Nutrient Management And Nutrient Cycling Raymond C. Ward, President Ward Laboratories, Inc Kearney, NE www.rayward@wardlab.com Take Good Soil Samples to Evaluate Soil Fertility Status Take soils samples for nutrient assessment by sampling the top 8 inches of soil. We want a 10 to 15 uniform slices of soil to mix together for a representative composite sample. Sub-soil samples for mobile nutrients should be taken from the 8 to 36 inches. Take 8 to 10 subsamples to mix together for a representative composite sub-soil sample. Soil ph, EC, OM, nitrate, phosphorus, potassium, sulfur, zinc, manganese, copper, iron, calcium, magnesium, sodium, chloride, and boron are the nutrients that can be evaluated. It will depend on the soil, crop, and management. Many soils are supplied with adequate micronutrients. If you have not tested your soil for all nutrients you may want to do that every 5 to 10 years just to observe trends in nutrient supply. Nutrients removed by the harvested crop are shown in Table 1. Multiply your yield in bushels per acre times the values per bushel in Table 1. Table 1. Nutrient removal in grain of several crops. Nutrient Wheat, Corn, Milo, Soybean, Alfalfa, lb/ton Sunflower, lb/100 lbs N 1.2 0.75 0.81 3.7 55.0 3.6 P2O5 0.52 0.32 0.35 0.77 12.0 1.2 K2O 0.26 0.23 0.25 1.4 50.0 1.1 S 0.12 0.09 0.12 0.37 5.0 0.22 Zinc 0.003 0.001 0.001 0.002 0.11 0.005 Manganese 0.0002 0.0006 0.0008 0.001 0.11 0.002 Copper 0.0007 0.0004 0.0003 0.001 0.02 0.002 Calcium 0.03 0.01 0.05 0.38 28.0 1.2 Magnesium 0.17 0.05 0.08 0.20 5.3 0.2
Soil test value ratings are shown in Table 2. Nitrate soil tests are not rated because the goal is to keep nitrate low so it does not leach to the groundwater. The goal is to have 5 ppm or less nitrate-n left in the root zone after harvest. If residual nitrate tests are higher than 5 ppm nitrate-n, the normal N fertilizer application rate for the next crop must be reduced. Table 2. Soil test ratings for several plant nutrients. Soil test rating Very Low Low Medium High Very High Best Use of Test Nutrient ppm ppm ppm ppm ppm Phosphorus (Olsen) P 0-3 4-9 10-16 17-30 30+ Wide range of soils Phosphorus (Bray) P-1 0-5 6-12 13-25 26-50 51+ Neutral, acidic Phosphorus (Meh) P-3 0-5 6-12 12-25 26-50 51+ Wide range of soils Potassium (exch.) K 0-40 41-80 81-120 121-200 201+ Wide range of soils Magnesium (exch.) Mg 0-10 11-20 21-35 36-50 51+ Wide range of soils Sulfate (Ca-P) S 0-4 5-7 8-11 12-15 16+ Wide range of soils Zinc (DTPA) Zn 0-0.25 0.26-0.50 0.51-0.75 0.76-1.00 1.01+ Wide range of soils Iron (DTPA) Fe 0-1.0 1.1-2.0 2.1-4.5 4.6-10.0 10.1+ Alkaline soils Copper (DTPA) Cu 0-0.10 0.11-0.20 0.21-0.30 0.31-0.60 0.61+ Wide range of soils Manganese (DTPA)Mn 0-0.5 0.6-1.0 1.1-1.5 1.6-4.0 4.1+ Alkaline soils Boron B 0-0.10 0.11-0.25 0.26-0.50 0.51-2.00 2.1+ Wide range of soils Fertilizer guidelines are available from many sources. We have developed our fertilizer recommendations from several Universities. Our guidelines can be found at www.wardlab.com. Click on WardGuide and check the table of contents for the proper pages to follow. It is always best to measure the fertility status of your soils with soil tests. Be sure to do a good and consistent job of taking soil samples. If soil tests are not available then some guessing has to be done. For N, apply 1.2 times the crop removal. For other needed nutrients assume a medium range soil test and apply ½ of the crop removal rate. If you think your soil is low in a nutrient then use total crop removal value for that nutrient as a guide to application rate.
Relationship between Soil Carbon and Nitrogen Soil organic matter (OM) is the decomposed plant residues and microorganisms not the residues left on the soil surface. The real carbon sequestration occurs in the decomposed plant residues. The stable organic matter has a carbon to nitrogen (C:N) ratio in the range of 10:1 to 12:1. If a soil has 3% OM in the top 8 inches of soil there is 36 tons of OM per acre. Organic matter is 58% carbon (C) so 3% OM means there is 21 tons of C per acre. So a C:N of 10:1 means 21 tons of C and 2.1 tons of nitrogen (N) or 4200 lbs of N per acre. How much of this N is available each year? It depends on the cropping system. The release is approximately 0.5% for grassland, 1.0% for conventional tilled small grain, 2.0% for conventional tilled row crops and 4.0% for conventional tilled summer fallow. For zero till systems, the release of N is probably about ½ of conventional till systems. What happens to N when crop residue and N fertilizer are added to the soil? If the crop residue has a C:N ratio of greater than 30, the microbial population will use the available soil N to decompose the residue. This process is referred to as immobilization of N. On the other hand, if the C:N ratio of crop residue is less than 20 the microbial population will release additional available N. This process is referred to as mineralization. Legume and legume cover crops have a high concentration of N (protein). The C:N ratio will be less than 20 if crude protein is greater than 12 %. Therefore, legumes start releasing nitrate as soon as decomposition begins. This is the reason that legumes are important in no till. Corn stalks have about 5 % crude protein so the C:N is about 70:1. As the residue begins to decompose, the microorganisms immobilize nitrate-n. In general, small grain straw and corn stalks tie up from 18 to 30 pounds of N per ton of residue. Immobilization and mineralization are equal after 3 or 4 years of no till. However, any legume in rotation makes mineralization positive for the next crop year. Drought and/or freezing also have an effect on rate of mineralization. Nitrate mineralization is more rapid after a drought or after freezing. This moisture stress may account for the high rate of mineralization of nitrate-n in early spring or after a prolonged drought. In no till we do not disturb the soil so the rate of mineralization of organic matter should be close to the grassland release rate. When we hear about tilling no till land we can assume that we will get a tremendous release of available N from the organic N on or very near the soil surface. However, it will only take one year to release most of this N from the organic matter and then the process will have to start over again. The short-term benefit could be very costly to future cropping.
