Critical Points of Influence in Reproductive Crops

Adapted from a talk given by John Kempf, founder of Advancing Eco Agriculture

I came up with the phrase “critical points of influence” to describe a window in a plant’s growth cycle in which a great deal of its future yield potential or disease susceptibility is being determined. These points are the times when a knowledgeable manager can best influence both a crop and the operation’s bottom line by delivering the right amount of the right nutrition in the right form.


This is complex, interesting stuff, and I will try to pare it down to essentials while providing enough of the science to help you in the field. 


All living organisms go through periods of expansion and contraction. Let’s use a tomato for example, and think in terms of energy concentration. A tremendous amount of energy is compressed in the seed. As it germinates, the energy expands and a seedling emerges. The energy is much more diluted as the plant builds vegetative biomass. And then the energy concentrates again to form a bud and flower. As the fruit begins growing and filling, energy expands. It contracts again as the fruit itself forms seeds, and so on. So you have a constant cycle of expansion and contraction, male energy and female energy, vegetative growth and reproductive growth.


Certain nutrients are associated with each type of energy or growth. Nitrate, calcium, potassium and chloride feed the vegetative process, building biomass and crop frame. Three powerful nutrients on the reproductive side are manganese, phosphorus and ammonium. Trace minerals are also critical.


It may be useful to visualize the expansive (male, vegetative) energy and the compressed (female, reproductive) energy as separate, twisting strands of one rope. Both strands are always present and are necessary to the integrity of the whole, but at any one time one of the strands will be dominant, showing at the top of the rope.


Likewise, plants never have just reproductive or just vegetative energy. There is always a blend of the two, existing in a ratio that narrowly favors first one type of energy and then the other. What shifts the plant between the two is just a slight tip in the balance. 


Plants probably have more expansion and contraction cycles than you think. Corn, for instance, has 13 during its lifespan. The reason these cycles matter is because yield, genetic expression and disease resistance all are directly connected to how well a plant is able to transition from one type of energy to another. In reproductive crops like tomatoes or corn, the number of buds a plant can set is determined at one critical point of influence. Later on is another critical point of influence that determines the pollination success rate. After that is a point where available nutrition decides how many ears or tomatoes a plant will keep. So, to produce a high-yielding crop of marketable fruit, we need to focus on supplying nutrition at all the points where a plant is “making decisions” about its reproduction.


I’ve observed that on many crops the reproductive parts of plants are the weakest. They are the most susceptible to insects and diseases because they have the highest mineral nutrition requirements. Reproductive buds, an embryo, and a blossom all have zinc, manganese, boron, and other trace mineral concentrations as much as 80 to 100 times greater than leaf tissue. When trace minerals aren’t available in adequate supply, the plant becomes susceptible to pests.

“Think of the male vegetative energy as the orange strand, and the female reproductive energy as the green strand. Both are always present but switch dominance throughout the season.”

We have learned that the transition from vegetative to reproductive growth is a critical time in plant development, and it is just prior to these transitions that you have a window to make a difference—critical points of


Let’s look at another example, this time using perennial fruit crops. A fruit tree or berry vine goes into dormancy in the winter and is in an essentially vegetative state until the weather warms in the spring. Then the plant becomes reproductive-dominant and begins pollinating and starts to form blossoms. An apple tree will pollinate and set fruit in a five- to seven-day time period, during which the tree experiences a tremendous hormonal shift to support its “pregnancy.” The tree needs high concentrations of trace minerals, and calcium. If it doesn’t get what it needs to support the hormonal shift, it begins to sabotage itself, switching from protein synthesis to proteolysis (protein breakdown). This makes it an attractive food source for insects and diseases. Many of the yield-limiting problems in fruit production—apple scab, codling moth, plum curculio, and so on—result from the plant not having the right nutrition at the right time.


