Are you eager to start your outdoor seeds and plantings? Many like broad beans, calendula and cilantro can go in the ground right now, provided that the soil temperature is at least 50°F. (Did I mention that a soil thermometer is a great investment?)
Before you plant, prepare your garden beds. Loosen the soil and either remove or turn in unwanted matter like annual weeds. And you’ve taken care of any necessary soil texture and pH amendments for your plant choices, of course. So, you’re ready to go, right?
Not so fast … remember the Rapitest I recommended last year? The one that tests soil pH? If you bought a Rapitest, you noticed that it includes three other tests: N-P-K. These three letters stand for nitrogen (N), phosphorus (P), and potassium (K) – three elements that plants need for healthy growth. While these three are not the only nutriments that plants use, they are the three that plants need in abundance. Fortunately, all are amenable to home testing without the time and expense of a professional soil test.
These three elements are behind the numbers on fertilizer packages (e.g., 3-2-2) that denote the amounts of N, P and K, respectively in a fertilizer. Fertilizers that supply two or more of the three elements are called ‘all purpose.’ The actual numbers indicate the percent of the fertilizer’s weight that each element can claim. The numbers don’t add up to 100 percent because of fillers and other substances. The key to selecting a fertilizer is to examine the relative amount of each element, not the absolute amount. The absolute numbers will affect the rate at which you apply the fertilizer. You already knew that you are supposed to read the information on application rate, right?
Let’s tackle the first number. You might remember from high school chemistry that nitrogen is a prominent component of many organic compounds, along with carbon, hydrogen and oxygen. It’s no surprise, then, that adequate amounts of nitrogen result in more plant tissue – meaning plant growth. Plants use nitrogen to build leaves, stems and roots. Nitrogen is also a notable element in chlorophyll, so it is necessary for photosynthesis. When you think of nitrogen, think primarily ‘green stuff.’
Although our atmosphere contains abundant nitrogen, plants cannot absorb or use nitrogen directly from it. They can only absorb nitrogen that has been washed into the soil and converted to a usable organic compound by bacteria in the soil or in the roots of ‘nitrogen-fixing’ plants. Since leaves contain large amounts of nitrogen once it’s been converted, digging that ‘green stuff’ back into the soil either directly or via composting will also supply nitrogen that can be used by new plants.
This is behind the practice of ‘cover cropping.’ Cover crops are sown on barren ground in early autumn, left to grow over the remainder of the year, and then tilled back into the ground in advance of direct sowing or planting of new plants. Legumes like peas and beans and a couple of other nitrogen-fixing cover crops like radish are especially valuable for supplying rapidly available nitrogen as you till them into the soil and they break down. Clearly, cover crops are not used in perennial beds. But cover cropping is a good practice for annual garden beds and for preparing a new garden patch. Keep this in mind next late summer and early autumn as you harvest or think of preparing a new planting bed.
Without cover cropping or amendments, our constant cultivation of new plants can easily delete the amount of usable nitrogen in the soil. Even without soil testing, you might notice a nitrogen deficiency in poor growth or yellow leaves. Subsequently you, dear gardener, might need to supply nitrogen-deficient soil with additives like kelp, composted chicken manure or a commercial fertilizer.
But please don’t add nitrogen unless you know that your soil is deficient. Excessive growth from nitrogen can result in poor flowering and fruiting because the bulk of the plant’s resources are devoted to using all that nitrogen for green growth. That might be great for your leafy greens (remember green stuff?), but not for your ornamental flowers or root crops.
In the soil, nitrogen also combines with other elements to produce mineral salts which can block a plant’s uptake of water. Therefore, excess ground nitrogen can result in plants wilting despite generous watering.
And if that’s not bad enough, excess nitrogen can seep through the watershed and pollute coastal waters. That’s more of an issue with large-scale agricultural use of nitrogen-rich fertilizers, but there is no need for us to add to the problem.
There’s more to plant nutriments than that, but this will give you some insight into why soil testing and choice of amendments is critical for successful gardening.
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