2011-12-22 / Editorial

The Island Garden

Understanding fertilizers


BY ROGER MARSHALL BY ROGER MARSHALL We’ve all been into the hardware store and seen bags of fertilizer piled high. We may also have succumbed to the temptation to buy a few of the bags. Of course, nobody in their right mind attempts to lift them. That’s why Jamestown Hardware has a bunch of young men running around. Their sole function is to lift heavy things for us older folks.

Each bag of fertilizer has a code on the side. It consists of three numbers and if you’ve been gardening for 40 years or more, you’ll probably have learned that these numbers represent how much of each nutrient is in the fertilizer.

For example, the number might read 15-10-5. These numbers tell the gardener how much nitrogenphosphorus potassium the fertilizer contains. Just to make it more confusing, the numbers are referred to as the N-P-K number. They are a national standard. Thus, if you have a 50-pound bag of fertilizer, in the example above 15 percent of it will be nitrogen, 10 percent will be phosphorus, and 5 percent will be potassium. The remainder is inert material to help you spread it easily.

OK, so now that you understand that part of the equation, you might want to know what that does to your plants? Frankly, it helps them to grow, provided the pH or soil acidity is correct. Plants like to grow in soil with the right acidity levels. For example, rhododendrons and hydrangeas love acidic soil. This means that they take up fertilizer better when the soil is acidic. Hydrangea flowers, for example, are blue when they are growing in acidic soil and turn pink when they grow in alkaline soils. You can change their color by adding acidic compost such as coffee grounds, or fertilizing with an acidic fertilizer.

Around here most soils are acidic simply because we get a lot of acid rain. However, the best way to determine your soil acidity is to get a soil test. To counter soil acidity, you need to add lime. That’s why we put lime on our lawns. Lime is not a fertilizer – it merely changes soil acidity. When a plant is growing in its ideal pH range, it grows at its best.

So we’ve corrected soil acidity levels, now we need to know what fertilizers work best for our plants. In general, lawns like high nitrogen fertilizer. That’s why in spring you’ll see fertilizers with numbers like 55-10-5 for lawns. In fall the numbers change slightly because you want to promote root growth so you may see a fertilizer that is more balanced, such as 30- 20-20. In general, leafy vegetables like high nitrogen fertilizers: cabbage, broccoli, spinach and other greens. Plants such as tomatoes and peppers require less nitrogen and more phosphorus such as 15- 30-15. Root crops tend to need more potassium so the last number should be slightly higher. Potassium also helps to strengthen roots and plant structure.

These numbers apply to both inorganic and organic fertilizers, but if you want to use solely organic fertilizers, you will need to understand what type of organic fertilizers contain what level of nutrients. Putting animal manure on your garden might make things grow but it won’t give you many tomatoes. Animal manure is high in nitrogen, as is blood meal. High nitrogen levels give you large leafy green tomato plants, but they will not set many tomatoes. Bone meal, on the other hand, is high in phosphorus and is good for tomatoes and peppers. (You should be aware that in the 1990s bone meal was identified as a vector for mad cow disease and you should wear a mask when applying it and any other dusty fertilizers.) Rock phosphate and rock potash are both good sources of potassium.

Compost, if it is well made, in the most balanced fertilizer that you can make yourself. Usually, it would rate about 5-5-3, but more importantly, compost contains high levels of micronutrients that are not present in inorganic fertilizers. Compost also contains high levels of vegetable matter that helps to hold water at the root level and makes it easier for your plants to take up nutrients. Adding compost to your soil is probably the most important thing you can do in your vegetable garden in spring. So, this year instead of tossing leaves and garden trash into the garbage, turn it into compost and reap the rewards next spring.

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