During daylight of each 24-hour photoperiod, plants absorb carbon dioxide from the air and water from the soil, catalyzing them with sunlight energy to form a broad array of sugars (photosynthates) which are sent from the leaves via the plant’s sugar transport system (phloem) to the plant’s “sugar sinks.” The three major sugar sinks are filling fruit, new growth in the upper part of the plant, and adding to the root system. The hormone auxin determines where the sugars go. For example, when a plant reaches the fruit-fill stage, the fruit starts producing very high levels of auxin and the plant sends all of the sugar to the fruit and seeds, often resulting in a decline of the sugar-deprived root system. This is bad for plant health.


Two antagonistic groups of plant hormones, cytokinins and auxins, are key in managing and maintaining plant health. (see chart below) Cytokinins are reproductive hormones produced in growing root tips, hopefully during every 24-hour photo cycle. When cytokinins move to the upper part of a plant, they slow vegetative growth. A plant with regular, strong cytokinin production will express it in closely spaced internodes and strong bud initiation.


Auxins are vegetative hormones produced in the growing shoot tips. They cause very rapid top growth with long internodes, and they suppress bud initiation. When auxins move down to the root system, they shut off root growth.


So you have these two teams of hormones waging constant battle inside a plant. Cheer for cytokinins; healthy plants are always cytokinin dominant. They have more root growth below the soil than they have shoot growth at the top of the plant.

We’re beginning to come to the crux of the matter; hang in there.


Calcium is the control switch on auxin production. On the day that your soil is not able to supply enough calcium through a plant’s water and nutrient transport system (xylem), auxin production will explode at the top of the 

plant, resulting in a burst of vegetative growth. We’ve seen this happen time and again in commercial tomato production. About eight weeks after the plants have been in the field, they will be 36 to 42 inches tall with nice, tight canopies. They will size and fill the first several clusters of fruit when all of a sudden comes this rapid shoot of growth to the tops of the plants, triggered by seeds in the fruit producing auxins, which summon the sugars from the leaves into the fruit. Now the plant’s sugar supply is going to fill fruit and to top growth, with little left to supply energy to the root system. This slows down root growth, resulting in less cytokinin production, causing further disparity in the hormone balance in the plant. The root system starts going downhill and the new top growth fails to get adequate nutrition from the soil. This is when we have the greatest level of disease susceptibility. 


A daily supply of calcium is critical to a healthy plant, and not just in its regulation of auxin. Calcium also determines, or limits, fruit size during the 14-day cell division stage after pollination. If a plant gets a good supply of calcium to build cell membranes during this stage, the fruit will be large and of higher quality. During the cell expansion stage that follows cell division, potassium is king, serving as the locomotive that moves sugars and water into the fruit to make them firm and tasty.


The problem for farm managers is that calcium and potassium antagonize each other. Applying potassium early in a plant’s life cycle can prevent the plant’s uptake of calcium. For that, we have a simple solution: Stop applying potassium early. Wait until you are well into the fruit-fill period. 


As for getting calcium to your crop on a daily basis throughout the growing season, if you rely on foliar feeding for delivery, you will be in trouble after the cell division stage. However, if you supply calcium to the root system, and it moves up through the xylem to the upper part of the plant and into the fruit, it will ratchet back the auxin production. You won’t get rapid shoot growth from the top of the plant. You won’t create disease susceptibility. Fruit will begin to grow larger sooner and will fill out more slowly, heavily, and consistently. You will have more marketable fruit of higher quality.


I will tell you right now that calcium nitrate is not a calcium fertilizer. Many growers we’ve worked with have used calcium nitrate in irrigation systems and foliar sprays and we have never seen a calcium response on sap analysis. So we need to use other very available forms of calcium in the drip system to make sure that we have really aggressive biology to supply calcium to the root every day. 


Boron improves calcium absorption in a crop and helps get calcium to move into the fruit and into the upper part of the plant. We’ve experimented with growers using higher than normal concentrations of boron and calcium into the fruit fill stage yielded much better fruit quality without rapid shoot growth.


Another important element in achieving cytokinin dominance is cobalt, which positively impacts root tip growth. When we started incorporating cobalt into our products and programs, we saw almost immediate results in more aggressive plant growth below the soil.


The concept of critical points of influence, and understanding nutrient interactions at these critical points is the foundation for producing both high levels of marketable yield and healthy plants that resist insects and diseases.