The amount of nitrogen fertilizer to apply depends on the nitrogen supplying capability of the soil. The pool of available nitrogen sources includes 1) organic N sources such as animal manure, sewage sludge, and compost, 2) nitrogen fixing Rhizobium associated with legume plants, 3) nitrogen fixing microbes, 4) nitrogen fertilizer and 5) fixed soil ammonium. All of these sources of nitrogen are mineralized to nitrate in time. Since nitrate is the predominate form of N used by plants, a measure of residual nitrate in the root zone before planting the intended non-legume crop is a good method of estimating N fertilizer needed. The residual nitrate test is a good measure of available nitrate in most Great Plains soils where nitrate leaching is minimal. Nitrate is soluble therefore it is mobile in soil water. Where rainfall is great enough to move water deeper than the soil root zone during the growing season, the residual soil nitrate test can be used after harvest to estimate proper application of N fertilizer. Nitrate in poorly drained, wet soils can be lost by denitrification if the soil becomes anaerobic (lack of oxygen) for a period of time. Nitrification requires oxygen. If oxygen is low in the soil, nitrification will be low. Therefore, oxygen is necessary for nitrification. Phosphorus (P) Cycle and Availability Soil phosphorus exits as: 1) solution P; 2) surface or adsorbed P; 3) organic P held in organic matter; and 4) fixed or crystalline P. Fixed P is bound tightly in some of the soil compounds mentioned previously and is unavailable to the crops. I consider fixed P to be found inside the crystals. Surface P, on the other hand, is the phosphorus held on the surface of soil particles and crystals. Surface P is easily diffused from the crystal surface into the soil solution. Solution P is the small amount of P that is in soil water at any given time, usually less than one pound per acre. However, as the plant takes up P from the soil water, more P diffuses from the surface P. Surface P is also known as the active pool of P. Organic P is mineralized by microorganisms and enzymes to phosphate ions that crops can use. Some of the organic matter is easily mineralized and some is very resistant. The P mineralized from organic matter becomes part of the surface P. The organic P that is highly resistant to mineralization is considered part of the fixed P. Surface P (active P) pool determines the availability to crops. Soil texture influences the size of the surface P pool. Clay and organic matter are the reactive portions of the soil. Clay contains the aluminum and iron that are reactive with P. In alkaline soils, calcium and magnesium are reactive with P. Soils with more clay will have a larger surface P pool to supply P than sandy soils, and therefore have more P-supplying capability. Phosphorus soil tests have been developed to estimate P availability. The tests estimate the amount of P held as surface P. The common P soil tests for the Great Plains have been Bray P-1 and Olsen. The Bray P-1 test is well suited for noncalcareous (no free lime) soils, but is useless on soils with more than 4 % free lime because the lime
neutralizes acid in the Bray P-1extractant rendering the test invalid. The Olsen test is well suited for calcareous soils, and also performs well in other soils of the Great Plains. We use Mehlich P-3 test for measuring available P in our soils. It works in well in all soils. All P soil tests work well when used within the limits of the test. Suggested ratings for different P soil test values P Soil Test Method Low Medium High ppm P ppm P ppm P Bray & Kurtz (Bray P-1) 0-12 13-25 26-50 Olsen Bicarbonate P 0-9 10-16 17-30 Mehlich P-3 0-12 13-25 26-50 Sulfur Cycle Several years ago we began making higher sulfur fertilizer recommendations because we found that no till and reduced tillage slowed mineralization of sulfate from organic matter. Most sulfates are held in the organic phase of the soil. When we no till to build organic matter, we get less release of sulfate from the organic matter. Organic matter contains the plant nutrients that are present in the crop residue left on the soil surface. So, as organic matter or carbon (C) is sequestered or stored in the soil, nitrogen (N), phosphorus (P), sulfur (S), etc are also sequestered. The C:N:P:S ratio then is about 100C: 8N: 1P: 1S. The point is that as organic carbon is increased in the soil, other plant nutrients are also increased meaning they are not used for crop growth. We have dropped the amount of sulfur from organic matter factor from our recommendations, which increases the sulfur recommendation. Other reasons for more sulfur being recommended is:) less sulfur from the air because of clean air standard and 2) less sulfur in more refined commercial fertilizers. These factors are reasons for recommending higher sulfur fertilizer to our clients. To evaluate the level of sulfur in no till fields, soils samples should be taken at 0-8 inches for all nutrients including sulfate and 8 to 36 inches for nitrate and chloride and in some cases, sulfate.