Soil Testing & Fertility Management

Soil & Tissue Testing

Soil tests provide the best way to determine pre-plant requirements for lime and fertilizers. For perennial fruit crops, leaf tissue or petiole analysis are the best ways to determine nutrient status for established crops. With the information from these tests, growers can make informed decisions about fertilizing and liming small fruit crops to achieve optimum yield and quality and to safeguard water quality in a cost-effective manner. You can contact Soil Solutions for all your soil testing needs. It is best to use local labs because they are calibrated for local soils and recommendations are tailored to Ghana’s climatic conditions.

Soil Solutions Soil Testing Kits and Labs
soil testing (1), leaf tissue analysis (2), compost testing (3), manure testing (4)

Taking a Soil Sample 

Although soil samples can be taken any time, many prefer to take samples in summer or fall because this allows time to apply any needed lime, plan a fertility program and order materials well in advance of spring planting. Avoid sampling when the soil is very wet or soon after a lime or fertilizer application. If a field is uniform, a single composite sample is sufficient. A composite sample consists of 10 to 20 sub-samples taken from around the field and mixed together. To obtain sub-samples, use a spade to take thin slices of soil representing the top 6” to 8” of soil. (A soil probe is faster and more convenient to use than a spade.) Put the slices into a clean container and thoroughly mix. Take about one cup of the mixture, dry it at room temperature, put it in a zip lock bag and tightly close it. Label each sample on the outside of the bag. To get accurate recommendations, make sure to download and fill out sample submission forms from the lab where samples will be sent.  

In many cases, fields are not uniform. There are many reasons for this including: uneven topography, wet and dry areas, different soil types and areas with varying previous crop and fertilizing practices. In such cases, the field should be subdivided and composite samples tested for each section.

Soil testing laboratories vary somewhat in their services and prices. Soils should be tested for organic matter content every two or three years. Be sure to request this if it is not part of the standard test. For more information, check with your state’s laboratory or Extension Specialist.

Cation Exchange Capacity 

Cation exchange capacity (CEC) is a measure of the soil’s ability to retain and supply nutrients, specifically the positively charged nutrient ions called cations. These include the cations calcium (Ca2+), magnesium (Mg2+), potassium (K1+), ammonium (NH4+), and many of the micronutrients. Cations are attracted to negatively charged surfaces of clay and organic particles called colloids. CEC is reported in units of milli-equivalents per 100 grams of soil (meq/100 g) and can range from below 5 meq/100 g in sandy, low organic matter soils to over 15 meq/100 g in finer textured soils and those high in organic matter. Low CEC soils are more susceptible to cation nutrient loss through leaching, and may not be able to hold enough nutrient cations for a whole season of crop production.

Base Saturation 

The cations calcium (Ca2+), magnesium (Mg2+), potassium (K+), hydrogen (H+) and aluminum (Al3+) account for the vast majority of cations adsorbed on the soil colloids in New England soils. Hydrogen (H+) and aluminum (Al3+) are considered acidic cations because they tend to lower soil pH while calcium (Ca2+), magnesium (Mg2+), and potassium (K+) are considered basic cations and have no direct influence on soil pH. Base saturation is the portion (expressed as a percentage) of the soil’s cation exchange capacity occupied by calcium (Ca2+), magnesium (Mg2+), and potassium (K+).  At one time, many labs provided fertilizer recommendations to achieve very specific ideal potassium, calcium, and magnesium saturation ratios. This approach was never well supported by data. Research conducted over the last several decades indicates that an ideal basic cation ratio does not exist and fertilizing to achieve a prescribed level of potassium, calcium, and magnesium saturation is unjustified. Still, base saturation can provide useful information. Your report will include the base cation saturation values observed for your sample and the ranges typically observed in New England soils. When base saturation is well outside of these ranges it is typically associated with deficient or excessive potassium or very acidic or alkaline soil conditions. Following the fertilizer and lime recommendations provided with your report will typically result in base saturation values within normal ranges.

