Soil Test in the Fall for Next Year's Fertility
Each September morning is a little darker, a little cooler, and a little dewier as the earth wraps its way towards another autumn. Although you may be tired, ragged and bleary-eyed from a hot, dry season, autumn is the ideal time to turn your remaining energy towards next year's soil fertility plan.
The first step towards a soil fertility plan is taking a soil test. This can happen at any time of the year, but maintaining annual consistency is important as some nutrients fluctuate seasonally. On a practical level, fall soil testing is preferred by many growers because it allows the entire winter to plan crop rotations and to budget for cover crops and soil amendments for the following season.
Soil Test Methods
Even if you have been growing on a field or in a garden for many years and have a strong intuitive sense of your soil health, soil testing is an important tool for understanding the underlying chemical and biological dynamics of your soil. Although soil analysis is just one piece of information that can help guide your management decisions, it is a critical first step to take before you apply any soil amendments.
Most soil analyses for vegetable production provide a lime recommendation and a measure of nutrient availability. Soil testing labs use different methods, and it is best to select one appropriate to your region. This will allow your state extension agent and fertilizer dealers to interpret the results accurately since different labs use different extraction procedures and report results using different units. Furthermore, some states also have regulations specifying what type of soil testing you must use (be sure to check with your state's agency of agriculture). In New England, most state university labs offer test for around $14 and measure phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). Micronutrients commonly measured by a soil test include boron (B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn). Depending on which lab you use, there are many types of soil tests and many different options for what you should have tested. For most growers, starting with a standard soil test plus organic matter analysis should suffice for a basic soil fertility plan.
Good sampling techniques are not challenging and make all the difference in obtaining accurate results. First, determine what your field "management" units are. These units are simply based on how you manage your land, whether that is by physical boundaries, soil type, crop rotations, or some combination thereof. You can sample 100 square feet to 20 acres as one "unit." However, be sure to get a representative sample of the field by following a zig-zag pattern and taking 12-15 samples across the entire field. Stay away from edges of the field or any anomalous features, such as depressions or wet areas. If you can, use a sampling tube - this is easy, accurate, and efficient; otherwise use a spade or trowel, but be sure to just take a vertical column of soil from the shovel, discarding the edges. Sampling depth should be six to eight inches. Mix all the cores in a clean bucket, breaking up any clumps. Once well mixed, one cup of the soil can go in a labeled sample bag and sent to your lab.
Soil Test Interpretation
Soil test results are of little use if you don't understand them - and you may not have the time or desire to painstakingly decipher the results. Your state's extension is a great resource and should be able to help you interpret the results and offer nutrient recommendations. If you are working with a fertilizer dealer, they can also help translate the data into a plan (but remember it is their job to sell fertilizer!).
You will immediately notice that soil tests don't measure nitrogen levels in the soil. This is because nitrogen availability in the soil is always changing due to dynamic processes affected by temperature, moisture, biological activity and tillage practice. Most nutrient recommendation offer nitrogen application guidelines based on our crop needs BUT they do not account for nitrogen credits that exist in your soil. It is important not to underestimate your soil's nitrogen, which is mineralized over time from soil organic matter, previous applications of compost, manure, fertilizer and cover crops.
Your soil test should indicate available levels of the other nutrients your crops need: phosphorus, potassium, calcium, magnesium, sulfur, and many micronutrients. It is important to be mindful in your selection of soil amendments as well as your timing and method of application. Some amendments, like potassium sulfate, are inexpensive, quickly available to the crops, and have a relatively benign effect on the environment. Other amendments, like those high in phosphorus such as manure or compost, can boost your soil phosphorus levels, but can also have a negative impact on water quality if there is a heavy rain and runoff after an application.
Some amendments can be toxic if applied in excess or onto the wrong crops - so the message is ALWAYS know the safest practices for applying amendments and to consult with a trusted source before you make decisions. It is also always interesting to experiment in the field with your soil amendments - for instances, leave a few plants at the end of a row unfertilized and see what happens. Or try adding a little extra fertilizer to a couple of plants and take notes on your yields. You might be surprised to learn how your soil responds to the amendments you add.
So indulge in a walk around your farm or garden on a sunny afternoon in the fall and take some soil samples! It is a small job with potentially great payoffs - and it will put you and your farm on a lifelong journey towards ever better soil health.