Soils can never produce top yields without sufficient nitrogen to do so. There are no shortcuts; a pound of nitrogen required by the plant is a pound of nitrogen that must be supplied from some source, or the plant suffers loss.
What do you do when soil tests show that nitrogen is the greatest limiting factor: to achieve best results for high nitrogen-requiring vegetables such as sweet corn, tomatoes, cucumbers, squash, carrots and members of the cabbage family, use approximately 200 lbs. of actual nitrogen per acre (220 kilograms/hectare) plus a pound of nitrogen for every 100 lbs. of undecomposed residue. For legumes such as beans and peas, consider 100 lbs. of nitrogen less the amount supplied by the humus content.
Many will maintain that this amount is far too much nitrogen, and it is if your soil is in poor condition. But for soil with the proper nutrients to grow high yields, anything less in terms of nitrogen will limit both yield and quality.
Phosphorus
This nutrient is considered of primary importance for growing crops. It is called “the workhorse” of plant nutrition. Phosphate, the form required to be of use to plants, is necessary for seed germination and plant growth, inducing extensive root systems, producing much larger leaves (when supplied to excellent levels) and needed for water uptake. Adequate phosphorus also helps plants withstand cooler temperatures. The nutritional value of the plants we eat is also diminished when the soil is lacking in phosphate. We see this with livestock. When soils deficient in phosphate are supplied the proper amount, a feed analysis will show an increase in phosphate as well. What is true of cattle feed proves true of the foods we eat as well.
When it comes to organic production, those who have access to good compost can solve any phosphate deficiency when enough is applied. The problem is that many who use compost to grow organically believe that if enough is good, more is better. Even with the best compost, such thinking is a serious mistake. Too much compost will soon build up excess phosphate in the soil.
Excessive phosphate affects the availability of other needed elements for plant uptake and use. Too much phosphate limit’s the availability of sulfur to growing plants. When excessive, it also ties up the zinc and when extremely excessive limits copper availability as well. In plant nutrition, sulfur is a key to palatability, and one of the signs of excessive phosphorous is a lack of taste. When the problem becomes severe, many types of vegetables begin to have a bitter taste.
It is well-known that zinc is commonly lacking in plants grown in soils excessive in phosphate. Zinc is essential for moisture absorption by plants, and too much phosphate in the soil can affect zinc to the point of requiring additional water to properly grow the plants.
As for copper, which is very deficient in most soils of the world, an excess of phosphate can cause plant deficiencies even in soils which appear to have sufficient copper levels. When tomatoes crack around the stem it is a sure sign of copper deficiency in the plants. When potatoes exhibit grayish colored spots as they are being peeled or once they are boiled for eating, it is due to a lack of copper. Some believe the problem is a lack of calcium, but calcium only solves the problem to the extent that it aids copper uptake when there in sufficient amounts present to meet the need of the potato plants.
If the plants we grow do not have sufficient uptake of the necessary nutrients, how can we expect to find these nutrients in plants?