A vibrant and prolific vegetable garden starts with well-tilled soil that has been amended to increase beneficial microbes and nutritive balance. A little information, some organic additives and some elbow work will produce the perfect soil balance for your vegetables. Consideration should be given to the type of original soil, varieties of vegetables, number of plants, drainage and the zone in which the vegetables will be grown. The goal is a perfect balance of soil structure and nutrients. A sandy loam works best for most vegetables.
Carbon and Nitrogen Ratio
The perfect soil combination will have a high concentration of carbon and nitrogen. These need to be replenished annually, as the previous year's vegetables will have used up some of these nutrients. Nitrogen is especially hard to supply, as nitrogen-based organics are resistant to decomposition and therefore don't release the compound as readily as easily decomposed carbons. In order to determine if you need to add more nitrogen or carbon to your soil, you can take a sample to a garden center or Master Gardener's clinic or use a home test kit. The carbon is important because it is food for the beneficial organisms in soil such as earthworms. You can easily add more carbon with mulch, old plant material or manure. The addition of nitrogen is best done by planting a cover crop in the Fall of a legume and then tilling it in after the danger of frost has passed.
The best soil for growing vegetables will be comprised of 40 percent sand, 40 percent silt and 20 percent clay. Too much clay will impede drainage and rot out the little plants, so adding sand will help create breaks in the clay to allow water to drain. Include plenty of organic matter that will also break up soil and allow drainage and penetration of oxygen to roots. Organic matter can be peat, sawdust, hay or any other dry organic material. Working these compounds into the soil in the Fall can increase decomposition and ensure garden-ready soil in the Spring. Almost any compostable materials added in Fall will increase the organic composition of the soil, and that will generate beneficial bacteria, act as food for good organisms such as earthworms and create a humus-rich environment for vegetable seed germination and growth. To get a visual assessment of what comprises your soil, put a cup of dry soil in a canning jar and add a teaspoon of non-foaming dishwasher detergent. Fill the jar two-thirds of the way with water, and shake thoroughly. Let it sit where it won't be moved for a few days or until the water is clear. The soil at the bottom of the water will separate into its parts. The top layer will be clay, then silt and finally sand. You will know what proportions the soil is made of and what to add to make the perfect mix.
Acid or Alkaline?
The pH of the soil is important to keep in check. If it is too acidic it can burn tender young roots, and if it is too alkaline it can impede the decomposition cycle that creates nutrition for plants. You can test the pH yourself with a test kit or take in a dry sample of soil to a garden center for assessment. Most vegetables prefer slightly acidic soil; the pH can be raised by adding lime. However, it is hard to decrease acidity once it has been raised too much, so avoid adding much lime. To increase your pH by 1.0 point and make your soil more alkaline:
Add 4 ounces of hydrated lime per square yard in sandy soils.
Add 8 ounces of hydrated lime per square yard in loamy soils.
Add 12 ounces of hydrated lime per square yard in clay soils.
Add 25 ounces of hydrated lime per square yard in peaty soils.
Alkaline soils are easier to achieve. You can add sulfur or just mix in composted oak leaves, wood chips, sawdust, peat moss or cottonseed meal.
Best Vegetables for Different Soil Types
Some vegetables demand more of one component of good soil. Brassicas, for instance, need more clay in the soil to help with water retention. Tubers and root vegetables need an equal sand-silt mixture to increase drainage and keep underground produce from rotting. Some root vegetables like carrots and parsnips need a little bit of clay in the sand-silt mixture to preserve moisture; however too much clay will keep the roots from pushing through the soil and will yield stunted or forked vegetables. No matter what produce is planted, make sure there is plenty of organic matter added to the soil to produce nutrients for the plants.
Time Frame for Amending Soil
Ideally vegetable garden soil should begin to be amended in the Fall. If there was a garden previously, then the old greens (minus any seeding fruits, vegetables or flowers) can be tilled into the ground. A layered compost of grass clippings and leaves will add carbon and nitrogen in almost equal amounts to the soil. It can be left on top of the soil or tilled in, but it will need to be tilled again in the Spring. Fall is also the time to start a cover crop. Chose red clover or any legume to add nitrogen. Till the crop in well in the Spring. You can test your pH in the Fall too, but keep in mind that acidic soils may take many seasons to amend so you will have to plan accordingly and perhaps put in raised beds with more alkaline soil for a season. Putting a little time and effort into the composition of your garden soil will increase your yield, save water and save time next season.