.

Fertigation - Greenhouse plants and other plants are sometimes fed by injecting fertilizers directly into the irrigation or hydroponics water supply. An EC meter can be used to monitor fertilizer concentrations at various points along the system to ensure optimum plant response.

Corn, wheat, milo and other field crops - You can get very close to an exact nitrogen (N) requirement by using a formula that includes your yield goal, amount of N used by your crop, soil organic matter content, and amount of crop residue. Your fertilizer dealer, Cooperative Extension agent or private consultant should be able to help you with this calculation the first time through. Remember, soybeans are legumes and add N to the soil. Soybeans typically don't need supplemental N fertilizers.

Forage crops - If you sell forage off the farm, or supplement animal rations with purchased feed, spreading manure may result in under-or over-fertilized forage crops. Testing both your soil and your manure can indicate when supplemental fertilizers are needed.

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Nutrient Definitions

Plants need a specific diet of nutrients to perform at the highest levels. "Macronutrients" (N, P and K) are needed in large amounts by many plants. "Micronutrients" and "trace" nutrients are required in smaller quantities.

Nitrogen (N) – Often deficient in soils after a few seasons of production. N easily leaches out of sandy soils, polluting ground and surface water. Where leaching is a risk, multiple applications of small amounts is better than a single application of a large amount. N can have a negative effect on yields if over-applied.

Phosphorus (P) – Doesn't move through soil easily, so be sure to work or irrigate P applications into the soil to reach the root zone of your plants. Preventing soil erosion is the best way to prevent P from contaminating surface water systems.

Potassium (K) – K, like N, leaches out of sandy soils easily. Proper N/K balance in fertilizer applications helps reduce the amount of N needed to achieve maximum performance.

Calcium, magnesium and sulfur are also supplied to plants through the soil and can be added through commercial fertilizers and other soil amendments such as lime and gypsum. Testing can tell you how much, if any, is needed.

Boron, chloride, copper, iron, manganese, molybdenum and zinc are required by soils in small amounts. Proper testing is important to correct deficiencies and prevent toxicity from excesses.

Trace nutrients:
Aluminum, cobalt, fluorine, iodine, sodium and lithium are found in soils in varying amounts and are either not essential to plant growth, or required only in minute quantities.

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