Autumn planting: soil chemistry

Now that it is officially fall, it's time to get serious about the upcoming California native planting season.

Seasonal plant sales are just around the corner and it's a perfect time to consider replacing turf with something that demands less water. 

Last month, we reviewed some of the conditions that guide us towards matching plants to the right location. Conditions such as microclimate (sun/shade patterns, wind etc.) and soil are two variables present in every landscape. We offered some methods for assessing soil type and drainage (see Getting Native on Facebook or at www.gettingnative.com).

There are some remaining attributes of soil that we did not discuss, namely chemistry and biology.

Years of geological formation and accumulation of organic materials give us the stuff we call soil, a rich matrix of components, and a characteristic profile that defines its chemistry. Gardeners are interested in this because it tells us what nutrients are present and if the soil is acidic or alkaline, otherwise known as its pH.  Measurement of pH is expressed on a numerical scale—where pH7 is neutral, below pH7 is acidic and above is alkaline. Most soils range between pH5 and pH8.

Needless to say, you can't tell the pH just by looking at the soil. You will need to test it, either by sending a sample to a soil lab or by means of a simple DIY kit from your local garden store.

The soil lab results will give you a detailed breakdown not only of pH, but also a profile of the macro- and micronutrients and a basic result cost approximately $100.

Macronutrients are the main plant foods, the primary ones being nitrogen, phosphorous and potassium (often abbreviated to 'NPK' and appearing as the three main numbers on fertilizer containers - as in 5-5-10 for instance). Calcium, sulfur and magnesium are considered secondary macronutrients. Lesser constituents like iron, boron, zinc and others are considered micronutrients but are also essential plant foods.

A more thorough (and expensive) lab test will give you the nutrient analysis plus a breakdown of the soil structure and drainage. Running around $165, this may be a good choice if you are planning extensive renovation.

DIY kits from your local garden store will approximate the macronutrients as well as the pH and is a good and cost-effective option for many gardeners.

Many Mediterranean climate native plants prefer nutrient poor soils and it is worth checking on this before selecting plants. Some also have a specific preference for either acidic or alkaline soils so this is also worth checking. Thankfully many have wide latitude when it comes to pH tolerance and for most home landscapes this is not a major constraint.

Each nutrient, whether macro or micro has a specific function in the plant metabolism and therefore, plant health relates ultimately back to the soil. Discoloration of the leaves and general demeanor of a plant is often an indication of some imbalance of nutrients. pH plays into this as different nutrients can be more or less available to the roots according to the level of acidity or alkalinity. Adding calcium to acid soil will help raise the pH. Conversely, adding sulfur will lower the pH.

The subject increases in complexity the more you look into it but we should not neglect the role of the organic systems in the soil. Macro- and microorganisms are at work full time ingesting and converting organic matter into plant nutrients. This is why organic matter is so important to food crops for instance. For ornamental plants, organic matter in varying amounts in always beneficial. Plants that are native to desert and chaparral soils have evolved in less rich organic soils while plants native to riparian and woodland environments are used to high amounts. This information can guide you toward matching plants to your soil or towards amending your soil with organic matter depending on what the plant needs.

Next time we will talk more about how plant spacing can save you money and lower your maintenance costs.