A3030 Optimum soil test levels for Wisconsin oil test results for phosphorus, Maintaining nutrients at optimum potassium, and other mineral levels avoids economically damaging Selements are interpreted on nutrient stress throughout the grow- Wisconsin soil test reports in terms ing season while avoiding excesses ranging from very low to excessively that can cause agronomic or environ- high. This publication explains the mental problems. The best combina- meaning of the ratings and how they tion of economic return and mainte- are derived for various crops and soils. nance of environmental quality is K.A. Kelling Farmers and others can use this publi- provided by considering nutrients L.G. Bundy cation along with their soil test results from all sources. This means storing S.M. Combs to evaluate the overall fertility status somewhat lower quantities of nutri- J.B. Peters of their farms, estimate the likelihood ents in the soil and meeting nutrient of profitable fertilizer responses from needs from both fertilizer applications the fields, and decide where to invest and soil reserves. in lime and fertilizer for the greatest economic return and crop quality. Understanding soil test interpretations Why test soils? Soil test interpretations estimate the The goal of the fertilizer recommenda- likelihood of a profitable yield tions generated by the Wisconsin soil increase when a given nutrient is test program is to suggest appropriate added. The interpretation categories nutrient levels for specific crops. are described in table 1. The tests have When nutrient levels are deficient or been calibrated so that the addition of excessive, the crop suffers. recommended amounts of nutrients Nutrient shortages markedly lower are strongly suggested when the tests crop yield and quality. For example, are at or below the optimum level. At potassium deficiencies have been these levels, the likelihood of obtain- linked to poor winter survival of ing a profitable economic response to alfalfa, lowered disease resistance, applied nutrients is very good and increased lodging in corn and (greater than 30%). other grains. Insufficient amounts of The optimum soil test level for a nitrogen or sulfur can reduce protein given nutrient depends on a number levels in forages. Low calcium levels of factors, including crop to be grown, in fruits and vegetables can increase soil type, and contributions from the their susceptibility to several diseases. subsoil. Excesses of some elements can reduce Interpretive levels for soil pH are yields by causing imbalances. given graphically on the soil test Excessive amounts of boron, man- report in relation to the target pH for ganese, copper, and zinc can lead to the most acid-sensitive crop in the toxicities. Also, once soil tests reach indicated rotation. Table 2 lists the the high level, adding more nutrients optimum pH levels for crops grown is of little economic benefit. Excess in Wisconsin. nutrients build up when more fertil- izer or manure is added than is Most routine soil testing programs removed by the harvested portions of give no interpretations for nitrogen or the crop. It is important to know organic matter. Under normal or when to cut back on certain nutrients higher rainfall and optimum fertiliza- as well as when to add more. tion programs, nitrogen usually does 2 OPTIMUM SOIL TEST LEVELS FOR WISCONSIN not accumulate in soil. Because nitro- of nutrients approximately equivalent Subsoil fertility groups are also used gen may leach over winter, attempt- to the amount present in the har- to determine nutrient buffering ing to build up nitrogen in the soil is vested part of the crop. capacities or how much phosphate or neither practical nor environmentally potash is required to raise soil test P wise. Recommended application rates Subsoil or K to the optimum level. As shown given in the routine soil test report are contributions in figure 1, soil in subsoil group D estimates of crop nitrogen needs for requires 18 pounds of P O per acre to Nutrients present in the subsoil can 2 5 the indicated soil and assume good change soil test P by 1 part per million contribute significantly to the nutri- soil management practices are used. (ppm). A soil in subsoil group E, on tion of crops. Roots that reach down The recommended rates of nitrogen the other hand, requires only into the subsoil can use the nutrients were determined through experi- 12 pounds of P O per acre to raise stored there, so the level of phospho- 2 5 ments that measured plant response soil test P by 1 ppm. rus and potassium present in the on various soils. These studies plow layer becomes slightly less showed that for some crops, including important. For example, recent Secondary nutrients corn, the optimum rate of nitrogen on research at Arlington showed that and micronutrients a given soil was similar in both