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2022 Midwest Vegetable Guide

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Soil Fertility and Nutrient Management Midwest Veg Guide 2022 19 4 to gaseous forms (ammonia, nitrous oxides, N gas) and evaporate into the air. Ammonium in the soil solution exists in equilibrium with ammonia gas (NH 3 ). The amount of each compound depends to a large extent on the soil pH. At lower pH, there is more ammonium and less ammonia gas. At pH 7, the equilibrium condition is 99 percent ammonium and 1 percent ammonia. At pH 8, the equilibrium is about 90 percent ammonium and 10 percent ammonia gas. Volatilization from N fertilizers that contribute ammonium to the soil (such as urea) is likely to be high at higher soil pH levels. However, depending on soil temperature and moisture, volatilization can be significant at lower soil pH levels, too, especially if the soil is dry and the fertilizer is not incorporated. To minimize volatilization, apply N in just the quantities plants need during the growing season, incorporate it into the soil, and use slow-release sources when possible. Soil pH is also an important factor in the N nutrition of legumes. Plants in this family are able to fix N from the soil with the help of several genera of soil bacteria known collectively as Rhizobia. As soils become more acidic, Rhizobia decline in activity, fixing less N. Phosphorus (P) Plants absorb P from the soil solution in the form of soluble phosphates. At any time, the amount of P in solution is usually extremely low — often less than 1 pound per acre — because P joins with other elements in the soil to form stable minerals. The type of mineral that gets formed in the soil depends on the soil's pH. In alkaline soils, P in fertilizers such as mono- ammonium phosphate (11-55-0) usually react with Ca to form calcium phosphate minerals. The P in calcium phosphate minerals is not available to plants, but as plants remove P from the soil solution, the minerals gradually dissolve to replenish the supply of P in the soil solution. Greenhouse and field research has shown that more than 90 percent of the fertilizer P tied up this year in calcium phosphate minerals will be available to crops in future years. In acid soils, P usually reacts with Al and Fe, instead of Ca. Aluminum and iron phosphates do not dissolve as readily as calcium phosphates, so in acid soils, applied P tends to be tied up more than in alkaline soils. Potassium (K) In soils with certain types of clay, K is fixed at specific sites between clay layers. This tends to be reduced under acid conditions, presumably because Al occupies the binding sites that would otherwise trap K. Because of this, one might think that raising the pH by liming would reduce the availability of K. However, this is not the case — at least in the short term. Liming increases K availability, probably because Ca displaces K on exchange sites. Sulfur (S) Plants absorb sulfur as sulfate (SO 3 2- ). Sulfate is little affected by soil pH. Micronutrients Micronutrients are elements plants need in very small amounts. The availability of the micronutrients Mn, Fe, copper (Cu), Zn, and B, decreases as soil pH increases. The exact mechanisms responsible for reducing availability differ for each nutrient. Deficiencies of these micronutrients are more likely at high pH, and toxicities are more likely at low pH. The availability of the micronutrient molybdenum (Mo) is reduced under acid conditions. Mo deficiency is more likely to occur in acid soils. Types of Lime Several types of lime that may be used to manage soil pH and/or Ca and Mg are described below. After each discussion, the percentage of CaO and MgO in a typical batch of lime is given. Calcitic lime (also called high-calcium lime — 50-56% CaO, 1-4% MgO) is the most soluble form and is the preferred type when soil Ca is low and soil Mg is high. It generally reacts the fastest and is the most common form available in some areas. Magnesian lime (also called hi-mag lime — 32-42% CaO, 5- 15% MgO) is intermediate in solubility and is the preferred type when pH, Ca, and Mg are low. The continued use of high-Mg liming materials increases the base saturation of Mg and decreases Ca saturation, which may result in Ca deficiencies during stress periods. Dolomitic lime (30% CaO, 20% MgO) is the preferred type when Mg is particularly low. Dolomitic lime is the least soluble of the materials. Hydrated lime (60% CaO, 12% MgO) reacts most rapidly with the soil, but unlike the ground limestones described above, it does not continue to provide liming activity over a period of years. Hydrated lime is caustic to humans and plants, and applicators must take care not to burn plants. Use hydrated lime only in emergencies when rapid changes in soil pH are needed. Gypsum is not a liming material and does not affect soil pH. It is a crude calcium sulfate product consisting chiefly of calcium sulfate with combined water (CaSO 4 2H 2 O). Although gypsum is not capable of neutralizing soil acidity, it is a source of calcium and sulfur. Fluid lime is a suspension of finely ground limestone in water, and may contain other dispersing agents. Finely

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