All Hydro 101

Soil Conditioning 101: Better growing through chemistry

At the beginning of every planting season many gardeners begin the ritual of tilling up soil and preparing both new and old garden beds. Before hauling out the old pickaxe and shovel, consider some simple, chemical properties of soil that can be manipulated to increase its quality. Through the use of a few agricultural substances, a lifeless plot of dirt can be cultivated into a thriving garden without breaking the bank or your back.

Initial Soil Assessment

Preparing soil to grow healthy, productive plants is not rocket science, but does require some planning and forethought. It is imperative that your soil’s chemical qualities are assessed before cultivation. An inexpensive soil test can provide an accurate representation of the soil. A composite sample should be collected by combining several smaller samples from the area, and sent to a local agricultural lab to have it tested for basic parameters such as NPK concentration, calcium, micronutrient concentration, and pH. The pH of the soil can be tested at home using a digital pH meter or chemical pH-indicating test paper. Simply gather a completely dry soil sample and hydrate it slowly with distilled water until it is fully saturated with as little excess water as possible. Then insert the probe or test strip into the soil paste.

Modification of Soil pH

Since some of the most common problems tend to be pH-related nutrient deficiencies, the initial pH testing can reveal the changes that must be made. Soils which are more basic (higher pH), are composed of layers of clay which possess a net negative charge, meaning that cationic (positively-charged) plant nutrients such as calcium, magnesium, and iron will adsorb to their surfaces and potentially become fixed, or unavailable to plants. Conversely, acidic (lower pH) soils are made of particles with a net positive charge, causing important anionic (negatively-charged) plant nutrients such a nitrate, phosphate, and sulfate to become fixed. Additionally, acidic soils may provide toxic amounts of metals such as aluminum, iron, and manganese. In order to prevent crippling nutrient deficiencies in most crops, the soil pH will need to be decreased if above 7 or increased if below 6.

Increasing the pH (liming) of an acidic soil is relatively straightforward. There are many inexpensive compounds available for this purpose. Such compounds include pure calcium carbonate (CaCO3) or agricultural limestone (the mineral in which calcium carbonate naturally occurs), magnesium-containing dolomitic limestone, wood ashes, and ground oyster shells. These substances neutralize hydronium ions, the concentration of which is exactly what pH measures, in the soil solution. Agricultural limestone is perhaps the most practical of these liming agents due to cost, but some of the others may provide additional nutrients.

Acidifying alkaline desert soils is notably more difficult due to the time required for acidifying materials to break down and take effect. Elemental sulfur is a direct way of lowering soil pH, but requires 4-6 weeks to break down with the help of ubiquitous soil bacteria. When elemental sulfur is incorporated into the soil, it is first present in the reduced form, but over time it oxidizes after exposure to oxygen and water, and releases hydronium ions into the soil solution. Ammonium (NH4+) fertilizers will also oxidize to release acid after soil application, providing much-needed nitrogen to the soil. It should be understood that gypsum (calcium sulfate, CaSO4) or any sulfate fertilizers do not decrease soil pH as many gardeners assume. The sulfate anion is already present in an oxidized form, and will not release hydronium to the soil solution. While not usually the primary reason for its incorporation into soil, organic matter can also be used to decrease pH. Addition of organic matter to the soil produces CO2 in pore spaces, which subsequently creates carbonic acid in the soil solution.

Soil’s Physical Properties

Unfortunately, altering a soil’s physical properties is not as practical as changing its chemical properties such as nutrient concentration or pH. However, some simple steps can be taken to change the most important of these physical parameters such as porosity, water-holding capacity, and aggregation, or the way in which clay particles are distributed.

Regardless of initial soil type it is always beneficial to add organic matter such as compost, crop residue, or organic material to introduce carbon. While these materials provide plant-available nutrients and foster microbiology, organic matter improves the water holding capacity and porosity of soils. Additionally, soil carbon behaves similarly to clay in that it is surrounded by negative charge which allows it to store many cationic nutrients for later use by the plants. This phenomenon, known as cation-exchange capacity (CEC), helps regulate soil fertility and can prevent some nutrients from being washed away.

The presence of sodium in many soils is a natural but highly problematic condition which prevents effective water penetration. Soils high in sodium form a crust at the surface which causes water to roll off before it has a chance to percolate downwards. Although gypsum does not affect soil pH, it is very effective at removing sodium. Additionally, adding gypsum is very inexpensive way to improve the water penetration and structure of clay soils. After dissolving in the soil solution, the calcium sulfate present in gypsum ionizes and the calcium cation begins to compete with sodium cations for adsorption to clay surfaces. In the process of flocculation, the attraction between calcium and the negatively-charged clay surface is much more energetically favored than that of sodium, and the sodium ions are displaced. To obtain the greatest benefit, apply the gypsum and dig it into the first few inches of the soil. The soil should then be heavily irrigated.

Soil Fertility

Nutrient concentration in soil (or lack thereof) is often the most limiting factor in terms of plant growth besides water availability. Of the essential plant nutrients, nitrogen is the most critical and often most deficient. Nitrogen is extremely soluble and leaches away from soil quite easily. Application of a nitrogen-containing fertilizer during soil preparation is essential to ensure productive growth. Nitrogen is also one of the most diverse nutrients as it comes in many forms, both inorganic and organic. While only the inorganic forms (NO3 and NH4+) can be assimilated by plants, the nitrogen in organic sources such as fish emulsion or animal manure will eventually convert to plant-available forms. Unlike synthetic nitrogen fertilizers, organic materials provide the additional benefit of contributing other nutrients as well as soil carbon.

Nutrient concentration in soil (or lack thereof) is often the most limiting factor in terms of plant growth besides water availability.

Phosphorus is often the second-most limiting plant nutrient, but unlike nitrogen which washes away with ease, it is extremely immobile in soil. In fact, Phosphorus is so immobile that plant roots cannot utilize it unless it is located extremely close to the root surface. Phosphorus fertilizer should not be applied until after plants have begun to grow, and should be applied in thick bands right near the root zone. Unfortunately, many natural phosphorus fertilizers like bone meal or rock phosphate are unsuitable for use in high-pH soils, as they form incredibly insoluble compounds. Instead, synthetic ammonium-phosphate fertilizers or animal manures are recommended. Animal manure can be surprisingly high in Phosphorus and if it is being used to meet a plant’s nitrogen demand, chances are it will provide an appreciable amount of Phosphorus if applied near the root zone.

With the exception of potassium, in heavily leached soils, nitrogen and Phosphorus are typically the only two nutrients that need to be applied to most soils in major quantities. The rest of the essential plant nutrients are usually present in high enough concentrations to meet plant needs and do not need to be added, as their availabilities are dictated by pH. For example, calcium and metal micronutrients like iron and zinc are more than abundant in many Western United States soils, but do not become available until soil pH decreases. In the acid soils of the Pacific Northwestern, Midwestern and the Eastern Coast states, calcium requirements may be met simply by applying lime. To reiterate an earlier point, a soil test will indicate exactly which nutrients are present or lacking in a soil and many labs will make recommendations for fertilizing as well. Lastly, when applying any type of product to soil, whether it’s organic fish emulsion or ammonium polyphosphate, it is important to follow the manufacturer’s listed application rates to avoid damaging plants.

– Connor Osgood, Botanicare

Continued in Water Quality for Hydroponics ⟶