high- alfalfa obtained about 100 lb of potash Soil tests are available upon request and low-yielding years. For this rea- (K O equivalent) per year from the for secondary nutrients calcium, mag- son, recommended nitrogen rates for 2 subsoil. nesium, and sulfur as well as trace corn are not based on expected yield nutrients zinc, boron, and manganese. but are soil specific. Use of special Some subsoils are higher in phospho- The interpretations for these tests are tests (e.g., spring preplant profile rus and potassium than others. To given in table 5. nitrate test and presidedress nitrogen reflect this difference, the soil test test) can more precisely determine the report uses the subsoil fertility groups The sulfur availability index in table 5 specific nitrogen need. illustrated in figure 1 to distinguish is calculated by summing sulfur con- different soils. These groups are based tributions from estimates of sulfur Soil organic matter levels are con- on soil samples collected at a depth of released from organic matter, precipi- trolled by factors such as soil aeration, 8–30 inches from every county in a tation, subsoil, and manure as well as drainage and tillage systems and can- 1960 statewide survey. sulfate sulfur (SO -S) determined by not be increased easily without large 4 the soil test. The procedures for esti- additions of manure or other organic When sending in soil samples for test- mating the amounts of sulfur con- material or by switching to reduced ing and fertilizer recommendations, tributed from these sources are tillage. include the soil name on the informa- described in Extension publication tion sheet. The soil name is used to Soil Test Recommendations for Field, assign the correct subsoil group and Crop demand levels Vegetable, and Fruit Crops (A2809). Crops differ in their need for nutri- to interpret soil test phosphorus and ents. The optimum potassium level potassium data (tables 3 and 4). If the Available manganese is influenced for alfalfa is higher than that needed name is not given, the computer both by soil pH and organic matter. for red clover. To account for different “guesses” the soil group based on soil When organic matter exceeds 6%, phosphorus and potassium needs, pH, soil texture, organic matter, and manganese availability is predicted crops have been placed in one of six county of origin. This procedure obvi- from soil pH rather than the man- demand levels: (1) corn; (2) soybeans ously does not permit as precise a fer- ganese test itself. This interpretation is and low-demand field crops; tilizer recommendation as when soil shown in table 5. name is given. For soil name informa- (3) alfalfa, irrigated field crops, and Presently, there are no soil tests for tion contact your county Extension low-demand vegetable crops; (4) red copper, iron, molybdenum, and chlo- office or Natural Resource Conserva- clover and other medium-demand rine calibrated for Wisconsin soil con- tion Service (NRCS). A list of the sub- field crops; (5) high-demand veg- ditions. The likelihood of deficiencies soil fertility groups for each of the 699 etable crops; and (6) potatoes. of these micronutrients is too rare to soil types currently recognized in justify developing soil tests for them. The demand level assignments for the Wisconsin may be found in Extension If you suspect deficiencies of these various crops are given in table 2. publication Soil Test Recommendations nutrients, plant analysis should be These demand levels were established for Field, Vegetable, and Fruit Crops used to confirm the need for making so that if the soil test is in the opti- (A2809). mum range, then crop yield and profit an application. are optimized by adding the quantity 3 Figure 1. General subsoil fertility groups, based on available phosphorus and potassium in subsoils Subsoil Nutrient Nutrient buffering capacityb a group Legend supplying power P2O5 K2O A P high, K medium 18 7 B P medium, K medium 18 7 C P low, K high 18 7 D P medium, K low 18 7 E P variable, K low 12 6 O * P variable, K low 18 5 X * P low 18 — *Scattered throughout the state. aAll data refer to subsoils (8" to 30") only. Low, medium and high ratings are relative and are not defined in absolute units. Adapted from M.T. Beatty and R.B. Corey, 1961. bThe soil nutrient buffering capacity is the approximate amount of fertilizer in lb/a (oxide basis) required to change the soil test level (ele- mental basis) by 1 ppm. 4 OPTIMUM SOIL TEST LEVELS FOR WISCONSIN Table 1. Codes and descriptions of soil test interpretation categories Probability ——Category—— of yield Name Symbol Description increasea (%) Very low VL Substantial quantities of nutrients are required >90 to optimize crop yield. Buildup should occur over a 5- to 8-year period. Response to secondary or micronutrients is likely or possible for high or medium demanding crops, respectively.