It seems so obvious, and yet it can be a challenge for even an experienced gardener at times. Plants need nutrients. Most soils contain at least some of the necessary nutrients for plant growth, but sometimes there are deficiencies. The soil can lack nitrogen, phosphorus, or potassium – one of the essential nutrients. Sometimes we will see a deficiency of one of the less important nutrients (less important in that plants need smaller amounts), but that is not common.

You may think that a new garden won’t need fertilizer. That can be true, but not always. Some of our city soils are pretty depleted! In the Demo Garden right now, we have some good examples of plants that are healthy and plants that are hungry – they could use some more fertilizer.

These are some cherry tomatoes that are in gallon pots. (They are for a youth gardening project I’ll be starting next week.) The potting soil doesn’t have fertilizer, and I haven’t been doing a good job of watering with fertilizer either. The plants have kind of a sickly yellow tinge to them. Some of the lower leaves also show some purple color. The yellow is indicative of nitrogen deficiency, and the purple indicates phosphorus deficiency. However, to clearly see the deficiency, let’s compare to a nice, healthy, dark green tomato plant.

Can you see the difference? This is the row of cherry tomatoes, and we incorporated compost and a pound of an 11-15-9 granular fertilizer before planting. These plants are dark green, healthy, and growing well. They are not over-fertilized because they are setting fruit, but they certainly aren’t starving!

The other place we are seeing nutrient deficiencies is in our Family of 4 Garden. The reason for this is that we have had it continuously planted since the spring of 2008. This garden has hardly had a break! Because of that, it didn’t get very much compost this spring, nor has it had a full dose of fertilizer. Look at these peppers. Can you see the differences? The back two rows are nice and green, while the front two rows are small and yellowish. I fertilized the back rows almost 3 weeks ago, and they have really taken off. The front two rows were planted amongst the lettuce plants, and didn’t get fertilized until a week ago after the lettuce was harvested. They are starting to green up, but they are still behind.

The tomatoes in the Family of 4 Garden are also a little bit yellow. They just haven’t been growing as well as the other tomatoes in the Demo Garden. This is a good example of the dangers of interplanting different crops. Undoubtedly these plants need some additional fertilizer. However, cabbage plants are a notoriously heavy nitrogen feeder. They are probably using a lot of the available nitrogen, depriving the tomatoes!

The best solution to deficiencies like these is to sidedress a granular fertilizer (either synthetic or organic) around the plants. Don’t use too much, or you’ll burn the plants! I’ll probably use about 1/6 pound in the  4 x 4 tomato area. You can also use a water soluble fertilizer if you prefer. Topdressing with an inch or two of compost can also help provide additional nutrients.

I’d better go fertilize before I forget!