Midwest Vegetable Guide - 2022 - 19

Soil Fertility and Nutrient Management
to gaseous forms (ammonia, nitrous oxides, N gas) and
evaporate into the air. Ammonium in the soil solution exists
in equilibrium with ammonia gas (NH3). 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 monoammonium
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 (SO3
by soil pH.
Micronutrients
2-). Sulfate is little affected
4
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, 515%
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 (CaSO4 2H2O).
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
Midwest Veg Guide 2022
19
Midwest Vegetable Guide - 2022

Table of Contents for the Digital Edition of Midwest Vegetable Guide - 2022
