A good, quality soil sample is the critical first step in determining a crop nutrition plan. But once the sample is analyzed, what should be done with the report?
The information on that soil test is essential to the crop consultant, agronomist or grower understanding the condition of the soil and how to determine nutrient recommendations.
An example soil test report from Midwest Labs. The report is complete, showing organic matter, pH, cation exchange capacity, base saturation and nutrient levels — including micronutrients.
The first section of the soil test to review is the pH, cation exchange capacity (CEC) and percent-base-saturation section. We get a general indication of the soil texture by the CEC. A soil with a CEC below 8 is considered sandy, whereas a soil with a CEC between 8 and 14 is a medium-textured or loamy soil. When a soil has a CEC higher than 14 there is a fairly high clay content.
Those values aren’t hard-and-fast rules, but generally the higher the CEC the more clay and organic matter the soil contains.
Soil pH has a direct effect on nutrient availability.
This chart shows relative nutrient availability at various soil pH levels. Most nutrients are readily available when the soil pH is 6-7.5. Notable exceptions are aluminum, where availability drops substantially at pH levels greater than 5, and iron & manganese where availability drops starting at pH 6 and higher. It is also important to note that most bacteria and fungi are most active in soils with pH above 5.5.
Base saturation is the ratio or proportion of the amounts of potassium, magnesium, calcium, hydrogen and sodium in the soil. Having the soil’s calcium base saturation level between 65-75%, magnesium level between 10-18% and potassium level between 35% provides the best opportunity for all nutrients in the soil to be available, good soil structure and water-holding capacity and good microbial activity.
When those nutrients are in the desired range, soil pH is usually in the upper 6s to 7s. Simple calculations are available help determine the amount of calcium, magnesium or dry potash amendments needed to make base saturation adjustments.
Understanding Crop Nutrition
Let’s turn our attention to the crop nutrition we will need to provide during the growing season:
Nitrogen recommendations have been well researched for many crops. Most crops have nitrogen-recommendation equations that account for yield goals and soil residual nitrogen. Other factors to consider may include nitrogen credits for previous crops.
Nitrate nitrogen is a very mobile nutrient in the soil because it has a negative charge and doesn’t attach to soil particles. Although the recommendation equations are a very good starting point, environmental factors such as rainfall, temperatures and water saturation will influence nitrogen presence in the soil.
Phosphorous plays a major role in crop production — from the earliest stages of growth through fruit or grain production and maturity. When recommending phosphorus, or any nutrient, the yield goal is important.
Phosphorus recommendations depend on yield goals and the readily available phosphorus in the soil. That value is found in the Bray P1 column when soil pH is less than 7, and the Olsen bicarbonate column when the soil pH is greater than 7. Some labs use the Mehlich 3 extraction process to determine available phosphorus, which is not dependent on soil pH.
Phosphorus recommendations also take into account whether there is low, adequate or high levels of phosphorus in the soil. Applying the amount of phosphorus needed to grow the crop and taking advantage of the nutrients already in the soil will provide for good crop production and reduced potential for environmental problems.
The actual amount of phosphorus needed to grow the crop will vary by crop and yield goal, but if the phosphate level is less than 30 parts per million (ppm) most crops will respond to supplemental phosphorus application.
Crops that are often planted in cool, moist soils, such as corn, will benefit from a small amount of phosphate applied at planting, even in high-phosphorus soils.
Many crops — especially legumes, fruits and vegetables — have a high demand for potassium. It is necessary for production and water relationships in the plant, among many other functions.
When recommending potassium, the yield goal is the first piece of information to be collected. But the soil CEC also figures into recommendations for potassium. In sandy soils a potassium level of 150 - 175 ppm is considered adequate for most crops, and in higher-CEC soils that value is upwards of 200-225 ppm.
Those values are reasonable for row crops such as corn, soybeans and wheat, but may not be adequate for crops that have a high demand for potassium.
Sulfur is vital to high-yielding, high-quality crops. It’s not required in as high of rate as nitrogen, phosphorus or potassium in most crops, but it’s just as vital to plant health.
Sulfur recommendations are based on several factors, including CEC, organic matter and pH. A rule of thumb to use for determining sulfur need is that most crops require 1 pound of sulfur for every 10 pounds of nitrogen the plant needs. Most crops will respond to sulfur applications when soil sulfur levels are below 25 ppm.
The last section of the soil test is the micronutrient section, but the term “micro” does not mean that they are unimportant. In fact, having the proper amounts of micronutrients available to the crop is as critical to yield and quality as nitrogen, phosphorus and potassium.
Micronutrients are needed in much smaller amounts than other nutrients, but again, they’re every bit as important as the other nutrients for proper crop growth and production.
Crops such as corn and soybeans respond very well to additions of zinc, boron and manganese when the soil test calls for it. Crops such as wheat respond most to iron and manganese. Small amounts in the soil are extremely important, however excessive amounts can be phytotoxic.
There is a lot of information on that soil analysis. For additional information, or assistance in using soil test reports to develop a crop nutrition plan, contact an AgroLiquid retail partner, or AgroLiquid representative. Find a retail partner near you at agroliquid.com.
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