By Kirk Van Leuven, Territory Sales Manager (California)
The role of the crop consultant is changing with regular and severe impacts of climate and water issues. Pest and disease problems can even be overshadowed by abiotic stress. This new complexity in the crop consultant’s job calls for upgraded tools and understanding. As a technology supplier, I’m hearing from PCA organizations that they need more training in agronomy and plant physiology to deal with what happens to plants under stress.
At Stoller, I’ve associated for 15 years now with an international network of over 400 agronomists, 20 Ph.Ds. and over 300 research professionals from about 70 countries where we do business. Our knowledge of the components of yield and crop physiology grows with each crop cycle in every country. The founder of the company, Jerry Stoller, spent a lot of time traveling and listening to growers’ needs. Growers shared that they generally know what to do about weeds, pests and diseases, but that often the greater impact on the crop is climate and weather events. Stoller went to work developing a toolset to lessen this impact of abiotic stress on yield and quality. The technology focuses on understanding each physiological stage of development and how stress might impact the crop at each point in its lifecycle.
The concepts of genetic expression and components of yield were developed for commodity crops like grains and soybeans, but yield for any crop depends on how that crop fares through each stage of its lifecycle. Stress or imbalance at any stage will irreversibly reduce genetic expression and crop performance.
A lot of this can be anticipated based on history and conditions for a particular crop and field location, and season-long programs can be developed. For the rest of this article, the focus will be on deciduous tree crops like fruits and nuts, but the concept of season-long programs for stress management can be applied as well to crops of all kinds.
Manage Stress by Growth Stage
Plants physiologically shift gears through different growth stages. I learned to drive on an old farm truck with a manual four-speed and twospeed axle shifter. Clutching and shifting took a little practice, especially when your legs didn’t quite reach the pedals. If you ended up grinding it or not getting into gear, you lost speed and would sometimes even have to go back down a gear. In a worst-case scenario, you might even have to stop and start over. The same concept applies to the physiological stages of crops. The genetic code of the plant wants to keep advancing through the life cycle, but stress or imbalances can slow or stall the development and effect flowering, fruit set, plant growth, fruit size, color or quality.
There are only two ways that a plant can grow: cell division and cell enlargement. Since cell division only takes place in the meristematic tissue, the main points of growth are the root tips and the apical growth points of the shoots. These two meristematic zones have potential for a longer period of cell division when environmental and nutritional conditions are met. New growth points can develop also in the buds. But when these develop into flowers and set fruit with their new embryos, the time for cellular division is relatively short before the shift is made to cell enlargement. Planning to optimize nutritional resources and managing stress through this critical cell division stage is an important feature of a good season-long program.
Deciduous fruit and nut trees start out with more than one source of stress in the spring. When we think of a typical bloom period for almonds or cherries for example, weather can be unpredictable. Soils are still below 60 degrees F and might be wet or dry. Nutrient uptake is limited. Without leaves and photosynthesis, the flowering and early growth of the fruit depends solely on nutritional and hormonal reserves from the prior season. Additionally, whether by design or by short supply, watering may not be started up yet. If water was limited also by CDI or lack of water at the end of the prior growing season, or over a dry winter, flowering and fruit set and early season growth can be severely affected. The flowering process itself is exhaustive of tree energy and nutritional reserves. If all of this sounds like a deprivation, it shouldn’t surprise anyone that only a small percentage of flowers set fruit, and then when competition within the tree causes a hunger, a drop will occur.
Stay Ahead of the Game
Economically, a season-long program of stress management and nutrition is not expensive when compared to the potential losses from inadequate water or nutrition, or from stress of any kind. Growers and consultants who understand crop nutrition and physiology with the key timings for components of yield will see positive returns on their investment to preserve and finish the crop.
Transplant shock is one of the most obvious forms of stress. Getting a new plant past the establishment stage to be actively growing in both the roots and the apical shoots is job one. Permanent crops planted from nursery stock are no different than a small vegetable transplant in this regard. Growing conditions are not always ideal at the time of planting. Having berms or beds and irrigation prepped and providing appropriate nutrition and stress management from the start is the fastest way to get your planting to early production. The best plant architecture requires appropriate vigor and fast establishment. For most varieties, medium vigor is better than too much. Vigorous scion/root combinations can have a problem settling into early production and high yields.
The two main delivery systems for fertilizer inputs on permanent crops are foliars and fertigation. When selecting the right fertilizer for the crop, having the choice of delivery systems for a particular timing is important. Foliar nutrients can be tank mixed when the timing lines up with other sprays. Some consultants are now timing pesticidal applications according to crop physiological timings rather than piggy-backing nutrient sprays on pest control. A good argument can be made for either case, but when crop phenology and pest and disease timings line up, there is nothing more efficient than tank-mixing the nutrients required by the plant for that timing. There are a lot of good foliar materials on the market, and even selected generic fertilizers can be appropriately used.
Protect the Roots
Some of the best foliar fertilizers have systemic activity, meaning within a short time, sometimes within hours, positive effects are visible in the plant. Examples of systemic activity is fruit that gains brix within 24 hours of an application, or the phosphite and phosphate foliars, which have observable effects in the root system after foliar applications. Root stimulation is one of the best ways fight plant stress. But when water deficits occur or root systems are compromised by water quality or mounting problems with soil chemistry, the root hairs die off, rendering an imbalance in the plant.
A famous quote by Jerry Stoller is that the “roots are the brains of the plant.” In particular, it is the root tips that are the brains. The root caps respond in the soil environment with biofeedback to the plant whether the requirements for growth are present. The key nutrient required for cell division and root growth is calcium. Root tips stop growing within hours when available calcium runs out in the soil solution, and in 7 to 14 days without replenishing this important ion, the root cap will die off. Calcium is required throughout the plant for cell division, so especially through the first half of the growing season this ion should be supplied in the rootzone. Not all calcium sources are equally available to the plant.
Calcium’s unique stress-fighting abilities in the plant result from two or three important ways that the plant uses it on a cellular level. As a structural building block of the cell wall, it is important for resistance to stress. It is also found in important levels in the cellular organelles and in the cytoplasm. Movements of the calcium ion inside the cell creates signaling for important plant processes. As a messenger, calcium has been compared to the plant hormones. In the rootzone, the calcium ion is recognized and actually preferred by the plant, and calcium uptake is prioritized above other ions including sodium. This blocks potential harm to the plant tissues. Providing available calcium in the soil solution can be a first line of defense against plant stress.
Stacking some of these fertilizing concepts together with specific timings for a season-long program is an effective way to improve crop performance and help with yield preservation under abiotic stress. When it comes to crop development, the timing and availability of nutrients is key. What may at first look like an intense nutritional managementt system is not that difficult to implement because of the ability to use either foliars or fertigation in different timings. Growers and consultants who are making these changes are finding that results with a season-long program is more consistent and the return-on-investment to learn it is significant.
