Vegetable Gardening (108 page)

How to detect too much:
A plant with too much nitrogen has soft-textured, dark green foliage and an underdeveloped root system, and flowering and fruiting are delayed. Too much nitrogen fertilizer, especially when plants are young, can actually burn the roots and kill them. Because nitrogen leaches out of the soil quickly, if you have too much, stop fertilizing, wait, and eventually the problem will solve itself.

How to add it to your soil:
You can supplement soil nitrogen by adding organic fertilizers, decomposing organic matter, or composted manure.

Phosphorus (P) helps promote good root growth, increased disease resistance, and fruit and seed formation. It's less available in cooler soils, so adding a fertilizer containing phosphorus in spring is a good idea.

How to detect too little:
Plants lacking in phosphorus are stunted and sport dark green foliage and purplish stems and leaves (on the older leaves first). Soil pH affects the availability of phosphorus to plants; a pH of 6 to 7.5 keeps it available.

How to detect too much:
Too much phosphorous is a problem because it can cause zinc and iron deficiencies, and it can leach out of the soil and pollute streams and lakes. Signs of zinc or iron deficiencies appear as discolored or poorly formed leaves and reduced growth and production.

How to add it to your soil:
Sources of phosphorus include minerals, organic matter, inorganic fertilizers (such as rock phosphate), and organic fertilizers (such as bone meal).

Unlike nitrogen, phosphorus doesn't move quickly through soil. So, add a fertilizer containing phosphorus to the root zone before planting (instead of sprinkling it on the soil surface).

Potassium (K) promotes vigorous growth and disease resistance.

How to detect too little:
The first sign of a deficiency is slowed growth; brown leaf edges and deformed yellow leaves can indicate a severe deficiency.

How to detect too much:
Too much potassium can create nutrient imbalances in the plant and stunt its growth.

How to add to your soil:
Soil minerals provide potassium, as do organic matter and inorganic fertilizers, such as green sand and granite dust.

Secondary nutrients and micronutrients

Calcium, magnesium, and sulfur are known as
secondary nutrients.
Plants need them in substantial quantities, but not to the same extent that they need nitrogen, phosphorus, and potassium.

How to detect too little:
Most alkaline soils contain these secondary nutrients naturally, and few soils are deficient in sulfur. Most home garden soils contain these nutrients. But if you've eliminated other possibilities and you suspect a problem, a soil lab can do tests on your plant leaves to detect any deficiencies.

How to add them to your soil:
In regions where the soil is acidic, liming to keep your soil's pH in a good growth range provides adequate calcium and magnesium (the latter if you use dolomitic limestone). See Chapter 14 for more on liming.

Micronutrients
(iron, manganese, copper, boron, molybdenum, chlorine, and zinc) are elements that plants need in tiny amounts. Too much of one of these elements is often as harmful as too little.