Soil pH and Exchangeable Acidity 

One of the most valuable pieces of information you can get from soil testing is a measure of the soil pH. Soil pH is an indicator of the soil’s acidity which is a primary factor controlling nutrient availability, microbial processes, and plant growth.  A pH of 7.0 is neutral, less than 7.0 is acidic, and greater than 7.0 is alkaline. Maintaining proper soil pH is one of the most important aspects of soil fertility management. Most Ghana soils are naturally acidic and need to be limed periodically to keep the pH in the range of 6.0 to 7.0 desired by most crops and ornamental plants. When the soil is acidic, the availability of nitrogen, phosphorus, and potassium is reduced, and there are usually low amounts of calcium and magnesium in the soil. Under acidic conditions, most micronutrients are more soluble and are therefore more available to plants. Under very acidic conditions aluminum, iron, and manganese may be so soluble they can reach toxic levels. Soil acidity also influences soil microbes. For example, when soil pH is low (below 6.0), bacterial activity is significantly reduced. Acidic soil conditions also reduce the effectiveness of some herbicides.

When soil pH is maintained at the proper level, plant nutrient availability is optimized, solubility of toxic elements is minimized, and beneficial soil organisms are most active.  While most plants grow best in soil with a pH between 6 and 7, there are some notable acid-loving exceptions, including blueberry and rhododendron, which perform best in soils with a pH near 5.

Due to the climate and geology of Ghana, soils here tend to be naturally acidic (4.5-5.5). The most effective way to increase soil pH is to apply agricultural limestone. The quantity of lime required is determined by the target pH (based on crops to be grown) and the soils buffering capacity. Buffering capacity refers a soil’s tendency to resist change in pH. Soil pH is a measure of active acidity, based on the concentration of hydrogen ions (H+) in soil solution, and is an indicator of the current soil condition. When lime is added to a soil, active acidity is neutralized by chemical reactions that remove hydrogen ions from the soil solution. However, there are also acidic cations (H+ and Al3+) adsorbed on soil colloids (the CEC) which can be released into the soil solution to replace those neutralized by the lime. This exchangeable acidity, which is reported in units of meq/100 g, is directly related to the quantity of lime required to increase the pH from its current level to the target level determined by the selected crop. Soils such as clays or those high in organic matter have a high cation exchange capacity (CEC) and a potential for large amounts of exchangeable acidity. These soils are said to be well buffered. The lab determines buffering capacity and lime requirement by estimating the exchangeable acidity.

Occasionally soil pH must be lowered, because either the plant requires acid soil or the soil was previously over-limed. Incorporating elemental sulfur (S) is the most effective way to lower soil pH. Once applied, the sulfur oxidizes to sulfuric acid. Applying 5 to 10 lbs. of sulfur per 1000 sq. ft. will lower the pH of most Ghana soils by approximately half a unit. (Use the lower rate for very sandy soils.) No more than 15 lbs. of sulfur per 1000 sq. ft. should be applied at any one time. Retest the soil after 4 to 6 months to determine if more sulfur is needed.

Buffer pH

In addition to soil pH, many soil tests provide a reading called buffer pH (sometimes called lime index). Soil pH is a measure of hydrogen ion (H+) concentration in the soil solution. This is called active acidity. It is an indicator of current soil conditions. When lime is added to a soil, active acidity is neutralized by chemical reactions that remove hydrogen ions from the soil solution. However, there are also hydrogen ions attached to soil particles which can be released into the soil solution to replace those neutralized by the lime. This is called reserve acidity. Soils such as clays or those high in organic matter have a high cation exchange capacity (CEC) and a potential for high reserve acidity. To effectively raise the soil pH, we must neutralize both active and reserve acidity. Buffer pH is a measure of reserve acidity and is used by the soil testing laboratory to estimate lime requirement. Low buffer pH readings indicate high amounts of reserve acidity, and therefore, high amounts of lime will be recommended. The soil pH should always be lower than the buffer pH except on some alkaline soils. Instead of using buffer pH, some laboratories calculate lime requirement from CEC and base saturation, and others make this determination based on aluminum level.