Yep, same time every year is best. Fall is the optimum time for several reasons. First, the soil is likely to be as depleted as it will get. Second, it gives you a chance to apply some amendments before putting the garden to bed. Some amendments take time to work, like lime or sulfur.
You can certainly have different soil profiles depending on how you garden. It's important to take samples from each area and have them tested separately.
pH is critical to many soil nutrients. Here is an example:
In a garden with a pH of 4.9 the gardener suffers a near total loss of his tomato crop to BER (blossom end rot) and blossom drop. Why?
pH is a measurement of how acidic or alkaline the soil is. Why is that important? If you think about the terms acidic and alkaline, the answer starts to become clear. Soil is composed of many minerals and metals such as potassium, calcium, zinc, boron, aluminum, and even lead. In simple terms, an acidic soil is just that, acidic. If the pH level is low enough, the soil acid levels will be strong enough to dissolve the aluminum component of the soil. This free aluminum will now be in a form that is capable of occupying CEC sites, restricting the amount of calcium available to plants, especially in low Ca soils.
The CEC is basically a reserve of certain cations in the soil. Cations are positively charged ions such as Calcium (Ca), Potassium (K), Magnesium (Mg), and Hydrogen (H). These cations become available as they are dissolved by soil acids and become soluble. These cations attach to particles in the soil such as clay and are then transferred to plant roots. The soil only has a certain number of points for the cations to attach too. The measurement of this number of points is called the Cation Exchange Capacity, or CEC.
Another part of the CEC are the Base Saturation and Nutrient Saturation, or how many exchange sites are occupied by what cations. This is important to know because plants can only get these certain nutrients from the CEC sites.
Now, let's put this all together so we can see why a low pH can cause BER in tomatoes, squash and zucchini.
As the pH level drops, the soil becomes more acidic and eventually reaches the point that aluminum becomes soluble. Plants have no use for aluminum. Soluble Al is a positive cationuh oh. What do positive cations do? They attach to CEC sites. In this case the soluble Al attaches to sites that would normally be occupied by calcium. Calcium is used by plants to build strong cell walls, among other things. Since Calcium is also immobile in plant tissue a Ca deficiency will be first noticed in growth tips and the ends of fruits where the deficiency is greatest. The weaker cell walls collapse and die resulting in sunken dark areasBER.
So, in our low pH soil, we have excessive soluble Al taking up the CEC spaces that Ca needs to occupy. The plant is not getting adequate Ca and suffers BER and poor growth.
Remember that this is only one possible cause of BER. Low soil calcium can also cause it in soils with a normal pH because there simply isn't enough Ca in the soil to begin with. This is rare however.
Yes a soil sample is 1/2 pound representing tons of soil. But the average garden has tons of soil that is exactly the same.
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