• Soil pH is the foundation to proper fertility. Soil nutrient availability and fertilizer use effectiveness is pH sensitive.
• Fall is a great time to apply lime to balance pH.
• Grid sampling provides the best method for measuring pH differences due to its uniform acre size/sample area size.
• Field variance of pH drop is governed by yield variation across the time period since last correction, ammonium fertilizer applied and manure use in some cases.
• The irregular shape and size of predetermined zones of zone sampling lead to unseen pH variation that can result in over application of lime in some areas of the field and under application in others.

Balanced soil pH is the foundation to obtaining the most from soil fertility and maximizing yield.Extreme soil acidity or alkalinity can affect both nutrient (Diagram 1) and fertilizer availability.Additionally, soil pH outside the range of 6.0-7.5 can create an unfavorable environment for soil microbial life, adversely affecting soil health.Phosphorus is one of the more sensitive crop nutrients with its pH sweet spot for maximum availability around 6.7-6.8.At soil pH levels outside this range phosphorus begins to form plant unavailable compounds with iron and aluminum at lower ranges and calcium at higher soil pH levels.Even major nutrients like nitrogen begin to loose availability at 6.0.Likewise, too high or too low of soil pH can greatly effect herbicide activity, and soil persistence of product.Fall is a great time to conduct soil tests and the ideal time to correct soil pH.

Diagram 1- Soil pH effect on nutrient availability

Grid sampling is the best way to systematically and geospatially get a measure of soil pH differences within a field.Sample areas representing 2.5 acre areas (or smaller) and of equal size provides an unbiased random method to develop zones and represent soil pH.

Soils are acidified by addition of H⁺ ions to the soil through the use of ammonium containing fertilizer, manure application, as organic matter mineralized, and from plants roots as the they take up nutrients. As base cations (Ca⁺², Mg⁺², K⁺) released from the cation exchange capacity are brought into the plant, CE-sites are left open for H⁺ ions to attach and acidify the soil.

Why grid sample versus zone sample?Soil pH variation can be extremely dynamic. Many reasons exist but the number one factor is sample area represented, their uniformity of size and ability to unbiasedly create zones of application.Zones typically are predetermined and vary in size.In addition, field pH for a given field is a complex mix of 4 years of crop nutrient removals, fertility applied and spatial yield differences. Across years between pH corrections, yield has spatially varied accompanied by hydrogen root exudates. Likewise, fertilizer application may have varied and applied fertilizer may have migrated from the area of application.Larger sample zones can lead tounseen soil pH differences leading to under or over application of lime resulting in too high or too low pH that complicate nutrient availability.

Soil pH is a pivotal factor in maximizing crop yield as is illustrated by the NRCS publication table below (Table 1).For maximum nutrient availability and top yields in a corn-soybean rotation, soil pH should be in the 6.2-7.2 range.