Organic Vegetable Garden Soil Preparation

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Soil quality is one of the most important factors in achieving high harvest yields, but is often the most overlooked for the beginner gardener. When I first took up gardening again as an adult I thought you worked the ground, planted your seeds, weeded and watered and presto! –  everything else took care of itself.  At least that’s how simple it was to grow my little garden when I was a kid. At that age, I wasn’t really concerned with the quality or measure of the bounty. As long as I got to eat a handful of sweet strawberries or chomp on a few carrots, it was all good. 

Preparing your garden soil involves adding items that improve the water retention and nutrient balance along with improving aeration (air flow). So let’s talk about each of these.

Soil Nutrients

The three primary nutrients used by plants are nitrogen, phosphorus and potassium. Testing your soil before you start will give you a very good idea of what nutrients your soil is deficient in, before you plant. For instance a couple, three years ago we added a new raised bed and filled it with a pretty good quality “vegetable-garden-ready compost mix” purchased from our local municipal supported composting company.  You would think that soil labeled as such would be good to go, but after testing it we discovered a severe deficiency of nitrogen. Soil testing is easy. There are inexpensive home soil test kits or you can contact your local extension office for low-cost soil testing.

Nitrogen is an especially important nutrient needed by your vegetables during their growth phase. The plant doesn’t actually use nitrogen, but actually consumes nitrates, which is created by specific bacteria that converts the nitrogen into nitrates. It degrades and washes away fairly quickly and therefore must be replenished before and during the growing season. To add nitrogen to deficient garden soil, amend it with items such as BloodMeal, well-composted organic material or worm castings from your vermicomposter.  Note : Too much nitrogen can result in huge, healthy plants, but at the expense of fruiting. So for fruit-bearing plants, such as tomatoes and peppers, don’t go overboard on the nitrogen. Also, many of you have probably heard that adding coffee grounds is great for your plants. The reason for this is because coffee beans are rich in nitrogen – however, it will also raise the acidity level of the soil. So be careful using coffee grounds, if you already have acidic soils.  More about acidic soils later.

Phosphorus is necessary for strong, healthy root formation as well as the flowering stage of plants. Add phosphorus to bulbs in the fall, root crops and to fruiting plants like tomatoes. During the flowering stage of tomatoes, for example, nitrogen levels should be lower and phosphorus should be higher – the reverse is true during the heavy growth or vegetative stage. If your soil is deficient in phosphorus, amend your garden bed with steamed BoneMeal or phosphorous-rich seabird or bat guano (not all guano is the same, so be sure the nitrogen level of any guano you choose is low).

Potassium (Potash) provides for a plant’s overall health by supporting the immune system and helps the plant produce carbohydrates and proteins needed to grow well. Like nitrogen, potassium is also water soluble and requires replacement every so often. If your soil has a severe deficiency, add rock potash which provides a 10-11%  of slow-released potassium. If the deficiency is minor, amend your garden soil with seaweed meal or a liquid kelp solution.  You might have heard that wood ash is a great organic source of potassium, which is true, however, use caution. Wood ash increases soil acidity, so only add it to alkaline soil.

Speaking of acid vs. alkaline soil leads us into another aspect of soil condition – pH. In simple terms, pH is the level of acidity of your soil. The more acidic soil is, the lower the pH. The reverse is true for alkaline soil. pH is measured on a scale of 1.0 – 14.0, with 7.0 being neutral. Nutrients are absorbed best by plants when the pH is within a certain range. Every plant type prefers a different range. For example, tomatoes are grown in pH range of 5.5-7.5, while broccoli likes a range of 6.0-7.0. As a general rule of thumb, most veggies can fit in a range between 6.2 - 6.8. There are some plants that really thrive in acidic soils however, such as blueberries that likes a soil pH of 4.0-5.0. To adjust soil pH up to be more alkaline, amend soil with agricultural lime (calcium carbonate) or dolomite (magnesium carbonate & calcium carbonate). To lower soil pH to be more acidic, add sulfur to your garden beds. pH can also be easily measured with a home test kit.

In addition to the 3 main plant nutrients, trace minerals should be considered as well. These include calcium, magnesium, zinc, molybdenum, copper and more. Fish meal, kelp and worm castings tend to be good sources of trace minerals as well as crushed volcanic rock. You can purchase trace minerals that might be sold as “micronutrients” or “soil booster”. Just read the labels to be sure you’re getting natural ingredients and not some weird synthetic concoction.

Soil Structure

Soil structure describes the density, water retentiveness and texture of the garden soil. Optimal garden soil retains moisture well and evenly while at the same time has good drainage of any excess water. This means a good percentage of the soil must be made up of fibrous, organic materials – approximately 30%. This comes from composted organic matter like leaves or peat or coir.

Coconut coir is a great amendment for adding moisture-retaining texture to your soil, has a neutral pH and is a highly renewable resource and is considered the “new peat”.

Peat is also a great soil amendment for the same reasons. Just a note on peat though – Mountain peat is harvested from high-altitude bogs and takes hundreds of years to replenish itself. The practice of harvesting mountain peat is damaging to the water and mountain ecosystems. In addition, it has a finer texture and higher pH, which doesn’t make this peat a good choice. On the other hand, Sphagnum peat is mined from Canadian and Northern U.S. bogs and can be revegetated and grow back relatively quickly. It has a coarser texture than mountain peat and a lower pH. I still personally opt for coconut coir over sphagnum when given the option because it is a more sustainable product. Both peat and coir offer a lighter texture, allowing for better air circulation within the soil.

Another great soil amendment are worm castings - for several reasons. It provides for a light, fluffy texture allowing for good soil aeration, beneficial bacteria, nutrients and trace minerals and retains a good level of moisture. Keep a vermicomposter and to have fresh worm castings available at your fingertips.

Sand is another amendment material that provides good drainage, however, if added to a high-content clay type soil, it can actually make it harder – like concrete.

Many gardeners add wood products to soil to give it the texture and moisture-retentive properties desired, but be advised that wood fibers tend to absorb and hold on to nitrogen. If you opt to use wood material, you’ll need to add more nitrogen than usual.

If you live in a rainy area, you should cover your garden soil at least 2 weeks before planting so it can dry out a bit. Digging in and working soggy, drenched soil, can ruin the soil structure. Keep the beds covered until it’s time to plant. A nice side effect of covering the soil is that it warms up earlier.

Now, given the discussion above, there are things you can do in the fall and winter to help prepare and improve your soil over the winter which minimizes the amount of soil preparation needed each spring. Later this year, I’ll talk about that more.

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