Monday, November 7, 2011

Rhizosphere Interactions

OK so you have had soil tests pulled, and fertilized accordingly to prepare for planting. Your background or bulk soil nutrition is where you need it to have a successful crop – weather permitting. You saved money and time by only using the type and amount of fertilizer needed.
So why do some crops fail to establish themselves and take off with vigorous growth? This question has many answers we hear all of the time but don’t pay much attention to. Solubility, availability, nutrient mobility in the soil, and the health and activity of the microbial populations in the soil are a few pieces of the puzzle.

We need to look specifically at the roots and the soil around and near the root zone also called the rhizosphere. This area of soil to root contact is where the action is.

Let’s look at solubility, availability, and we can’t forget the resource that is always running short - TIME. It takes time for that pre-plant fertilizer to dissolve into the soil solution and wind up in the right form for uptake by the crop. If the microbial populations in the soil are not diverse enough it could take even more time to access those nutrients. If it is cold and dry it could take even longer!

I would like to use phosphate as an example and I won’t dwell too long on the pH connection to phosphate availability. I will just mention that phosphate availability is more drastically reduced at the following pH values: pH 3-4 when it likes to tie up with iron, pH 5-6 when plentiful aluminum is the problem, and pH 7-8 when calcium can get in the way. Phosphate exists in two forms – orthophosphate and polyphosphate. Plants take up the orthophosphate ion and it takes time for the polyphosphate to convert to orthophosphate. It can take up to 30 days to convert only half of a polyphosphate fertilizer to orthophosphate. What’s my point? Despite your commendable soil testing and responsible fertilization – there can be gaps in nutrient availability that can lower yields and quality.

Avoid nutrient availability gaps. Use starter fertilizers with highly available nutrients placed in and near the root zone or where the roots will be very soon.

Your crop is not just sitting there idly waiting for the good stuff to come to it. Those plants can help themselves to an extent. Plant roots are like two-way streets. Water and nutrients are headed in, and organic acids, carbohydrates, and enzymes are leaking out. These outbound substances help acidify the soil zone directly surrounding the roots, dissolving nutrients, and providing sustenance for the microbial community which will continue to make even more nutrients available.

Years of farming practices can drastically alter the microbial health of your soil. Use a biological supplement with known beneficial organisms in it. Choose products that specify the organisms, population numbers, and proposed benefits.

The scientific literature is full of support for maintaining a thriving and diverse microbial community in the soil. Some benefits include disease suppression, increased mycorrhizal colonization and attachment to roots, and faster nutrient solubilization.

Let’s see how we’re doing… soil test – check, responsible fertilization – check, starter fertilizer – check, biological support – check. Looks good, but just like your crops roots, now is not the time to sit idly by and wait.

Monitor your crop and use plant and tissue sampling to see if things are headed in the right direction. When the reports come back from the lab, be prepared to take action. If there are nutrient deficiencies that will reduce yield or quality it is not too late. There are fast acting and highly available foliar nutrition products that can alleviate the deficiency.

Believe it or not, this practice of making foliar adjustments can make a big difference. In addition to supplying the plant with what it is lacking, foliar fertilization can increase the production of those desirable root exudates we discussed earlier. Choose foliar products with a small molecular weight carbon based carrier like the aromatic acids. Carbon is unique in its ability to bond with positively charged nutrient cations and negatively charged nutrient anions.

Anonymous quote from a crop adviser somewhere…

“All I ask is that you fertilize according to soil test recommendations, follow with starter fertilizers and biological support at planting, and make foliar adjustments according to tissue/plant samples."

These individual practices can seem too simple to make a difference, but when used as part of a combined program the results can be quite positive. These are some of the steps we use at Ag Tech Services, LLC to routinely boost spinach seed production here in the valley to quantities well above what the seed companies think a given variety can produce, and to push fresh market potato yields and quality to the next level.