David Kissel and Leticia Sonon University of Georgia Spatial Variability of Soil pH and its Relationship to N Reactions in Soil David Kissel and Leticia Sonon University of Georgia

Why is subsoil pH important?

Low subsoil pH can result in toxic levels of Al+3 that can restrict root growth and the amount of water available to the crop.

Relationship between pHCaCl2 and KCl Extractable Aluminum, 90 NAPT Soils 120 100 Critical pH 4.85 80 For Saturated Paste pH the value is 5.40 60 KCl Extr. Al (mg/kg) 40 20 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 Soil pH (1:1) CaCl2 Miller et al. 2003

Soil pHw with depth, Cabin Field Depth (inches) pH S pH N 0-12 5.5 12-24 6.2 5.2 24-36 6.6 36-48 6.1 5.4 48-60 5.0

Water Remaining (%), Cabin Field Date S 12” S 20” S 28” N 12” N 20” N 28” 6/12 85 90 92 100 99 6/22 70 72 64 93 95 7/1 48 2 62 69 7/8 25 1 37 7/18 78 11 87 7/29 55 84

Soil pH in Cabin Field (Cotton) Sampling Depth Short (55 cm) Medium (85 cm) Tall (130 cm)

Soil pHw vs depth, NWCrisp Co Depth inches Site 1 Site 2 Site 3 Site 4 Site 5 Site 6 0-6 5.8 6.2 5.7 5.3 6.1 6-12 5.6 6.0 5.2 5.9 12-24 4.9 5.5 4.8 5.0 24-36 36-48 4.5

What role might nitrogen nutrition have in the formation of subsoil acidity?

September 1998, ammonium (kg N/ha) distribution with depth. Depth (cm) North Center South 0-15 0.18 0.32 0.35 15-30 0.64 0.60 1.50 30-45 1.42 2.44 3.07 45-60 1.17 3.36 2.89 60-90 2.55 5.43 3.70 90-120 1.87 4.98 6.74

What is the possible role of high levels of ammonium on soil pH, especially for subsoil layers?

“Plant uptake of a cation must be accompanied by uptake of an anion of equal charge or by the extrusion of H+ or other cations. The reverse is true for uptake of anions.” (C. Tang and Z. Rengel, 2003. Handbook of Soil Acidity; Marcel Dekker, Inc.

Some ions in soils used by crops Cations (positive) potassium K+ calcium Ca++ Magnesium Mg++ zinc Zn++ Manganese Mn++ Iron Fe++ Copper Cu++ Ammonium NH4+ Anions (negative) nitrate NO3- sulfate SO4-- Phosphate HPO4 - Borate H2BO3- Molybdate MoO4-- Chloride Cl-

If nitrification is slow and a crop takes up a substantial proportion of its N as ammonium, the crop will likely take up an excess of cations and the crop roots will respond by emitting H+. If uptake of ammonium occurs from subsoil layers, the crop will therefore tend to acidify those soil layers.

Recent data of soil pH and available N with depth.

March 2006 samples Location & Depth pHCaCl2 LBC NO3-N Lb/A NH4+-N 5.0 F31D 0-6 5.0 500 5 4.5 6 - 12 4.9 442 3.4 12 - 24 4.7 306 16 6.6 24 - 36 4.4 690 24 8.2 F31up 0-6 4.8 194 4 3.7 209 3.1 314 10 8.6 491 12 7.0

March 2006 samples pHCaCl2 LBC NO3-N NH4+-N Location & Depth Lb/A 4.8 317 4 3.7 6-12 4.7 296 3.8 12-24 4.9 230 8 10.3 24-36 4.6 528 18 8.1 36-48 512 22 7.0 H Sandy 0-6 5.0 155 3 2.9 111 3.5 86 6 8.6 96 12 10.0 4.2 330 13.5

We propose that relatively high levels of NH4+ at depth are not uncommon in S. Georgia soils and that an acid surface layer makes the problem worse because it slows the rate of nitrification, allowing N to remain as NH4+ for a longer time, increasing the probability that leaching rains will move NH4+ deeper into the soil.

Nitrate formed from ammonium at 25 to 30 C in different soils (Frederick and Broadbent. 1966. Agr. Anh. NH3) Soil State pH ppm N/week Imperial clay California 8.0 308 Clermont sil Indiana 5.0 40 Clermont sil + 50 T lime 7.4 200 Leon fsand Florida 4.3 5.9 10

wheat Al & Mn Toxicity Mn & Fe Deficiency

Soil test manganese ratings Soil test manganese, lb/acre Recommend manganese if soil pH is equal to or above 10.0-10.9 6.5 11.0-11.9 6.6 12.0-12.9 6.7 13.0-13.9 6.8

H+ ion 1.6 to 1.7 x10-5 Ångstrom From Wikipedia 1.20 Ångstrom